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FLAM MARION'S POPULAR ASTRONOMY

Translated from the French by J. ELLARD GORE, F.R.A.S. With 3 Plates and 288 Illustrations.

" The six books into which the book is divided give a very lucid and accurate description of the knowledge which has been acquired of the moving bodies of space, both as respects their motions and physical constitutions. Of the translation we can only speak in terms of praise. Not only does it well represent the original, but Mr. Gore has added useful notes for the purpose of bringing the information up to date, and has also increased the number — already very considerable — of the excellent illustrations, so that the work is likely to become as popular in England as it has been in France."— ATHENAEUM.

"The work which Mr. J. E. Gore has translated into English has made for itself a name and reputation in France . . . and has gone into general circulation to the number of a hundred thousand copies. This last fact is proof how well within the bounds of possibility it is to make the latest discoveries of science comprehensible and fascinating to the common mind. M. Flammarion has attained this triumph through the grasp of his knowledge, the lucidity of his style, and his power of bringing home the most stupendous and complicated of the things revealed to us in the depths of space. M. Flammarion's pages should find almost as great acceptance in this country as in his own. Simplicity of arrangement and of statement are part of his charm and of his success." — SCOTSMAN.

" M. Flammarion's latest volume, if it does not displace its English rivals, may well take a high place in the rank to which they belong. It is full} lucid, and, thanks to Mr. Gore's careful revision, well up to date. . . . Mr. Gore's edition is so carefully brought abreast of the latest discoveries that the English student may now con- gratulate himself on being in an even better position than the countrymen of M. Flammarion." — DAILY CHRONICLE.

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" M. Flammarion is a sound practical astronomer ; he has rendered good and laborious service to the science, and he possesses a valuable faculty of popular exposition. . . . The volume is profusely and well illustrated, some of the best plates making here their first appearance." — SATURDAY REVIEW.

"A high place must be accorded to Flammarion's ' Popular Astronomy.' Never before has the science of the heavens been treated with such fulness and interest as in this fascinating book ; for Flamraarion is a man of letters as well as a man of science — a man of letters, too, endowed with the wondrous gifts of lucidity and charm which distinguish the best French writers. . . . Flammarion's book is much more absorbing than most novels, more romantic than most romances, more poetic than most poems, yet strictly and scientifically accurate." — LUDGATE MONTHLY.

" It must be confessed that M. Flammarion not only arrests the attention, but assists the reader to grasp astronomical theories— a task in which less popular writers often fail when they make the attempt."— LITERARY WORLD.

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LONDON : CIIATTO & WJNDUS, in ST. MARTIN'S LANE, W.C.

ASTRONOMICAL CURIOSITIES

FACTS AND FALLACIES

ASTRONOMICAL CURIOSITIES

FACTS AND FALLACIES

BY

J. ELLARD GORE

MEMBER OF THE ROYAL IRISH ACADEMY FELLOW OK THE ROYAL ASTRONOMICAL SOCIETY

CORRESPONDING MEMBER OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA

ETC.

AUTHOR OF "ASTRONOMICAL ESSAYS," "STUDIES IN ASTRONOMY" "THE VISIBLE UNIVERSE," ETC.

- or THE UNIVERSITY

•lfe«

LONDON CHATTO & WINDUS

1909

PRINTED BY

WILLIAM CLOWES AND SONS, LIMITED LONDON AND BECCLES

All rights reserved

PREFACE

THE curious facts, fallacies, and paradoxes con- tained in the following pages have been collected from various sources. Most of the information given will not, I think, be found in popular works on astronomy, and will, it is hoped, prove of interest to the general reader.

J. E. G.

September, 1900.

201609

CONTENTS

CHAPTER P1GB

I. THE SUN ... ... ... ... ... 1

II. MERCURY ... ... ... ... 10

in. VENUS ... ... ... ... ... 17

IV. THE EARTH ... ... ... ... 32

V. THE MOON ... ... ... ... 48

VI. MARS ... ... ... ... 59

Vn. THE MINOR PLANETS ... ... ... 68

vm. JUPITER ... ... ... ... 74

IX. SATURN ... ... ... ... ... 84

X. URANUS AND NEPTUNE ... ... ... 91

XI. COMETS ... ... ... ... ... 97

XII. METEORS ... ... ... ... 117

XIII, THE ZODIACAL LIGHT AND GEGEXSCHKIN ... 127

XIV. THE STARS ... ... ... ... 135

XV. DOUBLE AND BINARY STARS ... ... ... 160

XVI. VARIABLE STARS ... ... ... 170

XVII. NEBUUE AND CLUSTERS ... ... ... 191

XVIII. HISTORICAL ... ... ... ... 217

XIX. THE CONSTELLATIONS ... ... ... 239

XX. THE VISIBLE UNIVERSE ... ... ... 313

XXI. GENERAL ... ... ,.* ... ... 329

INDEX .,, 359

ILLUSTRATIONS

PAGE

AL-SUFI'S "EARTHEN JAR" ... .*• 247

AL-SUFI'S "FISHES" IN ANDROMEDA ... ... 249

0? THE

UNIVERSITY

OF

ASTRONOMICAL CURIOSITIES

CHAPTER I

The Sun

SOME observations recently made by Prof. W. H. Pickering in Jamaica, make the value of sunlight 540,000 times that of moonlight. This makes the sun's " stellar magnitude " minus 2()'83, and that of moonlight minus 12'5. Prof. Pickering finds that the light of the full moon is equal to 100,000 stars of zero magnitude. He finds that the moon's " albedo " is about 0-0909 ; or in other words, the moon reflects about one-tenth of the light which falls on it from the sun. He also finds that the light of the full moon is about twelve times the light of the half moon : a curious and rather unexpected result.

M. C. Fabry found that during the total eclipse of the sun on August 30, 1905; the light of the corona at a distance of five minutes of arc from the sun's limit, and in the vicinity of the sun's equator, was about 720 candle-power. Comparing this

B

2 ASTRONOMICAL CURIOSITIES

with the intrinsic light of the full moon (2600 candle-power) we have the ratio of 0*28 to 1. He finds that the light of the sun in the zenith, and at its mean distance from the earth, is 100,000 times greater than the light of a " decimal candle " placed at a distance of one metre from the eye.1 He also finds that sunlight is equal to 60,000 million times the light of Vega. This would make the sun's " stellar magnitude " minus 26*7, which does not differ much from Prof. Pickering's result, given above, and is probably not far from the truth.

From experiments made in 1906 at Moscow, Prof. Ceraski found that the light of the sun's limb is only 31*4 to 38'4 times brighter than the illumination of the earth's atmosphere very near the limb. This is a very unexpected result ; and considering the comparative faintness of the sun's corona during a total eclipse, it is not surprising that all attempts to photograph it without an eclipse have hitherto failed.2

From Paschen's investigations on the heat of the sun's surface, he finds a * result of 5961° (absolute), " assuming that the sun is a perfectly black body."3 Schuster finds that "There is a stratum near the sun's surface having an average temperature of approximately 5500° C., to which about 0*3 of the sun's radiation is due. The

1 Comptes Rendus, 1903, December 7.

• Nature, April 11, 1907.

3 Astrophysical Journal, vol. 19 (1904), p. 39.

THE SUN 3

remaining portion of the radiation has an intensity equal to that due to a black body having a temperature of about 6700° C." The above results agree fairly well with those found by the late Dr. W. E. Wilson.1 The assumption of the sun being " a black body " seems a curious paradox ; but the simple meaning of the state- ment is that the sun is assumed to act as a radiator as if it vvere a perfectly black body heated to the high temperature given above.

According to Prof. Langley, the sun's photo- sphere is 5000 times brighter than the molten metal in a " Bessemer converter." 2

Observations of the sun even with small tele- scopes and protected by dark glasses are very dangerous to the eyesight. Galileo blinded him- self in this way ; Sir William Herschel lost one of his eyes ; and some modern observers have also suffered. The present writer had a narrow escape from permanent injury while observing the transit of Venus, in 1874, in India, the dark screen before the eyepiece of a 3-inch telescope having blistered — that is, partially fused during the observation. Mr. Cooper, Markree Castle, Ireland, in observing the sun, used a " drum " of alum water and dark spectacles, and found this sufficient protection against the glare in using his large refracting telescope of 13%3-inches aperture.

1 Astrophysical Journal, vol. 21 (1905), p. 260.

2 Knowledge, July, 1902, p. 132.

4 ASTRONOMICAL CURIOSITIES

Prof. Mitchell, of Columbia University (U.S.A.), finds that lines due to the recently discovered atmospherical gases argon and neon are present in the spectrum of the sun's chromosphere. The evidence for the existence of krypton and xenon is, however, inconclusive. Prof. Mitchell suggests that these gases may possibly have reached the earth's atmosphere from the sun. This would agree with the theory advanced by Arrhenius that " ionised particles are constantly being repulsed by the pressure of light, and thus journey from one sun to another."1

Prof. Young in 1870, and Dr. Kreusler in June, 1904, observed the helium line D3 as a dark line " in the spectrum of the region about a sun-spot." a This famous line, from which helium was originally discovered in the sun, and by which it was long afterwards detected in terrestrial minerals, usually appears as a bright line in the spectrum of the solar chromosphere and "prominences." It has also been seen dark by Mr. Buss in sun-spot regions.3

The discovery of sun-spots was claimed by Hariotte, in 1610, and by Galileo, Fabricius, and Schemer, in 1611. The latter wrote 800 pages on them, and thought they were small planets revolv- ing round the sun ! This idea was also held by Tarde, who called them Astra Borbonia, and by

* Nature, April 30, 1903. 2 Ibid., May 18, 1905.

3 Ibid., May 18, 1905.

THE SUN 5

C. Malapert, who termed them Sydera Austricea. But they seem to have been noticed by the ancients.

Although in modern times there has been no extraordinary development of sun-spots at the epoch of maximum, it is not altogether impossible that in former times these spots may have occasionally increased to such an extent, both in number and size, as to have perceptibly darkened the sun's light. A more probable explanation of recorded sun-darkenings seems, however, to be the passing of a meteoric or nebulous cloud between the sun and the earth. A remarkable instance of sun-darkening recorded in Europe occurred on May 22, 1870, when the sun's light was observed to be considerably reduced in a cloudless sky in the west of Ireland, by the late John Birmingham ; at Greenwich on the 23rd ; and on the same date, but at a later hour, in North-Eastern France — " a progressive manifestation," Mr. Birmingham says, " that seems to accord well with the hypothesis of moving nebulous matter." A similar phenomenon was observed in New England (U.S.A.), on September 6, 1881.

One of the largest spots ever seen on the sun was observed in June, 1843. It remained visible for seven or eight days. According to Schwabe — the discoverer of the sun-spot period — its diameter was 74,000 miles, so that its area was many times

6 ASTRONOMICAL CURIOSITIES

that of the earth's surface. The most curious thing about this spot was that it appeared near a mjnwium of the sun-spot cycle ! and was there- fore rather an anomalous phenomenon. It was suggested by the late Daniel Kirkwood that this great spot was caiised by the fall of meteoric matter into the sun ; and that it had possibly some connection with the great comet of 1843, which approached the sun nearer than any other recorded comet, its distance from the sun at perihelion being about 65,000 miles, or less than one-third of the moon's distance from the earth. This near approach of the comet to the sun occurred about three months before the appear- ance of the great sun-spot ; and it seems probable that the spot was caused by the downfall of a large meteorite travelling in the wake of the comet.1 The connection between comets and meteors is well known.

The so-called blackness of sun-spots is merely relative. They are really very bright. The most brilliant light which can be produced artificially looks like a black spot when projected on the sun's disc.

According to Sir Robert Ball a pound of coal striking a body with a velocity of five miles a second would develop as much heat as it would produce by its combustion. A body falling into the sun from infinity would have a velocity of 1 Nature, June 29, 1871.

THE SUN 7

450 miles a second when it reached the sun's surface. Now as the momentum varies as the square of the velocity we have a pound of coal

developing 902( = ^-Y, or 8,100 times as much

heat as would be produced by its combustion. If the sun were formed of coal it would be con- sumed in about 3000 years. Hence it follows that the contraction of the sun's substance from infinity would produce a supply of heat for 3000 x 8100, or 24,300,000 years.

The late Mr. Proctor and Prof. Young believed " that the contraction theory of the sun's heat is the true and only available theory.'* The theory is, of course, a sound one; but it may now be supplemented by supposing the sun to contain a certain small amount of radium. This would bring physics and geology into harmony. Proctor thought the " sun's real globe is very much smaller than the globe we see. In other words the process of contraction has gone on further than, judging from the sun's apparent size, we should suppose it to have done, and therefore represents more sun work " done in past ages.

With reference to the suggestion, recently made, that a portion, at least, of the sun's heat may be due to radium, and the experiments which have been made with negative results, Mr. R. T. Strutt — the eminent physicist — has made some calculations on the subject and says, " even if all

8 ASTRONOMICAL CURIOSITIES

the sun's heat were due to radium, there does not appear to be the smallest possibility that the Becquerel radiation from it could ever be detected at the earth's surface." l

The eminent Swedish physicist Arrhenius, while admitting that a large proportion of the sun's heat is due to contraction, considers that it is probably the chemical processes going on in the sun, and not the contraction which constitiite the chief source of the solar heat.2

As the centre of gravity of the sun and Jupiter lies at a distance of about 460,000 miles from the sun's centre, and the sun's radius is only 433,000 miles, it follows that the centre of gravity of the sun and planet is about 27,000 miles outside the sun's surface. The attractions of the other planets perpetually change the position of the centre of gravity of the solar system; but in some books on astronomy it is erroneously stated that the centre of gravity of the system is always within the sun's surface. If all the planets lay on the same side of the sun at the same time (as might possibly happen), then the centre of gravity of the whole system would lie considerably more than 27,000 miles outside the sun's surface.

With reference to the sun's great size, Carl Snyder has well said, " It was as if in Vulcan's

1 Nature, October 15, 1903.

2 The Life of the Universe (1909). vol. ii. p. 209.

THE SUN 9

smithy the gods had moulded one giant ball, and the planets were but bits and small shot which had spattered off as the glowing ingot was cast and set in space. Little man on a little part of a little earth — a minor planet, a million of which might be tumbled into the shell of the central sun — was growing very small ; his wars, the con- vulsions of a state, were losing consequence. Human endeavour, human ambitions could now scarce possess the significance they had when men could regard the earth as the central fact of the universe." '

With reference to the late Prof. C. A. Young (U.S.A.) — a great authority on the sun — an American writer has written the following lines : —

" The destined course of whirling worlds to trace, To plot the highways of the universe, And hear the morning stars their song rehoarse, And find the wandering comet in his place ; This is the triumph written in his face, And in the gleaming eye that read the sun Like open book, and from the spectrum won The secrets of immeasurable space." 2

1 The World Machine, p. 234.

- Quoted in The Observatory, March 1908, p. 125.

CHAPTER II

Mercury

A the elongation of Mercury from the sun seldom exceeds 18°, it is a difficult object, at least in this country, to see without a telescope. As the poet says, the planet —

" Can scarce be caught by philosophic eye Lost in the near effulgence of its blaze."

Tycho Brahe, however, records several obser- vations of Mercury with the unaided vision in Denmark.

It can be occasionally caught with the naked eye in this country after sunset, when it is favourably placed for observation, and I have so seen it several times in Ireland. On February 19, 1888, I found it very visible in strong twilight near the western horizon, and apparently brighter than an average star of the first magnitude would be in the same position. In the clear air of the Punjab sky I observed Mercury on November 24-29, 1872, near the western horizon after sunset. Its appear- ance was that of a reddish star of the first magni- tude. On November 29 I compared its brilliancy

MERCURY 11

with that of Saturn, which was some distance above it, and making allowance for the glare near the horizon in which Mercury was immersed, its brightness appeared to me to be quite equal to that of Saturn. In June, 1874, I found it equal to Aldebaran, and of very much the same colour. Mr. W. F. Denning, the famous observer of meteors, states that he observed Mercury with the naked eye about 150 times during the years 1868 to 1905.1

He found that the duration of visibility after sunset is about lh 40m when seen in March, lh 30m in April, and lh 20m in May. He thinks that the planet is, at its brightest, " certainly much brighter than a first magnitude star.2 In February, 1868, he found that its brightness rivalled that of Jupiter, then only 2° or 3° distant. In November, 1882, it seemed brighter than Sirius. In 1876 it was more striking than Mars, but the latter was then " faint and at a considerable distance from the earth."

In 1878, when Mercury and Venus were in the same field of view of a telescope, Nasmyth found that the surface brightness (or " intrinsic bright- ness," as it is called) of Venus was at least twice as great as that of Mercury ; and Zollner found that from a photometric point of view the surface of Mercury is comparable with that of the moon.

1 The Observatory, September, 190C.

2 Nature, March 1, 1900.

12 ASTRONOMICAL CURIOSITIES

With reference to the difficulty of seeing Mercury, owing to its proximity to the sun, Admiral Smyth says, " Although Mercury is never in opposition to the earth, he was, when in the house of Mars, always viewed by astrologers as a most malignant planet, and one full of evil influences. The sages stigmatized him as a false deceitful star (sidus dolosum), the eternal torment of astronomers, eluding them as much as terres- trial mercury did the alchemists ; and Goad, who in 1686 published a whole folio volume full of astro-meteorological aphorisms, unveiling the choicest secrets of nature, contemptuously calls Mercury a * squinting lacquey of the sun, who seldom shows his head in these parts, as if he was in debt.' His extreme mobility is so striking that chemists adopted his symbol to "denote quick- silver." 1

Prof. W. H. Pickering thinks that the shortness of the cusps (or " horns ") of Mercury's disc indicates that the planet's atmosphere is of small density — even rarer than that of Mars.

The diameter of Mercury is usually stated at about 3000 miles ; but a long series of measures made by Prof. See in the year 1901 make the real diameter about 2702 miles. This would make the planet smaller than some of the satellites of the large planets, probably smaller than satellites III. and IV. of Jupiter, less than Saturn's satellite 1 Cycle of Celestial Objects, p. 96.

MERCURY 13

Titan, and possibly inferior in size to the satellite of Neptune. Prof. Pickering thinks that the density of Mercury is about 3 (water = 1). Dr. See's observations show " no noticeable falling off in the brightness of Mercury near the limb." There is therefore no evidence of any kind of atmospheric absorption in Mercury, and the observer " gets the impression that the physical condition of the planet is very similar to that of our moon." l '

Schroter (1780-1815) observed markings on Mercury, from which he inferred that the planet's surface was mountainous, and one of these mountains he estimated at about 11 miles in height ! 2 But this seems very doubtful.

To account for the observed irregularities in the motion of Mercury in its orbit, Prof. Newcomb thinks it possible that there may exist a ring or zone of " asteroids " a little " outside the orbit of Mercury" and having a combined mass of " one - fiftieth to oiie-three-hundredth of the mass of Venus, according to its distance from Mercury." Prof. Newcomb, however, considers that the existence of such a ring is extremely improbable, and regards it " more as a curiosity than a reality." 3

M. Leo Brenner thinks that he has seen the

1 Ast. Naeh. No. 3737.

2 Observatory, September, 1906.

3 Nature, November 29 and December 20, 1894.

U ASTRONOMICAL CURIOSITIES

dark side of Mercury, in the same way that the dark side of Venus has been seen by many observers. In the case of Mercury the dark side appeared darker than the background of the sky. Perhaps this may be due to its being projected on the zodiacal light, or outer envelope of the

sun.1

Mercury is said to have been occulted by Venus in the year 1737.3 But whether this was an actual occultation, or merely a near approach does not seem to be certain.

The first transit of Mercury across the sun's disc was observed by Gassendi on November 6, 1631, and Halley observed one on November 7, 1677, when in the island of St. Helena.

Seen from Mercury, Venus would appear brighter than even we see it, and as it would be at its brightest when in opposition to the sun, and seen on a dark sky with a full face, it must present a magnificent appearance' in the midnight sky of Mercury. The earth will also form a brilliant object, and the moon would be distinctly visible. The other planets would appear very much as they do to us, but with somewhat less brilliancy owing to their greater distance.

As the existence of an intra-Mercurial planet (that is a planet revolving round the sun within the orbit of Mercury) seems now to be very

1 Bulletin, Ast. Soc. de France, July, 1898.

2 Observatory, vol. 8 (1885), pp. 300-7.

r

MERCURY 15

improbable, Prof. Perrine suggests that possibly " the finely divided matter which produces the zodiacal light when considered in the aggregate may be sufficient to cause the perturbations the orbit of Mercury." l Prof. Newcomb, how ever, questions the exact accuracy of Newton's law, and seems to adopt Hall's hypothesis that gravity does not act exactly as the inverse square of the distance, and that the exponent of the distance is not 2, but 2'0000001574.2

Voltaire said, " If Newton had been in Portugal, and any Dominican had discovered a heresy in his inverse ratio of the squares of the distances, he would without hesitation have been clothed in a san benito, and burnt as a sacrifice to God at an auto da /<£." 3

An occultation of Mercury by Venus was observed with a telescope on May 17, 1737.4

May transits of Mercury across the sun's disc will occur in the years 1924, 1957, and 1970 ; and November transits in the years 1914, 1927, and 1940.5

From measurements of the disc of Mercury during the last transit, M. R. Jonckheere concludes that the polar diameter of the planet is greater

1 Nature, October 30, 1902.

2 Charles Lane Poor, TJie Solar System, p. 170.

3 Smyth, Celestial Cycle, p. 60.

4 Denning, Telescopic itfbrk for Starlight Evenings, p. 225.

5 The Observatory, 1894. p. 395.

16 ASTRONOMICAL CURIOSITIES

than the equatorial! His result, which is very curious, if true, seems to be supported by the observations of other observers.1

The rotation period of Mercury, or the length of its day, seems to be still in doubt. From a series of observations made in the years 1890 to 1909, Mr. John McHarg finds a period of 1-0121162 day, or ld Oh 17m 26S'8. He thinks that "the planet possesses a considerable atmosphere not so clear as that of Mars " ; that " its axis is very considerably tilted " ; and that it " has fairly large sheets of water." 2

1 Ast. Nach. 4333, quoted in Nature, July 1, 1009, p. 20.

2 English Mechanic, July 23, 1901).

CHAPTER III

Venus

VENUS was naturally — owing to its bright- ness— the first of the planets known to the ancients. It is mentioned by Hesiod, Homer, Virgil, Martial, and Pliny ; and Isaiah's remark about "Lucifer, son of the morning" (Isaiah xiv. 12) probably refers to Venus as a " morning star." An observation of Venus is found on the Nineveh tablets of date B.C. 684. It was observed in daylight by Halley in July, 1716.

In very ancient times Venus, when a morning star, was called Phosphorus or Lucifer, and when an evening star Hesperus ; but, according to Sir G. C. Lewis, the identity of the two objects was known so far back as 540 B.C.

When Venus is at its greatest brilliancy, and appears as a morning star about Christmas time (which occurred in 1887, and again in 1889), it has been mistaken by the public for a return of the " Star of Bethlehem." l But whatever " the star 1 Nature, December 22, 1892.

C

18 ASTRONOMICAL CURIOSITIES

of the Magi" was it certainly was not Venus. It, seems, indeed absurd to suppose that " the wise men " of the East should have mistaken a familiar object like Venus for a strange apparition. There seems to be nothing whatever in the Bible to lead us to expect that the star of Bethlehem will reappear.

Mr. J. H. Stockwell has suggested that the " Star of Bethlehem " may perhaps be explained by a conjunction of the planets Venus and Jupiter which occurred on May 8, B.C. 6, which was two years before the death of Herod. From this it would follow that the Crucifixion took place on April 3, A.D. 33. But it seems very doubtful that the phenomenon recorded in the Bible refers to any conjunction of planets.

Chacornac found the intrinsic brightness of Venus to be ten times greater than the most luminous parts of the moon.1 But this estimate is probably too high.

When at its brightest, the planet is visible in broad daylight to good eyesight, if its exact position in the sky is known. In the clear air of Cambridge (U.S.A.) it is said to be possible to see it in this way in all parts of its orbit, except when the planet is within 10° of the sun.2 Mr. A. Cameron, of Yarmouth, Nova Scotia, has, however, seen Venus with the naked eye three

1 Celestial Objects> vol. i. p. 52, footnote.

2 Ibid., p. 54.

VENUS 19

days before conjunction when the planet was only 61° from the sun.1 This seems a remarkable observation, and shows that the observer's eye- sight must have been very keen. In a private letter dated October 22, 1888, the late Rev. S. J. Johnson informed the present writer that he saw Venus with the naked eye only four days before conjunction with the sun in February, 1878, and February, 1886.

The crescent shape of Venus is said to have been seen with the naked eye by Theodore Parker in America when he was only 12 years old. Other- observers have stated the same thing ; but the possibility of such an observation has been much disputed in recent years.

In the Chinese Annals some records are given of Venus having been seen in the Pleiades. On March 16, A.D. 845, it is said that " Venus eclipsed the Pleiades.'* This means, of course, that the cluster was apparently effaced by the brilliant light of the planet. Computing backwards for the above date, Hind found that on the evening of March 16, 845, Venus was situated near the star Electra ; and on the following evening the planet passed close to Maia ; thus showing the accuracy of the Chinese record. Another " eclipse " of the Pleiades by Venus is recorded in the same annals as having occurred 011 March 10, A.D. 1002.2 H

1 Astronomy and Astrophysics, 1892, p. 618. - Nature, August 7, 1879.

20 ASTRONOMICAL CURIOSITIES

When Venus is in the crescent phase, that is near " Inferior conjunction " with the sun, it will be noticed, even by a casual observer, that the crescent is not of the same shape as that of the crescent moon. The horns or "cusps" of the planetary crescent are more prolonged than in the case of the moon, and extend beyond the hemisphere. This appearance is caused by refrac- tion of the sun's light through the planetary atmosphere, and is, in fact, a certain proof that Venus has an atmosphere similar to that of the earth. Observations further show that this atmosphere is denser than ours.

Seen from Venus, the earth and moon, when in opposition, must present a splendid spectacle. I find that the earth would shine as a star about half as bright again as Venus at her brightest appears to us, and the moon about equal in brightness to Sirius! the two forming a superb " naked eye double star " — perhaps the finest sight of its kind in the solar system.1

Some of the earlier observers, such as La Hire, Fontana, Cassini, and Schroter, thought they saw evidence of mountains on Venus. Schroter estimated some of these to be 27 or 28 miles in height! but this seems very doubtful. Sir William Herschel severely attacked these supposed discoveries. Schroter defended himself, and was supported by Beer and Madler, the famous 1 The World of Space, p. 56.

VENUS 21

lunar observers. Several modern observers seem to confirm Schroter's conclusions ; but very little is really known about the topography of Venus.

The well-known French astronomer Trouvelot — a most excellent observer — saw white spots on Venus similar to those on Mars. These were well seen and quite brilliant in July and August, 1876, and in February and November, 1877. The observations seem to show that these spots do not (unlike Mars) increase and decrease with the planet's seasons. These white spots had been previously noticed by former observers, including Biaiichini, Derham, Gruithuisen, and La Hire; but these early observers do not seem to have considered them as snow caps, like those of Mars. Trouvelot was led by his own observations to conclude that the period of rotation of Venus is short, and the best result he obtained was 23h 49m 28s. This does not differ much from the results previously found by De Vico, Fritsch, and Schroter.1

A white spot near the planet's south pole was seen on several occasions by H. C. Russell in May and June, 1876.2

Photographs of Venus taken 011 March 18 and April 29, 1908, by M. Quenisset at the Observatory of Juvissy, France, show a white polar spot. The

1 Nature, September 15, 1892.

2 Olservatory, 1880, p. 574.

22 ASTRONOMICAL CURIOSITIES

spot was also seen at the same observatory by M. A. Benoit on May 20, 1903.

The controversy on the period of rotation of Venus, or the length of its day, is a very curious one and has not yet been decided. Many good observers assert confidently that it is short (about 24 hours); while others affirm with equal con- fidence that it is long (about 225 days, the period of the planet's revolution round the sun). Among the observers who favour the short period of rotation are : D. Cassini (1667), J. Cassini (1730), Schroter (1788-93), Madler (1836), De Vico (1840?) Trouvelot (1871-79), Flammarion, Leo Brenner, Stanley Williams, and J. McHarg; and among those who support the long period are : Bianchini (1727), Schiaparelli, Cerulli, Tacchini, Mascari, and Lowell. Some recent spectroscopic observa- tions seem to favour the short period.

Flammarion thinks that "nothing certain can be descried upon the surface of Venus, and that whatever has hitherto been written regarding its period of rotation must be considered null and void"; and again he says, "Nothing can be affirmed regarding the rotation of Venus, inas- much as the absorption of its immense atmosphere certainly prevents any detail on its surface from being perceived." 1

The eminent Swedish physicist Arrhenius thinks, however, that the dense atmosphere and clouds 1 Knowledge, November 1, 1897, pp. 200, 2G1.

VENUS 23

of Venus are in favour of a rapid rotation 011 its axis.1 He thinks that the mean temperature of Venus may " not differ much from the calculated temperature 104° F." " Under these circumstances the assumption would appear plausible that a very considerable portion of the surface of Venus, and particularly the districts about the poles, would be favourable to organic life." 2

The " secondary light of Venus," or the visibility of the dark side, seems to have been first mentioned by Derham in his Astro Theology published in 1715. He speaks of the visibility of the dark part of the planet's disc "by the aid of a light of a somewhat dull and ruddy colour." The date of Derham's observation is not given, but it seems to have been previous to the year 1714. The light seems to have been also seen by a friend of Derham. We next find observations by Christfried Kirch, assistant astronomer to the Berlin Academy of Sciences, on June 7, 1721, and March 8, 1726. These observations are found in his original papers, and were printed in the Astronomische Nachrichten, No. 1586. On the first date the telescopic image of the planet was "rather tremulous," but in 1726 he noticed that the dark part of the circle seemed to belong to a smaller circle than the illuminated portion of the disc.3 The same effect was also noted by

1 Worlds in the Making, p. 61. 2 Ibid., p. 48.

3 Nature, June 1, 1876.

24 ASTRONOMICAL CURIOSITIES

Webb.1 A similar illusion is seen in the case of the crescent moon, and this has given rise to the saying, " the old moon in the new moon's arms."

We next come, in order of date, to an observa tion made by Andreas Mayer, Professor of Mathematics at Grief swald in Prussia. The observation was made on October 20, 1759, and the dark part of Venus was seen distinctly by Mayer. As the planet's altitude at the time was not more than 14° above the horizon, and its apparent distance from the sun only 10°, the phenomenon — as Professor Safarik has pointed out — " must have had a most unusual intensity."

Sir William Herschel makes no mention of having ever seen the " secondary light " of Venus, although he noticed the extension of the horns beyond a semicircle.

In the spring and summer of the year 1793, Von Hahii of Remplin in Mecklenburg, using excellent telescopes made by Dollond and Herschel, saw the dark part of Venus on several occasions, and describes the light as "grey verging upon brown."

Schroter of Lilienthal — the famous observer of the moon — saw the horns of the crescent of Venus extended many degrees beyond the semi- circle on several occasions in 1784 and 1795, and the border of the dark part faintly lit up by a dusky grey light. On February 14, 1806, at 1 Cel, Object*, vol. i. p. CC (5th Edition).

VENUS 25

7 P.M. he saw the whole of the dark part visible with an ash-coloured light, and he was satisfied that there was no illusion. On January 24 of the same year, 1806, Harding at Gottingen, using a reflector of 9 inches aperture and power 84, saw the dark side of Venus " shining with a pale ash-coloured light," and very visible against the dark background of the sky. The appearance was seen with various magnifying powers, and he thought that there could be no illusion. In fact the phenomenon was as evident as in the case of the moon. Harding again saw it on February 28 of the same year, the illumination being of a reddish grey colour, " like that of the moon in a total eclipse."

The " secondary light " was also seen by Pastorff in 1822, and by Gruithuisen in 1825. Since 1824 observations of the " light" were made by Berry, Browning, Guthrie, Langdon, Noble, Prince, Webb, and others. Webb saw it with powers of 90 and 212 on a 9*38-inch mirror, and found it " equally visible when the bright crescent was hidden by a field bar." l

Captain Noble's observation was rather unique. He found that the dark side was " always dis- tinctly and positively darker than the background upon which it is projected."

The "light" was also seen by Lymaii in America in 1867, and by Safarik at Prague. In 1 Celestial Objects, vol. i. p. 63 (5th Edition).

26 ASTRONOMICAL CURIOSITIES

1871 the whole disc of Venus was seen by Pro- fessor Winnecke.1 On the other hand, Winnecke stated that he only saw it twice in 24 years ; and the great observers Dawes and Madler never saw it at all ! 2

Various attempts have been made to explain the visibility— at times — of the " dark side " of Venus. The following may be mentioned3: — (1) Reflected earth-light, analogous to the dark side of the crescent moon. This explanation was advo- cated by Harding, Schroter, and others. But, although the earth is undoubtedly a bright object in the sky of Venus, the explanation is evidently quite inadequate. (2) Phosphorescence of the planet's atmosphere. This has been suggested by some observers. (3) Visibility by contrast, a theory advanced by the great French astronomer Arago. (4) Illumination of the planet's surface by an aurora borealis. This also seems rather inadequate, but would account for the light being sometimes visible and sometimes not. (5) Lumi- nosity of the oceans — if there be any — on Venus. But this also seems inadequate. (6) A planetary surface glowing with intense heat. But this seems improbable. (7) The Kunstliche Feuer (artificial fire) of Gruithuisen, a very fanciful theory. Flammarion thinks that the visibility of the dark side may perhaps be explained by its

1 Ast. Nacli. No. 1863. 2 Nature, June 1, 1876.

3 Ibid., June 8, 1876.

VENUS 27

projection 011 a somewhat lighter background, such as the zodiacal light, or an extended solar envelope.1

It will be seen that none of these explanations are entirely satisfactory, and the phenomenon, if real, remains a sort of astronomical enigma. The fact that the "light" is visible on some occasions and not on others would render some of the explanations improbable or even inadmissible. But the condition of the earth's atmosphere at times might account for its invisibility on many occasions.

A curious suggestion was made by Zollner, namely, that if the secondary light of Venus could be observed with the spectroscope it would show bright lines ! But such an observation would be one of extreme difficulty.

M. Hansky finds that the visibility of the "light" is greater during periods of maximum solar activity — that is, at the maxima of sun spots. This he explains by the theory of Arrhenius, in which electrified " ions emitted by the sun cause the phenomena of terrestrial magnetic storms and auroras." " In the same way the dense atmosphere of Venus is rendered more phos- phorescent, and therefore more easily visible by the increased solar activity." 2 This seems a very plausible hypothesis.

On the whole the occasional illumination of the

1 Nature, October 17, 1895. > 11ml, July 27, 1905.

28 ASTRONOMICAL CURIOSITIES

night side of Venus by a very brilliant aurora (explanation (4) above) seems to the present writer to be the most probable explanation. Gruithuisen's hypothesis (7) seems utterly im- probable.

There is a curious apparent anomaly about the motion of Venus in the sky. Although the planet's period of revolution round the sun is 224'7 days, it remains on the same side of the sun, as seen from the earth, for 290 days. The reason of this is that the earth is going at the same time round the sun in the same direction, though at a slower pace ; and Venus must continue to appear on the same side of the sun until the excess of her daily motion above that of the earth amounts to 179°, and this at the daily rate of 37' will be about 290 days/

Several observations have been recorded of a supposed satellite of Venus. But the existence of such a body has never been verified. In the year 1887, M. Stroobant investigated the various accounts, and came to the conclusion that in several at least of the recorded observations the object seen was certainly a star. Thus, in the observation made by Rcedickcer and Boserup on August 4, 1761, a satellite and star are recorded as having been seen near the planet. M. Stroobant finds that the supposed " satellite " was the star x* Orionis, and the " star " %. Orionis. 1 Cekstial Cycle, p. 107.

VENUS 29

A supposed observation of a satellite made by Horrebow 011 January 3, 1768, was undoubtedly 0 Librae. M. Stroobant found that the supposed motion of the " satellite " as seen by Horrebow is accurately represented by the motion of Venus itself during the time of observation. In most of the other supposed observations of a satellite a satisfactory identification has also been found. M. Stroobant finds that with a telescope of 0 inches aperture, a star of the 8th or even the 9th magnitude can be well seen when close to Venus.1

On the night of August 13, 1892, Professor Barnard, while examining Venus with the great 36-inch telescope of the Lick Observatory, saw a star of the 7th magnitude in the same field with the planet. He carefully determined the exact position of this star, and found that it is not in Argelander's great catalogue, the Durch- musterung. Prof. Barnard finds that owing to elongation of Venus from the sun at the time of observation the star could not possibly be an intra-Mercurial planet (that is, a planet revolving round the sun inside the orbit of Mercury) ; but that possibly it might be a planet revolving between the orbits of Venus and Mercury. As the brightest of the minor planets — Ceres, Pallas, Juno, and Vesta — were not at the time near the position of the observed object, the observation 1 Nature, October 6, 1887.

30 ASTRONOMICAL CURIOSITIES

remains unexplained. It might possibly have been a nova, or temporary star.1

Scheuten is said to have seen a supposed satellite of Venus following the planet across the sun at the end of the transit of June 6, 1761.2

Humboldt speaks of the supposed satellite of Venus as among " the astronomical myths of an uncritical age." 3

An occultation of Venus by the moon is mentioned in the Chinese Annals as having occurred on March 19, 361 A.D., and Tycho Bralie observed another on May 23, 1587.*

A close conjunction of Venus and Reguhi3 (a Leonis) is recorded by the Arabian astronomer, Ibii Yunis, as having occurred on September 9, 885 A.D. Calculations by Hind show that the planet and star were within 2' of arc on that night, and consequently would have appeared as a single star to the naked eye. The telescope had not then been invented.5

Seen from Venus, the maximum apparent dis- tance between the earth and moon would vary from about 5' to 31'.6

It is related by Arago that Buonaparte, when going to the Luxembourg in Paris, where the

1 Ast. Nach., No. 4106.

2 Copernicus, vol. ii. p. 168.

3 Cosmos, vol. iv. p. 476, footnote.

4 Denning, Telescopic Worlifor Starlight Kveningg, p. 153. * Ibid., p. 154.

6 Nature, July 13, 1876.

VENUS 31

Directory were giving a fete in his honour, was very much surprised to find the crowd assembled in the Rue de Touracour '* pay more attention to a region of the heavens situated above the palace than to his person or the brilliant staff that accompanied him. He inquired the cause and learned that these curious persons were observing with astonishment, although it was noon, a star, which they supposed to be that of the conqueror of Italy — an allusion to which the illustrious general did not seem indifferent, when he himself, with his piercing eyes, remarked the radiant body." The " star" in question was Venus.1

1 P. M. Ryves in Knowledge, June 1, 1897, p. 144.

CHAPTER IV

The Earth

THE earth being our place of abode is, of course, to us the most important planet in the solar system. It is a curious paradox that the moon's surface (at least the visible portion) is better known to us than the surface of the earth. Every spot on the moon's visible surface equal in size to say Liverpool or Glasgow is well known to lunar observers, whereas there are thousands of square miles on the earth's surface — for example, near the poles and in the centre of Australia — which are wholly unknown to the earth's inhabitants ; and are perhaps likely to remain so.

Many attempts have been made by "para- doxers " to show that the earth is a flat plane and not a sphere. But M. Ricco has found by actual experiment that the reflected image of the setting sun from a smooth sea is an elongated ellipse. This proves mathematically beyond all doubt that the surface of the sea is spherical ; for the reflection from a plane surface would be

THE EARTH 33

necessarily circular. The theory of a " flat earth" is therefore proved to be quite unten- able, and all the arguments (?) of the "earth flatteners" have now been — like the French Revolution — " blown into space."

The pole of minimum temperature in the northern hemisphere, or "the pole of cold," as it has been termed, is supposed to lie near Werchojansk in Siberia, where a temperature of nearly — 70° has been observed.

From a series of observations made at Annapolis (U.S.A.) on the gradual disappearance of the blue of the sky after sunset, Dr. See finds that the extreme height of the earth's atmosphere is about 130 miles. Prof. Newcomb finds that meteors first appear at a mean height of about 74 miles.1

An aurora seen in Canada on July 15, 1893, was observed from stations 110 miles apart, and from these observations the aurora was found to lie at a height of 166 miles above the earth's surface. It was computed that if the auroral "arch maintained an equal height above the earth its ends were 1150 miles away, so that the magnificent sight was presented of an auroral belt in the sky with 2300 miles between its two extremities." 2

" Luminous clouds " are bright clouds sometimes seen at night near the end of June and beginning

1 Bulletin, Ast. Soc. de France, August, 1905.

2 Xature, April 5, 1894.

34* ASTRONOMICAL CURIOSITIES

of July. They appear above the northern horizon over the sun's place about midnight, and evidently lie at a great height above the earth's surface. Observations made in Germany by Dr. Jesse, and in England by Mr. Backhouse, in the years 1885-91, show that the height of these clouds is nearly constant at about 51 miles.1 The present writer has seen these remarkable clouds on one or two occasions in County Sligo, Ireland, during the period above mentioned.

M. Montigny has shown that " the approach of violent cyclones or other storms is heralded by an increase of scintillation" (or twinkling of the stars). The effect is also very evident when such storms pass at a considerable distance. He has also made some interesting observations (especi- ally on the star Capella), which show that, not only does scintillation increase in rainy weather, but that "it is very evident, at such times, in stars situated at an altitude at which on other occasions it would not be perceptible at all ; thus confirming the remark of Humboldt's with regard to the advent of the wet season in tropical countries." 2

In a paper on the subject of " Optical Illusions " in Popular Astronomy, February, 1906, Mr.

1 Nature, May 14, 1896. Some have attributed these "luminous clouds " to light reflected from the dust of the Krakatoa eruption (1883).

- The Observatory, 1877, p, 90.

THE EARTH 35

Arthur K. Bartlett, of Batter Creek, Michigan (U.S.A.), makes the following interesting re- marks : —

" The lunar halo which by many persons is regarded as a remarkable and unexplained luminosity associated with the moon, is to meteorological students neither a mysterious nor an anomalous occurrence. It has been frequently observed and for many years thoroughly under- stood, and at the present time admits of an easy scientific explanation. It is an atmospheric ex- hibition due to the refraction and dispersion of the moon's light through very minute ice crystals floating at great elevations above the earth, and it is explained by the science of meteorology, to which it properly belongs ; for it is not of cosmical origin, and in no way pertains to astronomy, as most persons suppose, except as it depends on the moon, whose light passing through the atmosphere, produces the luminous halo, which as will be seen, is simply an optical illusion, originating, not in the vicinity of the moon — two hundred and forty thousand miles away — but just above the earth's surface, and within the aqueous envelope that surrounds it 011 all sides. ... A halo may form round the sun as well as the moon . . . but a halo is more fre- quently noticed round the moon for the reason that we are too much dazzled by the sun's light to distinguish faint colours surrounding its disc, and to see them it is necessary to look through smoked glass, or view the sun by reflection from the surface of still water, by which its brilliancy is very mvich reduced." . . .

" A ' corona ' is an appearance of faintly coloured ings often seen around the sun and moon when

36 ASTRONOMICAL CURIOSITIES

a light fleecy cloud passes over them, and should not be mistaken for a halo, which is much larger and more complicated in its structure. These two phenomena are frequently confounded by inexperienced observers." With these remarks the present writer fully concurs. Mr. Bartlett adds —

"As a halo is never seen except when the sky is hazy, it indicates that moisture is ac- cumulating in the atmosphere which will form clouds, and usually result in a storm. But the popular notion that the number of bright stars visible within the circle indicates the number of days before the storm will occur, is without any foundation whatever, and the belief is almost too absurd to be refuted. In whatever part of the sky a lunar halo is seen, one or more bright stars are always sure to be noticed inside the luminous ring, and the number visible depends entirely upon the position of the moon. More- over, when the sky within the circle is examined with even a small telescope, hundreds of stars are visible where only one, or perhaps two or three, are perceived with the naked eye."

It is possible to have five Sundays in February (the year must of course be a " leap year "). This occurred in the year 1880, Sunday falling on February 1, 8, 15, 22, and 29. But this will not happen again till the year 1920. No century year (such as 1900, 2000, etc.) could possibly have five Sundays in February, aud the Rev. Richard Campbell, who investigated this matter, finds

THE EARTH 37

the following sequence of years in which five Sundays occur in February: 1604, 1632, 1660, 1688, 1728, 1756, 1784, 1824, 1852, 1880, 1920, 1948, 1976.1

In an article on "The Last Day and Year of the Century: Remarks on Time Reckoning," in Nature, September 10, 1896, Mr. W. T. Lynn, the eminent astronomer, says, " The late Astronomer Royal, Sir George Airy, once received a letter requesting him to settle a dispute which had arisen in some local debating society, as to which would be the first day of the next century. His reply was, 'A very little consideration, will suffice to show that the first day of the twentieth century will be January 1, 1901.' Simple as the matter seems, the fact that it is occasionally brought into question shows that there is some little difficulty connected with it. Probably, however, this is in a great measure due to the circumstance that the actual figures are changed on January 1, 1900, the day preceding being December 31, 1899. A century is a very definite word for an interval respecting which there is no possible room for mistake or difference of opinion. But the date of its ending depends upon that of its beginning. Our double system of backward and forward reckoning leads to a good deal of inconvenience. Our reckoning supposes (what we know was not the case, but as an era the date 1 Popular Astronomy, vol. 11 (1903), p. 293.

38 ASTRONOMICAL CURIOSITIES

does equally well) that Christ was born at the end of B.C. 1. At the end of A.D. 1, therefore, one year had elapsed from the event, at the end of A.D. 100, one century, and at the end of 1900, nineteen centuries. ... It is clear, then, that the year, as we call it, is an ordinal number, and that 1900 years from the birth of Christ (reckoning as we do from B.C. 1) will not be completed until the end of December 31 in that year, the twentieth century beginning with January 1, 1901, that is (to be exact) at the previous midnight, when the day commences by civil reckoning." With these re- marks of Mr. Lynn I fully concur, and, so far as I know, all astronomers agree with him. As the discussion will probably again arise at the end of the twentieth century, I would like to put on record here what the scientific opinion was at the close of the nineteenth century.

Prof. E. Rutherford, the well-known authority on radium, suggests that possibly radium is a source of heat from within the earth. Traces of radium have been detected in many rocks and soils, and even in sea water. Calcula- tion shows that the total amount distributed through the earth's crust is enormously large, although relatively small " compared with the annual output of coal for the world." The amount of radium necessary to compensate for the present loss of heat from the earth "corre- sponds to only five parts in one hundred million

THE EARTH 39

millions per unit mass," and the " observations of Elster and Gertel show that the radio-activity observed in soils corresponds to the presence of about this proportion of radium." 1

The earth has 12 different motions. These are as follows : —

1. Rotation 011 its axis, having a period of 24 hours.

2. Revolution round the sun ; period 365J days.

3. Precession ; period of about 25,765 years.

4. Semi-lunar gravitation ; period 28 days.

5. Nutation ; period 18 J years.

6. Variation in obliquity of the ecliptic ; about 47" in 100 years.

7. Variation of eccentricity of orbit.

8. Change of line of apsides ; period about 21,000 years.

9. Planetary perturbations.

10. Change of centre of gravity of whole solar system.

11. General motion of solar system in space.

12. Variation of latitude with several degrees of periodicity.2

"An amusing story has been told which affords a good illustration of the ignorance and popular notions regarding the tides prevailing even among persons of average intelligence. 1 Tell me,' said a man to an eminent living English

1 Popular Astronomy, vol. 13 (1905), p. 226.

2 Nature, July 25, 1901 (from Flammftrion). -

40 ASTRONOMICAL CURIOSITIES

astronomer not long ago, * is it still considered probable that the tides are caused by the moon ? ' The man of science replied that to the best of his belief it was, and then asked in turn whether the inquirer had any serious reason for question- ing the relationship. 'Well, I don't know,' was the answer ; * sometimes when there is no moon there seems to be a tide all the same.' " ! 1

With reference to the force of gravitation, on the earth and other bodies in the universe, Mr. William B. Taylor has well said, "With each revolving year new demonstrations of its abso- lute precision and of its universal domination serves only to fill the mind with added wonder and with added confidence in the stability and the supremacy of the power in which has been found no variableness neither shadow of turning, but which— the same yesterday, to-day and for ever —

" Lives through all life, extends through all extent, Spreads undivided, operates unspent." 2

With reference to the habitability of other planets, Tennyson has beautifully said —

'* Venus near her ! smiling downwards at this earthlier earth of

ours,

Closer on the sun, perhaps a world of never fading flowers. Hesper, whom the poets call'd the Bringer home of all good

things ; All good things may move in Hesper ; perfect people, perfect

kings.

1 Popular Astronomy, Vol. 11 (1903), p. 496.

2 Kinetic Theories of Gravitation, Washington, 1877.

THE EARTH 41

Hesper — Venus—were we native to that splendour, or in Mars, We should see the globe we groan in fairest of their evening

stars. Could we dream of war and carnage, craft and madness, lust

and spite,

Roaring London, raving Paris, in that spot of peaceful light ? Might we not in glancing heavenward on a star so silver fair, Yearn and clasp the hands, and murmur, * Would to God that

we were there ! ' "

The ancient Greek writer, Diogenes Laertius, states that Anaximander (610-547 B.C.) believed that the earth was a sphere. The Greek words are : /UO-T/I/ re rrjv yrjv /ceicrtfat, Kevrpv rd&v €TT€\ovorav ovarav or^aipoetS^.1

With reference to the Aurora Borealis, the exact nature of which is not accurately known, " a good story used to be told some years ago of a candidate who, undergoing the torture of a vivd voce examination, was unable to reply satis- factorily to any of the questions asked. * Come, sir,' said the examiner, with the air of a man asking the simplest question, ' explain to me the cause of the aurora borealis.' 'Sir,' said the unhappy aspirant for t physical honours, * I could have explained it perfectly yesterday, but ner- vousness has, I think, made me lose my memory.' ' This is very unfortu/iate,' said the examiner ; ' you are the only man who could have explained this mystery, and you have forgotten it," ' 2 This was written in the year 1899, and probably the

1 The Observatory, June, 1894, p. 208.

2 Nature, June 8, 1899.

42 ASTRONOMICAL CURIOSITIES

phenomenon of the aurora remains nearly as great a mystery to-day. In 1839, MM. Bravais and Lottin made observations on the aurora in Norway in about N. latitude 70°. Bravais found the height to be between 62 and 93 miles above the earth's surface.

The cause of the so-called Glacial Epoch in the earth's history has been much discussed. The Russian physicist, Rogovsky, has advanced the following theory —

" If we suppose that the temperature of the sun at the present time is still increasing, or at least has been increasing until now, the glacial epoch can be easily accounted for. Formerly the earth had a high temperature of its own, but received a lesser quantity of heat from the sun than now ; on cooling gradually, the earth's surface attained such a temperature as caused a great part of the surface of the northern and southern hemispheres to be covered with ice ; but the sun's radiation increasing, the glaciers melted, and the climatic conditions became as they are now. In a word, the temperature of the earth's surface is a function of two quantities: one decreasing (the earth's own heat), and the other increasing (the sun's radiation), and conse- quently there may be a minimum, and this minimum was the glacial epoch, which, as shown by recent investigations, those of Luigi de Marchi (Report of G. Schiaparelli, Meteor olog. Zeitschr., 30, 130-136, 1895), are not local, but general for the whole earth " fc (see also M. Neumahr, Erdegeschicht).1

1 AstropliywalJournal, vol. 14 (1901), p. 238, footnote.

THE EARTH 43

Prof. Percival Lowell thinks that the life of geological palaeozoic times was supported by the earth's internal heat, which maintained the ocean at a comparatively warm temperature.1

The following passage in the Book of the Maccabees may possibly refer to an aurora —

" Now about this time Antiochus made his second inroad into Egypt. And it so befell that throughout all the city, for the space of almost forty days, there appeared in the midst of the sky horsemen in swift motion, wearing robes inwrought with gold and carrying spears, equipped in troops for battle ; and drawing of swords ; and on the other side squadrons of horse in array ; and encounters and pursuits of both armies ; and shaking of shields, and multitudes of lances, and casting of darts, and flashing of golden trappings, and girding on of all sorts of armour. Wherefore all men besought that the vision might have been given for food." 2

According to Laplace "the decrease of the mean heat of the earth during a period of 2000 years has not, taking the extremist limits, diminished as much as ^th of a degree Fahrenheit." 3

From his researches on the cause of the Preces- sion of the Equinoxes, Laplace concluded that " the motion of the earth's axis is the same as if the

1 Mars as the Abode of Life, p. 52.

2 Second Book of the Maccabees v. 1-4 (Revised Edition). 8 Humboldt's Cosmos, vol. i. p. 169 (Otte's translation).

44 ASTRONOMICAL CURIOSITIES

whole sea formed a solid mass adhering to its surface." l

Laplace found that the major (or longer) axis of the earth's orbit coincided with the line of Equinoxes in the year 4107 B.C. The earth's perigee then coincided with the autumnal equinox. The epoch at which the major axis was perpen- dicular to the line of equinoxes fell in the year 1250 A.D.2

Leverrier has found the minimum eccentricity of the earth's orbit round the sun to be 0*0047 ; so that the orbit will never become absolutely circular, as some have imagined.

Laplace says —

"Astronomy considered in its entirety is the finest monument of the human mind, the noblest essay of its intelligence. Seduced by the illu- sions of the senses and of self -pride, for a long time man considered himself as the centre of the movement of the stars ; his vain-glory has been punished by the terrors which his own ideas have inspired. At last the efforts of several centuries brushed aside the veil which concealed the system of the world. We discover ourselves upon a planet, itself almost imperceptible in the vast extent of the solar system, which in its turn is only an insensible point in the immen- sity of space. The sublime results to which this discovery has led should suffice to console us for our extreme littleness, and the rank which it assigns to the earth. Let us treasure with

1 Quoted by Grant in History of Physical Astronomy, p. 71.

2 Ibid., pp. 100, 101.

THE EARTH 45

solicitude, let us add to as we may, this store of higher knowledge, the most exquisite treasure of thinking beings." l

With reference to probable future changes in climate, the great physicist, Arrhenius, says —

" We often hear lamentation that the coal stored up in the earth is wasted by the present genera- tion without any thought of the future, and we are terrified by the awful destruction of life and property which has followed the volcanic erup- tions of our days. We may find a kind of consola- tion in the consideration that here, as in every other case, there is good mixed with evil. By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind." 2

The night of July 1, 1908, was unusually bright. This was noticed in various parts of England and Ireland, and by the present writer in Dublin. Humboldt states that " at the time of the new moon at midnight in 1743, the phosphorescence was so intense that objects could be distinctly recognized at a distance of more than 600 feet." 3

An interesting proof of the earth's rotation on its axis has recently been found.

1 Exposition du Systtme du Monde, quoted by Carl Snyder in The World Machine, p. 226.

2 Worlds in the Malting, p. 63. * Cosmos, vol. i. p. 131.

46 ASTRONOMICAL CURIOSITIES

" In a paper in the Proceedings of the Vienna Academy (June, 1908) by Herr Tumlirz, it is shown mathematically that if a liquid is flowing outwards between two horizontal discs, the lines of flow will be strictly straight only if the discs and vessel be at rest, and will assume certain curves if that vessel and the discs are in rotation, as, for example, due to the earth's rotation. An experimental arrange- ment was set up with all precautions, and the stream lines were marked with coloured liquids and photographed. These were in general accord with the predictions of theory and the supposition that the earth is rotating about an axis." l

In a book published in 1905 entitled The Rational Almanac, by Moses B. Cotsworth, of York, the author states that (p. 397), "The ex- planation is apparent from the Great Pyramid's Slope, which conclusively proves that when it was built the latitude of that region was 7°'l more than at present. Egyptian Memphis now near Cairo was then in latitude 37°'l, where Asia Minor now ranges, whilst Syria would then be where the Caucasus regions now experience those rigorous winters formerly experienced in Syria." But the reality of this comparatively great change of latitude in the position of the Great Pyramid can be easily disproved. Pytheas of Marseilles — who lived in the time of Alexander the Great, about 330 B.C. — measured the latitude of Marseilles by means of a gnomon, and found it to be about 42° 56'f. As the present latitude of Marseilles is

1 The Olsm-atory, June, 1009, p. 2G1.

THE EARTH 47

43° 17' 50", no great change in the latitude could have taken place in over 2000 years.1 From this we may conclude that the latitude of the Great Pyramid has not changed by 7°'l since its con- struction. There is, it is true, a slow diminution going on in the obliquity of the ecliptic (or inclina- tion of the earth's axis), but modern observations show that this would not amount to as much as one degree in 6000 years. Eudemus of Rhodes — a disciple of Aristotle (who died in 322 B.C.) — found the obliquity of the ecliptic to be 24°, which differs but little from its present value, 23° 27'. Al-Sufi in the tenth century measured the latitude of Schiraz in Persia, and found it 29° 30'. Its present latitude is 29° 36' SO",2 so that evidently there has been no change in the latitude in 900 years.

1 Astronomical Essays, pp. 61, G2.

2 Encyclopxdia Brilannica (Schiraz).

CHAPTER V

The Moon

THE total area of the moon's surface is about equal to that of North and South America. The actual surface visible at any one time is about equal to North America.

The famous lunar observer, Schroter, thought that the moon had an atmosphere, but estimated its height at only a little over a mile. Its density he supposed to be less than that of the vacuum in an air-pump. Recent investigations, however, seem to show that owing to its small mass and attractive force the moon could not retain an atmosphere like that of the earth.

Prof. N. S. Shaler, of Harvard (U.S.A.), finds from a study of the moon (from a geological point of view) with the 15-inch refractor of the Harvard Observatory, that our satellite has no atmosphere nor any form of organic life, and he believes that its surface " was brought to its present condition before the earth had even a solid crust." l

There is a curious illusion with reference to the 1 Monthly Notices, B.A.S., February, 1905.

THE MOON 49

moon's apparent diameter referred to by Proctor.1 If, when the moon is absent in the winter months, we ask a person whether the moon's diameter is greater or less than the distance between the stars 8 and e, and e and £ Orionis, the three well-known stars in the " belt of Orion," the answer will pro- bably be that the moon's apparent diameter is about equal to each of these distances. But in reality the apparent distance between S and c Orionis (or between c and £, which is about the same) is more than double the moon's apparent diameter. This seems at first sight a startling statement ; but its truth is, of course, beyond all doubt and is not open to argument. Proctor points out that if a person estimates the moon as a foot in diameter, as its apparent diameter is about half a degree, this would imply that the observer estimates the circumference of the star sphere as about 720 feet (360° x 2), and hence the radius (or the moon's distance from the earth) about 115 feet. But in reality all such estimates have 110 scientific (that is, accurate) meaning. Some of the ancients, such as Aristotle, Cicero, and Heraclitus, seem to have estimated the moon's apparent diameter at about a foot.2 This shows that even great minds may make serious mistakes.

It has been stated by some writer that the moon as seen with the highest powers of the great

1 Nature, March 3, 1870.

2 Hid., March 31, 1870, p. 557.

E

50 ASTRONOMICAL CURIOSITIES

Yerkes telescope (40 inches aperture) appears "just as it would be seen with the naked eye if it were suspended 60 miles over our heads." But this statement is quite erroneous. The moon as seen with the naked eye or with a telescope shows us nearly a whole hemisphere of its surface. But if the eye were placed only 60 miles from the moon's surface, we should see only a small portion of its surface. In fact, it is a curious paradox that the nearer the eye is to a sphere the less we see of its surface! The truth of this will be evident from the fact that on a level plain an eye placed at a height, say 5 feet, sees a very small portion indeed of the earth's surface, and the higher we ascend the more of the surface we see. I find that at a distance of 60 miles from the moon's surface we should only see a small portion of its visible hemisphere (about ^th). The lunar features would also appear under a different aspect. The view would be more of a landscape than that seen in any telescope. This view of the matter is not new. It has been previously pointed out, especially by M. Flammarion and Mr. Whit- mell, but its truth is not, I think, generally recognized. Prof. Newcomb doubts whether with any telescope the moon has ever been seen so well as it would be if brought within 500 miles of the earth.

A relief map of the moon 19 feet in diameter was added, in 1898, to the Field Columbian

THE MOON 51

Museum (U.S.A.). It was prepared with great care from the lunar charts of Beer and Madler, and Dr. Schmidt of the Athens Observatory, and it shows the lunar features very accurately. Its construction took five years.

On a photograph of a part of the moon's sur- face near the crater Eratosthenes, Prof. William H. Pickering finds markings which very much resemble the so-called "canals" of Mars. The photograph was taken in Jamaica, and a copy of it is given in Prof. Pickering's book on the Moon, and in Popular Astronomy, February, 1904.

Experiments made in America by Messrs. Stebbins and F. C. Brown, by means of selenium cells, show that the light of the full moon is about nine times that of the half moon ; * and that " the moon is brighter between the first quarter and full than in the corresponding phase after full moon." They also find that the light of the full moon is equal to "0*23 candle power,"1 that is, according to the method of measurement used in America, its light is equal to 0*23 of a standard candle placed at a distance of one metre (39'37 inches) from the eye.2

Mr. H. H. Kimball finds that no less than 52 per cent, of the observed changes in intensity of the " earth-shine " visible on the moon when at 01 near the crescent phase is due to the eccentricity

1 Prof. W. H. Pickering found 12 timea (eee p. 1). * Nature, January 30, 1908.

52 ASTRONOMICAL CURIOSITIES

of the lunar orbit, and " this is probably much greater than could be expected from any increase or diminution in the average cloudiness over the hemisphere of the earth reflecting light to the moon." l

v The "moon maiden" is a term applied to a fancied resemblance of a portion of the Sinus Iridum to a female head. It forms the " pro- montory " known as Cape Heraclides, and may be looked for when the moon's "age" is about 11 days. Mr. C. J. Caswell, who observed it on September 29, 1895, describes it as resembling " a beautiful silver statuette of a graceful female figure with flowing hair."

M. Landerer finds that the angle of polariza- tion of the moon's surface — about 33° — agrees well with the polarizing angle for many specimens of igneous rocks (30° 51' to 33° 46'). The polarizing angle for ice is more than 37°, and this fact is opposed to the theories of lunar glaciation advanced by some observers.2

Kepler states in his Somnium that he saw the moon in the crescent phase on the morning and evening of the same day (that is, before and after conjunction with the sun). Kepler could see 14 stars in the Pleiades with the naked eye, so his eyesight must have been exceptionally keen.

Investigations on ancient eclipses of the moon show that the eclipse mentioned by Josephus as

1 Nature, September 5, 1901. " Ibid., July 31, 1890.

THE MOON 53

having occurred before the death of Herod is probably that which took place 011 September 15, B.C. 5. This occurred about 9.45 p.m. ; and probably about six months before the death of Herod (St. Matthew ii. 15).

The total lunar eclipse which occurred 011 October 4, 1884, was remarkable for the almost total disappearance of the moon during totality. One observer says that " in the open air, if one had not known exactly where to look for it, one might have searched for some time without discovering it. I speak of course of the naked eye appear- ance." * On the other hand the same observer, speaking of the total eclipse of the moon on August 23, 1877, which was a bright one, says —

" The moon even in the middle of the total phase was a conspicuous object in the sky, and the ruddy colour was well marked. In the very middle of the eclipse the degree of illumination was as nearly as possible equal all round the edge of the moon, the central parts being darker than those near the edge."

In Roger de Hovedin's Chronicle (A.D. 756) an account is given of the occultation of " a bright star," by the moon during a total eclipse. This is confirmed by Simeon of Durham, who also dates the eclipse A.D. 756. This is, however, a mistake, the eclipse having occurred on the evening of November 23, A.D. 755. Calvisius supposed that 1 Nature^ October 16, 1884.

54. ASTRONOMICAL CURIOSITIES

the occulted " star " might have been Aldebaran. Pingre, however, showed that this was impossible, and Struyck, in 1740, showed that the planet Jupiter was the " star " referred to by the early observer. Further calculations by Hind (1885) show conclusively that Struyck was quite correct, and that the phenomenon described in the old chronicles was the occultation of Jupiter by a totally eclipsed moon — a rather unique phe- nomenon.1

An occultation of Mars by the moon is recorded by the Chinese, on February 14, B.C. 69, and one of Venus, on March 30, A.D. 361. These have also been verified by Hind, and his calculations show the accuracy of these old Chinese records.

It has been suggested that the moon may possibly have a satellite revolving round it, as the moon itself revolves round the earth. This would, of course, form an object of great interest. During the total lunar eclipses of March 10 and September 3, 1895, a careful photographic search was made by Prof. Barnard for a possible lunar satellite. The eclipse of March 10 was not very suitable for the purpose owing to a hazy sky, but that of September 3 was " entirely satisfactory," as the sky was very clear, and the duration of totality was very long. On the latter occasion " six splendid " photographs were obtained of the total phase with a 6-inch Willard lens. The result 1 Nature, February 19, 1885.

THE MOON 55

was that none of these photographs " show any- thing which might be taken for a lunar satellite," at least any satellite as bright as the 10th or 12th magnitude. It is, of course, just possible that the supposed satellite might have been behind the moon during the totality.

With reference to the attraction between the earth and moon, Sir Oliver Lodge says —

" The force with which the moon is held in its orbit would be great enough to tear asunder a steel rod 400 miles thick, with a tenacity of 30 tons to the square inch, so that if the moon and earth were connected by steel instead of gravity, a forest of pillars would be necessary to whirl the system once a month round their common centre of gravity. Such a force necessarily implies enormous tensure or pressure in the medium. Maxwell calculates that the gravitational stress near the earth, which we must suppose to exist in the invisible medium, is 3000 times greater than what the strongest steel can stand, and near the sun it should be 2500 times as great as that." x

With reference to the names given to " craters " on the moon, Prof. W. H. Pickering says,2 " The system of nomenclature is, I think, unfortunate. The names of the chief craters are generally those of men who have done little or nothing for selenography, or even for astronomy, while the men who should be really commemorated are

1 Nature, January 14, 1909, p. 323.

• Photographic Atlas of the Moon, Annah of Harvard Observatory vol. li. pp. 14, 15.

56 ASTRONOMICAL CURIOSITIES

represented in general by small and unimportant craters," and again —

"A serious objection to the whole system of nomenclature lies in the fact that it has apparently been used by some selenographers, from the earliest times up to the present, as a means of satisfying their spite against some of their contemporaries. Under the guise of pretend- ing to honour them by placing their names in perpetuity upon the moon, they have used their names merely to designate the smallest objects that their telescopes were capable of showing. An interesting illustration of this point is found in the craters of Galileo and Riccioli, which lie close together on the moon. It will be remembered that Galileo was the discoverer of the craters on the moon. Both names were given by Riccioli, and the relative size and importance of the craters [Riccioli large, and Galileo very small] probably indicates to us the relative importance that he assigned to the two men themselves. Other examples might be quoted of craters named in the same spirit after men still living. . . . With the exception of Maedler, one might almost say, the more prominent the selenographer the more insignificant the crater."

The mathematical treatment of the lunar theory is a problem of great difficulty. The famous mathematician, Euler, described it as incredibile studium atque indefessus labor.1

With reference to the "earth-shine" on the moon when in the crescent phase, Humbolclt says, " Lambert made the remarkable observation 1 Nature, January 18, 1906.

THE MOON 57

(14th of February, 1774) of a change of the ash- coloured moonlight into an olive-green colour, bordering upon yellow. The moon, which then stood vertically over the Atlantic Ocean, received upon its night side the green terrestrial light, which is reflected towards her when the sky is clear by the forest districts of South America." 1 Arago said, "II n'est done pas impossible, malgre tout ce qu'un pareil resultat exciterait de surprise au premier coup d'ceil qu'un jour les meteorologistes aillent puiser dans 1' aspect de la Lune des notions precieuses sur Vetat moyen de diaphanite de 1' atmosphere terrestre, dans les hemispheres qui successivement concurrent a la production de la lumiere cendree." 2

The " earth-shine " on the new moon was success- fully photographed in February, 1895, by Prof. Barnard at the Lick Observatory, with a 6-iiich Willard portrait lens. He says —

"The earth-lit globe stands out beautifully round, encircled by the slender crescent. All the ' seas ' are conspicuously visible, as are also the other prominent features, especially the region about Tycho. Aristarchus and Copernicus appear as bright specks, and the light streams from Tycho are very distinct." 3

Kepler found that the moon completely dis- appeared during the total eclipse of December 9,

1 Humboldt's Cosmos, vol. iv. p. 481. 2 Ibid,, p. 482.

3 Monthly Notices, B.A.S., June, 1895.

58 ASTRONOMICAL CURIOSITIES

1601, and Hevelius observed the same phenomenon during the eclipse of April 25, 1642, when " not a vestige of the moon could be seen." l In the total lunar eclipse of June 10, 1816, the moon during totality -was not visible in London, even with a telescope ! l

The lunar mountains are relatively much higher than those on the earth. Beer and Madler found the following heights : Dorfel, 23,174 feet ; Newton, 22,141 ; Casatus, 21,102 ; Curtius, 20,632 ; Callippus, 18,946 ; and Tycho, 18,748 feet.2

Taking the earth's diameter at 7912 miles, the moon's diameter, 2163 miles, and the height of Mount Everest as 29,000 feet, I find that

Everest 1 Dorfel 1

:,and

Earth's diameter 1440* moon's diameter 492

From which it follows that the lunar mountains are proportionately about three times higher than those on the earth.

According to an hypothesis recently advanced by Dr. See, all the satellites of the solar system, including our moon, were " captured " by their primaries. He thinks, therefore, that the " moon came to earth from heavenly space." 3

1 Humboklt's Cosmo*, vol. iv. p. 483 (Otte's translation).

- Grant, History of Physical Astronomy, p. 229.

3 Popular Astronomy, vol. xvii. No. 6, p. 387 (June- July, 1909).

CHAPTER VI

Mars

MARS was called by the ancients " the vanishing star," owing to the long periods during which it is practically invisible from the earth.1 It was also called Trupo'ei? and Hercules.

I have seen it stated in a book on the " Solar System" by a well-known astronomer that the axis of Mars " is inclined to the plane of the orbit " at an angle of 24° 50' ! But this is quite erroneous. The angle given is the angle between the plane of the planet's equator and the plane of its orbit, which is quite a different thing. This angle, which may be called the obliquity of Mars' ecliptic, does not differ much from that of the earth. Lowell finds it 23° 13' from observations in 1907.2

The late Mr. Proctor thought that Mars is " far the reddest star in the heavens ; Aldebaran and Antares are pale beside him." 3 But this does not

1 Nature, October 7, 1875.

2 Mars as an Abode of Life (1908), p. 281.

3 Knoioledge, May 2, 188G.

60 ASTRONOMICAL CURIOSITIES

agree with my experience. Antares is to my eye quite as red as Mars. Its name is derived from two Greek words implying " redder than Mars." The colour of Aldebaran is, I think, quite com- parable with that of the " ruddy planet." In the telescope the colour of Mars is, I believe, more yellow than red, but I have not seen the planet very often in a telescope. Sir John Herschel suggested that the reddish colour of Mars may possibly be due to red rocks, like those of the Old Red Sandstone, and the red soil often associated with such rocks, as I have myself noticed near Torquay and other places in Devonshire.

The ruddy colour of Mars was formerly thought to be due to the great density of its atmosphere. But modern observations seem to show that the planet's atmosphere is, on the contrary, much rarer than that of the earth. The persistent visibility of the markings on its surface shows that its atmosphere cannot be cloud-laden like ours ; and the spectroscope shows that the water vapour present is — although perceptible — less than that of our terrestrial envelope.

The existence of water vapour is clearly shown by photographs of the planet's spectrum taken by Mr. Slipher at the Lowell Observatory in 1908. These show that the water vapour bands a and near D are stronger in the spectrum of Mars than in that of the moon at the same altitude.1 1 Nature, March 12, 1908.

MARS 61

The dark markings on Mars were formerly supposed to represent water and the light parts land. But this idea has now been abandoned. Light reflected from a water surface is polarized at certain angles. Prof. W. H. Pickering, in his observations on Mars, finds no trace of polariza- tion in the light reflected from the dark parts of the planet. But under the same conditions he finds that the bluish -black ring surrounding the white polar cap shows a well-marked polarization of light, thus indicating that this dark ring is probably water.1

Projections on the limb of the planet have frequently been observed in America. These are known not to be mountains, as they do not re- appear under similar conditions. They are sup- posed to be clouds, and one seen in December, 1900, has been ^explained as a cloud lying at a height of some 13 miles above the planet's surface and drifting at the rate of about 27 miles an hour. If there are any mountains on Mars they have not yet been discovered.

The existence of the so-called " canals " of Mars is supposed to be confirmed by Lowell's photographs of the planet. But what these "canals" really represent, that is the question. They have certainly an artificial look about them, and they form one of the most curious and interesting problems in the heavens. Prof. Lowell says — 1 Bulletin, Ast. Soc. de France, April, 1899.

62 ASTRONOMICAL CURIOSITIES

" Most suggestive of all Martian phenomena are the canals. Were they more generally observ- able the world would have been spared much scepticism and more theory. They may of course not be artificial, but observations here [Flag- staff] indicate that they are ; as will, I think, appear from the drawings. For it is one thing to see two or three canals and quite another to have the planet's disc mapped with them on a most elaborate system of triangulation. In the first place they are this season (August, 1894) bluish- green, of the same colour as the seas into which the longer ones all eventually debouch. In the next place they are almost without exception geodetically straight, supernaturally so, and this in spite of their leading in every possible direc- tion. Then they are of apparently nearly uniform width throughout their length. What they are is another matter. Their mere aspect, however, is enough to cause all theories about glaciatioii fissures or surface cracks to die an instant and natural death." l

Some of the observed colour-changes on Mars are very curious. In April, 1905, Mr. Lowell observed that the marking known as Mare Erythrseurn, just above Syrtis, had "changed from a blue-green to a chocolate-brown colour." The season on Mars corresponded with our February.

Signor V. ' Cerulli says that, having observed Mars regularly for ten years, he has come to the conclusion that the actual existence of the " canals " is as much a subject for physiological

1 Astronomy and Astrophysics (1894), p. C49.

MARS 63

as for astronomical investigation. He states that " the phenomena observed are so near the limit of the range of the human eye that in observing them one really experiences an effect accompany- ing * the birth of vision.' That is to say, the eye sees more and more as it becomes accustomed, or strained, to the delicate markings, and thus the joining up of spots to form * canals ' and the gemination of the latter follow as a physiological effect, and need not necessarily be subjective phenomena seen by the unaccustomed eye." l

The possibility of life on Mars has been recently much discussed ; some denying, others asserting. M. E. Rogovsky says —

" As free oxygen and carbonic dioxide may exist in the atmosphere of Mars, vegetable and animal life is quite possible. If the temperature which prevails upon Mars is nearer to —36° C. than to —73° C., the existence of living beings like ourselves is possible. In fact, the ice of some Greenland and Alpine glaciers is covered by red algse (Sphcerella nivalis) ; we find there also different species of rotaloria, variegated spiders, and other animals on the snow fields illuminated by the sun; at the edges of glacier snows in the Tyrol we see violet bells of Soldanella pusilla, the stalks of which make their way through the snow by producing heat which melts it round about them. Finally the Siberian town Verkhociansk, near Yakutsk, exists, though the temperature there falls to — 69°*8 C. and the mean temperature of January to — 51°'2, and the mean pressure of the vapour of water is

1 Nature, April 20, 1905.

64 ASTRONOMICAL CURIOSITIES

less than 0*05mm. It is possible, therefore, that living beings have become adapted to the con- ditions now prevailing upon Mars after the lapse of many ages, and live at an even lower tem- perature than upon the earth, developing the necessary heat themselves."

M. Rogovsky adds, " Water in organisms is mainly a liquid or solvent, and many other liquids may be the same. We have no reason to believe that life is possible only under the same conditions and with the same chemical composition of organisms as upon the earth, although indeed we cannot affirm that they actually exist on Mars." l With the above views the present writer fully concurs.

Prof. Lowell thinks that the polar regions of Mars, both north and south, are actually warmer than the corresponding regions of the earth, although the mean temperature of the planet is probably twelve degrees lower than the earth's mean temperature.2

A writer in Astronomy and Astrophysics (1892, p. 748) says —

" Whether the planet Mars is inhabited or not seems to be the all-absorbing question with the ordinary reader. With the astronomer this query is almost the last thing about the planet that he would think of when he has an oppor- tunity to study its surface markings ... no

1 Astrophysical Journal, vol. 14 (1901), p. 258.

2 Nature, August 22, 1907.

MARS 65

astronomer claims to know whether the planet is inhabited or not."

Several suggestions have been made with reference to the possibility of signalling to Mars. But, as Mr. Larkin of Mount Lowe (U.S.A.) points out, all writers on this subject seem to forget the fact that the night side of two planets are never turned towards each other. " When the sun is between them it is day on the side of Mars which is towards us, and also day on the side of the earth Avhich is towards Mars. When they are on the same side of the sun, it is day on Mars when night on the earth, and for this reason they could never see our signals. This should make it ap- parent that the task of signalling to Mars is a more difficult one than the most hopeful theorist has probably considered. All this is under the supposition that the Martians (if there are such) are beings like ourselves. If they are not like us, we cannot guess what they are like." l These views seem to me to be undoubtedly correct, and show the futility of visual signals. Electricity might, however, be conceivably used for the purpose ; but even this seems highly improbable.

Prof. Newcomb, in his work Astronomy for Everybody, says with reference to this question, " The reader will excuse me from saying nothing in this chapter about the possible inhabitants of

1 Popular Astronomy, vol. 12 (1904), p. 679.

P

66 ASTRONOMICAL CURIOSITIES

Mars. He knows just as much about the subject as I do, and that is nothing at all."

It is, however, quite possible that life in some form may exist on Mars. As Lowell well says, " Life but waits in the wings of existence for its cue to enter the scene the moment the stage is set." x With reference to the " canals " he says —

" It is certainly no exaggeration to say that they are the most astonishing objects to be viewed in the heavens. There are celestial sights more dazzling, spectacles that inspire more awe, but to the thoughtful observer who is privileged to see them well, there is nothing in the sky so profoundly impressive as these canals of Mars." 2

The eminent Swedish physicist Arrhenius thinks that the mean annual temperature on Mars may possibly be as high as 50° F. He says, " Some- times the snow-caps on the poles of Mars dis- appear entirely during the Mars summer ; this never happens on our terrestrial poles. The mean temperature of Mars must therefore be above zero, probably about + 10° [Centigrade = 50° Fahren- heit]. Organic life may very probably thrive, therefore, on Mars." 3 He thinks that this excess of mean temperature above the calculated tem- perature may be due to an increased amount of carbonic acid in the planet's atmosphere, and says "any doubling of the percentage of carbon

1 Mars as an Abode of Life, p. 69. 2 Ibid., p. 146.

3 Worlds in the Making, p. 49.

MARS 67

dioxide in the air would raise the temperature of the earth's surface by 4° ; and if the carbon dioxide were increased fourfold, the temperature would rise by 8°." l Denning says, — 2

" A few years ago, when christening celestial formations was more in fashion than it is now, a man simply had to use a telescope for an evening or two on Mars or the moon, and spice the relation of his seeings with something in the way of novelty, when his name would be pretty certainly attached to an object and hung in the heavens for all time! A writer in the Astro- nomical Register for January, 1879, humorously suggested that 'the matter should be put into the hands of an advertising agent,' and 'made the means of raising a revenue for astronomical purposes.' Some men would not object to pay handsomely for the distinction of having their names applied to the seas and continents of Mars or the craters of the moon."

An occultation of Mars by the moon is recorded by Aristotle as having occurred on April 4, 357 B.C.3

Seen from Mars the maximum apparent distance between the earth and moon would vary from 3^' to nearly 17'.4

1 Worlds in the Making, p. 53.

2 Denning, Telescopic Work for Starlight Evenings, p. 158.

3 Ibid., p. 166.

4 Nature, July 13, 1876.

CHAPTER VII

The Minor Planets

UP to 1908 the number of minor planets (or asteroids) certainly known amounted to over 650.

From an examination of the distribution of the first 512 of these small bodies, Dr. P. Stroobant finds that a decided maximum in number occurs between the limits of distance of 2'55 and 2*85 (earth's mean distance from sun = 1), " 199 of the asteroids considered revolving in this annulus." He finds that nearly all the asteroidal matter is concentrated near to the middle of the ring in the neighbourhood of the mean distance of 2*7, and the smallest asteroids are relatively less numerous in the richest zones.1

There are some " striking similarities " in the orbits of some of the asteroids. Thus, in the small planets Sophia (No. 251 in order of dis- covery) and Magdalena (No. 318) we have the mean distance of Sophia 3*10, and that of Magdalena 3*19 (earth's mean distance = 1). 1 Nature, May 2, 1907.

THE MINOR PLANETS 69

The eccentricities of the orbits are 0'09 and 0'07 ; and the inclinations of the orbits to the plane of the ecliptic 10° 29' and 10° 33' respectively.1 This similarity may be — and probably is — merely accidental, but it is none the less curious and interesting.

Some very interesting discoveries have recently been made among the minor planets. The orbit of Eros intersects the orbit of Mars ; and the following have nearly the same mean distance from the sun as Jupiter : —

Achilles (1906 TG), No. 588, Patrocles (1906 XY), No. 617, Hector (1907 XM), No. 624,

and another (No. 659) has been recently found. Each of these small planets " moves approximately in a vertex of an equilateral triangle that it forms with Jupiter and the sun." 2 The minor planet known provisionally as HN is remarkable for the large eccentricity of its orbit (0*38), and its small perihelion distance (1'6). When discovered it had a very high South Declination (61J°), showing that the inclination of the plane of its orbit to the plane of the ecliptic is considerable.3

Dr. Bauschinger has made a study of the minor planets discovered up to the end of 1900.

1 Nature, May 30, 1907.

2 Publications of the Astronomical Society of the Pacific, August, 1908.

3 Monthly Notices, R.A.S., 1902, p. 291.

TO ASTRONOMICAL CURIOSITIES

He finds that the ascending nodes of the orbits show a marked tendency to cluster near the ascending node of Jupiter's orbit, a fact which agrees well with Prof. Newcomb's theoretical results. There seems to be a slight tendency for large inclinations and great eccentricities to go together ; but there appears to be no connection between the eccentricity and the mean distance from the sun. The longitudes of the perihelia of these small planets "show a well-marked maximum near the longitude of Jupiter's peri- helion, and equally well-marked minimum near the longitude of his aphelion," which is again in good agreement with Newcomb's calculations.1 Dr. Bauschinger's diameter for Eros is 20 miles. He finds that the whole group, including those remaining to be discovered, would probably form a sphere of about 830 miles in diameter.

The total mass of the minor planets has been frequently estimated, but generally much too high. Mr. B. M. Roszel of the John Hopkins University (U.S.A.) has made a calculation of the probable mass from the known diameter of Vesta (319 miles, Pickering), and finds the volume of the first 216 asteroids discovered. From this calculation it appears that it would take 310 asteroids of the 6th magnitude, or 1200 of the 7th to equal the moon in volume. Mr. Rosxel concludes that the probable mass of the whole 1 Monthly Notices, K.A.S., February, 1902, p. 291.

THE MINOR PLANETS 71

asteroidal belt is between ^th and r ^th of that of the moon.1 Subsequently Mr. Roszel extended his study to the mass of 311 asteroids,2 and found a combined mass of about ^th of the moon's mass.

Dr. Palisa finds that the recently discovered minor planet (1905 QY) varies in light to a considerable extent.3 This planet was discovered by Dr. Max Wolf on August 23, 1905 ; but it was subsequently found that it is identical with one previously known, (167) Urda.4 The light variation is said to be from the llth to the 13th magnitude.5 Variation in some of the other minor planets has also been suspected. Prof. Wendell found a variation of about half a magni- tude in the planet Eunomia (No. 15). He also found that Iris (No. 7) varies about a quarter of a magnitude in a period of about 6h 12m.6 But these variations are small, and perhaps doubtful. The variability of Eros is well known.

The planet Eros is a very interesting one. The perihelion portion of its orbit lies between the orbits of Mars and the earth, and the aphelion part is outside the orbit of Mars. Owing to the great variation in its distance from the earth the brightness of Eros varies from the 6th to the 12th magnitude. That is, when brightest, it is 250

1 Nature, May 24, 1894. " Ibid., February 14, 1895.

3 Ibid., September 14, 1905. 4 Ibid., September 21, 1905. 5 Ibid., September 28, 1905. 6 Ibid., July 13, 1905.

72 ASTRONOMICAL CURIOSITIES

times brighter than when it is faintest.1 This variation of light, is of course, merely due to the variation of distance ; but some actual variation in the brightness of the planet has been observed.

It has been shown by Oeltzen and Valz that Cacciatore's supposed distant comet, mentioned by Admiral Smyth in his Bedford Catalogue, must have been a minor planet.2

Dr. Max Wolf discovered 36 new minor planets by photography in the years 1892-95. Up to the latter year he had never seen one of these through a telescope ! His words are, " Ich selsbt habe noch nie einen meinen kleinen Planeten am Himmel geseheii." 3

These small bodies have now become so numerous that it is a matter of much difficulty to follow them. At the meeting of the Royal Astronomical Society on January 8, 1909, Mr. G. F. Chambers made the following facetious remarks —

" I would like to make a suggestion that has been in my mind for several years past — that it should be made an offence punishable by fine or imprisonment to discover any more minor planets. They seem to be an intolerable nuisance, and are a great burden upon the literary gentle- men who have to keep pace with them and record them. I have never seen, during the last few

1 Nature, November 3, 1898. - Ibid., July 14, 1881, p. 235.

3 Quoted in The Observatory, February, 1896, p. 104, from Ast. Nach., No, 3319.

THE MINOR PLANETS 73

years at any rate, any good come from them, or likely to come, and I should like to see the supply stopped, and the energies of the German gentlemen who find so many turned into more promising channels."

Among the minor planets numbered 1 to 500, about 40 " have not been seen since the year of their discovery, and must be regarded as lost." ]

1 Monthly Notices, R.A.S., February, 1909.

CHAPTER VIII

Jupiter

THIS brilliant planet— only inferior to Venus in brightness — was often seen by Bond (Jun.) with the naked eye in "high and clear sunshine " ; also by Denning, who has very keen eyesight. Its brightness on such occasions is so great, that — like Venus — it casts a distinct shadow in a dark room.1

The great " red spot " 011 Jupiter seems to have been originally discovered by Robert Hooke on May 9, 1664, with a telescope of 2 inches aperture and 12 feet focus. It seems to have existed ever since ; at least the evidence is, accord- ing to Denning, in favour of the identity of Hooke's spot with the red spot visible in recent years. The spot was also observed by Cassini in the years 1665-72, and is sometimes called '* Cassini's spot." But the real discoverer was Hooke.2

The Orbit of Jupiter is so far outside the earth's

1 Celestial Objects, vol. i. p. 163. - Nature, December 29, 1898.

JUPITER 75

orbit tliat there can be little visible in the way of "phase" — as in the case of Mars, where the " gibbous " phase is sometimes very perceptible. Some books on astronomy state that Jupiter shows no phase. But this is incorrect. A dis- tinct, although very slight, gibbous appearance is visible when the planet is near quadrature. Webb thought it more conspicuous in twilight than in a dark sky. With large telescopes, Jupiter's satellites II. and III. have been seen — in consequence of Jupiter's phase— to emerge from occultation "at a sensible distance from the limb." l

According to M. E. Rogovsky, the high " albedo of Jupiter, the appearance of the clear (red) and dark spots on its surface and their continual variation, the different velocity of rotation of the equatorial and other zones of its surface, and particularly its small density (1*33, water as unity), all these facts afford irrefragable proofs of the high temperature of this planet. The dense and opaque atmosphere hides its glowing surface from our view, and we see therefore only the external surface of its clouds. The objective existence of this atmosphere is proved by the bands and lines of absorption in its spectrum. The interesting photograph obtained by Draper, September 27, 1879, in which the blue and green parts are more brilliant for the equatorial zone than for the 1 Celestial Objects, vol. i. p. 166.

76 ASTRONOMICAL CURIOSITIES

adjacent parts of the surface, appears to show that Jupiter emits its proper light. It is possible that the constant red spot noticed on its surface by several observers, as Gledhill, Lord Rosse, and Copeland (1873), Russel and Bredikhm (1876), is the summit of a high glowing mountain. G. W. Hough considers Jupiter to be gaseous, and A. Ritter inferred from his formulae that in this case the temperature at the centre would be 600,000° C." l

The four brighter satellites of Jupiter are usually known by numbers L, II., III., and IV. ; I. being the nearest to the planet, and IV. the farthest. III. is usually the brightest, and IV. the faintest, but exceptions to this rule have been noticed.

With reference to the recently discovered sixth and seventh satellites of Jupiter, Prof. Perrine has suggested that the large inclination of their orbits to the plane of, the planet's equator seems to indicate that neither of these bodies was originally a member of Jupiter's family, but has been " captured by the planet." This seems possible as the orbits of some of the minor planets lie near the orbit of Jupiter (see "Minor Planets"). A similar suggestion has been made by Prof, del Marmol.2

Many curious observations have been recorded

1 Aatrophysical Journal, vol. 14 (1901), pp. 248-9. * Nature, August 27, 1908.

JUPITER 77

with reference to Jupiter's satellites ; some very difficult of explanation. In 1711 Bianchini saw satellite IV. so faint for more than an hour that it was hardly visible ! A similar observation was made by Lassell with a more powerful telescope on June 13, 1849. Key, T. T. Smyth, and Denning have also recorded unusual faintness.1 A very remarkable phenomenon was seen by Admiral Smyth, Maclear, and Pearson on June 26, 1828. Satellite II., "having fairly entered on Jupiter, was found 12 or 13 minutes afterwards outside the limb, where it remained visible for at least 4 minutes, and then suddenly vanished." As Webb says, " Explanation is here set at defiance ; demon- strably neither in the atmosphere of the earth, nor Jupiter, where and what could have been the cause ? At present we can get no answer." 3 When Jupiter is in opposition to the sun — that is, on the meridian at midnight — satellite I. has been seen projected on its own shadow, the shadow appearing as a dark ring round the satellite.

On January 28, 1848, at Cambridge (U.S.A.) satellite III. was seen in transit lying between the shadows of I. and II. and so black that it could not be distinguished from the shadows, " except by the place it occupied." This seems to suggest inherent light in the planet's surface, as the satellite was at the time illuminated by full

1 Webb's Celestial Objects, vol. i. p. 177.

2 Ibid., vol. i. p. 187.

78 ASTRONOMICAL CURIOSITIES

sunshine ; its apparent blackness being due to the effect of contrast. Cassini on one occasion failed to find the shadow of satellite I. when it should have been on the planet's disc,1 an observation which again points to the glowing light of Jupiter's surface. Sadler and Trouvelot saw the shadow of satellite I. double ! an observation difficult to explain — but the same phenomenon was again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of Detroit, Michigan (U.S.A.). He says that the satellite "was in transit nearing egress, and it appeared as a white disc against the dark southern equatorial belt; following it was the usual shadow, and at an equal distance from this was a second shadow, smaller and not so dark as the true one, and surrounded by a faint penumbra."2

A dark transit of satellite III. was again seen 011 the evening of December 19, 1891, by two observers in America. One observer noted that the satellite, when on the disc of the planet, was intensely black. To the other observer (Willis L. Barnes) it appeared as an ill-defined dark image.3 A similar observation was made on October 9 of the same year by Messrs. Gale and Innes.4

1 Celestial Objects, vol. i. p. 186.

• Astronomy and Astrophysics, 1892, p. 87.

3 Ibid., 1892, pp. 94-5.

4 Observatory, December, 1891.

JUPITER 79

A " black transit " of satellite IV. was seen by several observers in 1873, and by Prof. Barnard on May 4, 1886. The same phenomenon was observed 011 October 30, 1903, in America, by Miss Anne S. Young and Willis S. Barnes. Miss Young says —

" The ingress of the satellite took place at 8h 50|n (E. standard time) when it became invisible upon the background of the planet. An hour later it was plainly visible as a dark round spot upon the planet. It was decidedly darker than the equatorial belt." x

The rather rare phenomenon of an occultation of one of Jupiter's satellites by another was observed by Mr. Apple, director of the Daniel Scholl Observatory, Franklin and Marshall College, Lancaster, Pa. (U.S.A.), on the evening of March 16, 1908. The satellites in question were I. and II., and they were so close that they could not be separated with the 11 '5-inch telescope of the Observatory.2 One of the present writer's first observations with a telescope is dated May 17, 1873, and is as follows : " Observed one of Jupiter's satellites occulted (or very nearly so) by another. Appeared as one with power 133 " (on 3-inch refractor in the Punjab). These satellites were probably I. and II.

Jupiter has been seen on several occasions apparently without his satellites; some being

1 Popular Astronomy, vol. 11 (1903), p. 574.

2 Ibid., October, 1908.

80 ASTRONOMICAL CURIOSITIES

behind the disc, some eclipsed in his shadow, and some in transit across the disc. This phenomenon was seen by Galileo, March 15, 1611 ; by Molyneux, on November 12, 1681 ; by Sir William Herschel, May 23, 1802; by Wallis, April 15, 1826; by Greisbach, September 27, 1843 ; and by several observers on four occasions in the years 1867-1895.1 The phenomenon again occurred on October 3, 1907, No. 1 being eclipsed and occulted, No. 2 in transit, No. 3 eclipsed, and No. 4 occulted.2 It was not, however, visible in Europe, but could have been seen in Asia and Oceania.2 The phenomenon will occur again on October 22, 1913.3

On the night of September 19, 1903, a star of magnitude 6J was occulted by the disc of Jupiter. This curious and rare phenomenon was photo- graphed by M. Lucien Rudaux at the Observatory of Donville, France.4 The star was Lalande 45698 (=BAC8129).5

Prof. Barnard, using telescopes with apertures from 5 inches up to 36 inches (Lick), has failed to see a satellite through the planet's limb (an observation which has been claimed by other astronomers). He says, "To my mind this has

1 Bulletin, Ast. Soc. de France, August, 1907.

2 Nature, August, 29 1907.

3 Ibid., March 7, 1907.

* Bulletin, Ast. Soc. de France, June, 1904. 5 The Observatory, October, 1903, p. 392.

JUPITER 81

been due to either poor seeing, a poor telescope, or an excited observer." 1 He adds —

"I think it is high time that the astronomers reject the idea that the satellites of Jupiter can be seen through his limb at occultation. When the seeing is bad there is a spurious limb to Jupiter that well might give the appearance of trans- parency at the occultation of a satellite. But under first-class conditions the limb of Jupiter is perfectly opaque. It is quibbling and begging the question altogether to say the phenomenon of transparency may be a rare one and so have escaped my observations. Has any one said that the moon was transparent when a star has been seen projected on it when it ought to have been behind it?"

Prof. Barnard and Mr. Douglass have seen white polar caps 011 the third and fourth satellites of Jupiter. The former says they are "exactly like those on Mars." "Both caps of the fourth satellite have been clearly distinguished, that at the north being sometimes exceptionally large, covering a surface equal to one-quarter or one- third of the diameter of the satellite." 2 This was confirmed on November 23, 1906, when Signor J. Comas Sola observed a brilliant white spot sur- rounded by a dark marking in the north polar region of the third satellite. There were other dark markings visible, and the satellite presented the appearance of a miniature of Mars.8

1 Astronomy and Astrophysics, 1894, p. 277.

2 Nature, November 18, 1897.

3 Journal, B.A.A., January, 1907.

G

82 ASTRONOMICAL CURIOSITIES

An eighth satellite of Jupiter has recently been discovered by Mr. Melotte at the Greenwich Observatory by means of photography. It moves in a retrograde direction round Jupiter in an orbit inclined about 30° to that of the planet. The period of revolution is about two years. The orbit is very eccentric, the eccentricity being about one-third, or greater than that of any other satellite of the solar system. When nearest to Jupiter it is about 9 millions of miles from the planet, and when farthest about 20 millions.1 It has been suggested by Mr. George Forbes that this satellite may possibly be identical with the lost comet of Lexell which at its return in the year 1779 became entangled in Jupiter's system, and has not been seen since. If this be the case, we should have the curious phenomenon of a comet revolving round a planet !

According to Humboldt the four bright satellites of Jupiter were seen almost simultaneously and quite independently by Simon Marius at Ausbach on December 29, 1609, and by Galileo at Padua on January 7, 1610.2 The actual priority, there- fore, seems to rest with Simon Marius, but the publication of the discovery was first made by Galileo in his Nuncius Siderius (1610).3 Grant, however, in his History of Physical Astronomy,

1 Journal, B.A.A., February, 1909, p. 161.

2 Covmos, vol. ii. p. 703.

3 Ibid.

JUPITER 83

calls Simon Marius an " impudent pretender " ! (p. 79).

M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890, twenty minutes before sunset.1

Humboldt states that he saw Jupiter with the naked eye when the sun was from 18° to 20° above the horizon.2 This was in the plains of South America near the sea-level.

1 Denning, Telescopic Work for Starlight Evenings, p. 349.

2 Cosmos, vol. iii. p. 75.

CHAPTER IX

Saturn

TO show the advantages of large telescopes over small ones, Mr. C. Roberts says that "with the 25-inch refractor of the Cambridge Observatory the view of the planet Saturn is indescribably glorious ; everything I had ever seen before was visible at a glance, and an enormous amount of detail that I had never even glimpsed before, after a few minutes' observation." l

Chacornac found that the illumination of Saturn's disc is the reverse of that of Jupiter, the edges of Saturn being brighter than the centre of the disc, while in the case of Jupiter — as in that of the sun — the edges are fainter than the centre.2 According to Mr. Denning, Saturn bears satisfactorily "greater magnifying power than either Mars or Jupiter." 3

At an occultation of Saturn by the moon, which occurred on June 13, 1900, M. M. Honorat

1 Journal, B.A.A., June, 1896.

2 Celestial Objects, vol. i. p. 191.

3 Nature, May 30, 1901.

SATURN 85

noticed the great contrast between the slightly yellowish colour of the moon and the greenish tint of the planet.1

In the year 1892, when the rings of Saturn had nearly disappeared, Prof. L. W. Underwood, of the Underwood Observatory, Appleton, Wis- consin (U.S.A.), saw one of Saturn's satellites (Titan) apparently moving along the needlelike appendage to the planet presented by the rings. " The apparent diameter of the satellite so far exceeded the apparent thickness of the ring that it gave the appearance of a beautiful golden bead moving very slowly along a fine golden thread." 2

In 1907, when the rings of Saturn became invisible in ordinary telescopes, Professor Camp- bell, observing with the great Lick telescope, noticed " prominent bright knots, visible . , . in Saturn's rings. The knots were symmetrically placed, two being to the east and two to the west." This was confirmed by Mr. Lowell, who says, " Condensations in Saturn's rings confirmed here and measured repeatedly. Symmetric and permanent." This phenomenon was previously seen by Bond in the years 1847-56. Measures of these light spots made by Prof. Barnard with the 40 -inch Yerkes telescope show that the outer one corresponded in position with the outer edge

1 Bulletin, Ast. Soc. de France, August, 1900.

2 Astronomy and Astrophysics, 1892.

86 ASTRONOMICAL CURIOSITIES

of the middle ring close to the Cassini division, and the inner condensation, curious to say, seemed to coincide in position with the "crape ring." Prof. Barnard thinks that the thickness of the rings "must be greatly under 100 miles, and probably less than 50 miles," and he says —

"The important fact clearly brought out at this apparition of Saturn is that the bright rings are not opaque to the light of the sun — and this is really what we should expect from the nature of their constitution as shown by the theory of Clerk Maxwell, and the spectroscopic results of Keeler." 1

Under certain conditions it would be theo- retically possible, according to Mr. Whitmell, to see the globe of Saturn through the Cassini division in the ring. But the observation would be one of great difficulty and delicacy. The effect would be that, of the arc of the division Avhich crosses the planet's disc, "a small portion will appear bright instead of dark, and may almost disappear." 2

A remarkable white spot was seen on Saturn on June 23, 1903, by Prof. Barnard, and afterwards by Mr. Denning.3 Another white spot was seen by Denning on July 9 of the same year.4 From numerous observations of these spots, Denning found a rotation period for the planet of about

1 Astrophysical Journal, January, 1908, p. 35.

- Nature, May 22, 1902. 3 Ibid., July 0, 1908.

4 Ibid., July 1C, 1903.

SATURN 87

10h 39 m 21s.1 From observations of the same spots Signer Comas Sola found a period 10h 38m<4, a close agreement with Denning's result. For Saturn's equator, Prof. Hill found a rotation period of 10h 14m 23S>8, so that— as in the case of Jupiter — the rotation is faster at the equator than in the northern latitudes of the planet. A similar phenomenon is observed in the sun. Mr. Denning's results were fully confirmed by Herr Leo Brenner, and other German astronomers.2

Photographs taken by Prof. V. M. Slipher in America show that the spectrum of Saturn is similar to that of Jupiter. None of the bands observed in the planet's spectrum are visible in the spectrum of the rings. This shows that if the rings possess an atmosphere at all, it must be much rarer than that surrounding the ball of the planet. Prof. Slipher says that " none of the absorption bands in the spectrum of Saturn can be identified with those bands due to absorption in the earth's atmosphere," and there is no trace of aqueous vapour.3

In September, 1907, M. G. Fournier suspected the existence of a " faint transparent and luminous ring " outside the principal rings of Saturn. He thinks that it may possibly be subject to periodical fluctuations of brightness, sometimes being visible

1 Nature, September 24, 1903.

2 Ibid., October 8, 1903.

3 Astrophysical Journal, vol. 2G (1907), p. 60.

88 ASTRONOMICAL CURIOSITIES

and sometimes not.1 This dusky ring was again suspected at the Geneva Observatory in October, 1908.2 M. Schaer found it a difficult object with a 16-inch Cassegraiii reflector. Prof. Stromgen at Copenhagen, and Prof. Hartwig at Bamberg, however, failed to see any trace of the supposed ring.3 It was seen at Greenwich in October, 1908.

A "dark transit" of Saturn's satellite Titan across the disc of the planet has been observed on several occasions. It was seen by Mr. Isaac W. Ward, of Belfast, on March 27, 1892, with a 4 '3-inch Wray refractor. The satellite appeared smaller than its shadow. The phenomenon was also seen on March 12 of the same year by the Rev. A. Freeman, Mr. Mee, and M. F. Terby ; and again on November 6, 1907, by Mr. Paul Chauleur and Mr. A. B. Cobham.4

The recently discovered tenth satellite of Saturn, Themis, was discovered by photography, and has never been seen by the eye even with the largest telescopes ! But its existence is beyond all doubt, and its orbit round the planet has been calculated.

Prof. Hussey of the Lick Observatory finds that Saturn's satellite Mimas is probably larger than Hyperion. He also finds from careful measure- ments that the diameter of Titan is certainly

1 Nature, January 30, 1908.

2 Ibid., October 15, 1908.

3 Ibid., October 29, 1908.

4 Journal, B.A.A., March, 1908, and June 22, 1908.

SATURN 89

overestimated, and that its probable diameter is about 2500 miles.1

The French astronomer, M. Lucien Rudaux, finds the following variation in the light of the satellites of Saturn : —

Japetus from 9th magnitude to 12th Ehea „ 9 „ 10-6

Dione „ 9'5 „ 10'5

Tethye „ 9'8 „ 10-5

Titan „ 8 „ 8'6

The variation of light is, he thinks, due to the fact that the period of rotation of each satellite is equal to that of their revolution round the planet ; as in the case of our moon.2

The names of the satellites of Saturn are derived from the ancient heathen mythology. They are given in order of distance from the planet, the nearest being Mimas and the farthest Themis.

1. Mimas was a Trojan born at the same time as Paris.

2. Enceladus was son of Tartarus and Ge.

3. Tethys was wife of Oceanus, god of ocean currents. She became mother of all the chief rivers in the universe, as also the Oceanides or sea nymphs.

4. Dione was one of the wives of Zeus.

5. Rhea was a daughter of Uranus. She married Saturn, and became the mother of Vesta, Ceres, Juno, and Pluto.

1 Nature, June 25, 1903.

2 Bulletin, Aft. Soc. de France, June, 1901.

90 ASTRONOMICAL CURIOSITIES

6. Titan was the eldest son of Uranus.

7. Hyperion was the god of day, and the father of sun and moon.

8. Japetus was the fifth son of Uranus, and father of Atlas and Prometheus.1

9. Phcebe was daughter of Uranus and Ge.

10. Themis was daughter of Uranus and Ge, and, therefore, sister of Phoebe.

In a review of Prof. Comstock's Text Book of Astronomy in The Observatory, November, 1901, the remark occurs, " We are astonished to see that Mr. Comstock alludes with apparent serious- ness to the nine satellites of Saturn. As regards the ninth satellite, we thought that all astronomers held with Mrs. Betsy Prig on the subject of this astronomical Mrs. Harris." This reads curiously now (1909) when the existence of the ninth satellite (Phoebe) has been fully confirmed, and a tenth satellite discovered.

1 Pop. Asi., vol. 12, pp. 408-9.

CHAPTER X

Uranus and Neptune

FROM observations of Uranus made in 1896, M. Leo Brenner concluded that the planet rotates on its axis in about 8^ hours (probably 8h 27m). This is a short period, but considering the short periods of Jupiter and Saturn there seems to be nothing improbable about it.

Prof. Barnard finds that the two inner satellites of Uranus are difficult objects even with the great 36-inch telescope of the Lick Observatory ! They have, however, been photo- graphed at Cambridge (U.S.A.) with a 13-inch lens, although they are " among the most difficult objects known." l

Sir William Huggins in 1871 found strong absorption lines (six strong lines) in the spectrum of Uranus. One of these lines indicated the presence of hydrogen, a gas which does not exist in our atmosphere. Three of the other lines seen were situated near lines in the spectrum of atmospheric air. Neither carbonic acid nor sodhim 1 Nature, August 29, 1889.

92 ASTRONOMICAL CURIOSITIES

showed any indications of their presence in the planet's spectrum. A photograph by Prof. Slipher of Neptune's spectrum " shows the spectrum of this planet to contain many strong absorption bands. These bands are so pronounced in the part of the spectrum between the Fraunhofer lines F and D, as to leave the solar spectrum unrecog- nizable. . . . Neptune's spectrum is strikingly different from that of Uranus, the bands in the latter planet all being reinforced in Neptune. In this planet there are also new bands which have not been observed in any of the other planets. The F line of hydrogen is remarkably dark . . . this band is of more than solar strength in the spectrum of Uranus also. Thus free hydrogen seems to be present in the atmosphere of both these planets. This and the other dark bands in these planets bear evidence of an enveloping atmosphere of gases which is quite unlike that which surrounds the earth." l

With the 18-inch equatorial telescope of the Strasburgh Observatory, M. Wirtz measured the diameter of Neptune, and found from forty-nine measures made between December 9, 1902, and March 28, 1903, a value of 2"'303 at a distance of 30*1093 (earth's distance from sun = l). This given a diameter of 50,251 kilometres, or about 31,225 miles,2 and a mean density of 1*54 (water=l;

1 Attrophysical Journal, vol. 2G (1907), p. 62.

2 Bulletin, A*t. Soc.de France, January, 1904.

URANUS AND NEPTUNE 93

earth's mean density = 5'53). Prof. Barnard's measures gave a diameter of 32,900 miles, a fairly close agreement, considering the difficulty of measuring so small a disc as that shown by Neptune.

The satellite of Neptune was photographed at the Pulkown Observatory in the year 1899. The name Triton has been suggested for it. In the old Greek mythology Triton was a son of Neptune, so the name would be an appropriate one.

The existence of a second satellite of Neptune is suspected by Prof. Schaeberle, who thinks he once saw it with the 36-inch telescope of the Lick Observatory "on an exceptionally fine night " iii 1895.1 But this supposed discovery has not yet been confirmed. Lassell also thought he had discovered a second satellite, but this supposed discovery was never confirmed.1

The ancient Burmese mention eight planets, the sun, the moon, Mercury, Venus, Mars, Jupiter, Saturn, and another named Rahu, which is in- visible. It has been surmised that " Rahu " is Uranus, which, is just visible to the naked eye, and may possibly have been discovered by keen eyesight in ancient times. The present writer has seen it several times without optical aid in the West of Ireland, and with a binocular field- glass of 2 inches aperture he found it quite a con- spicuous object.

1 Humboldt'g Cosmos, vol. iv. p. 532.

94 ASTRONOMICAL CURIOSITIES

When Neptune was visually discovered by Galle, at Berlin, he was assisted in his observa- tion by Prof, d' Arrest. The incident is thus de- scribed by Dr. Dreyer, " On the night of June 14, 1874, while observing Coggia's comet together, I reminded Prof, d' Arrest how he had once said in the course of a lecture, that he had been present at the finding of Neptune, and that * he might say it would not have been found without him.' He then told me (and I wrote it down the next day), how he had suggested the use of Bremiker's map (as first mentioned by Dr. Galle in 1877) and con- tinued, * We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he said : " And then there is a star of the 8th magnitude, in such and such a position," whereupon I immediately exclaimed : " That star is not on the map," ' x This was the planet. But it seems to the present writer that if Galle or d' Arrest had access to Harding's Atlas (as they probably had) they might easily have found the planet with a good binocular field-glass. As a matter of fact Neptune is shown in Harding's Atlas (1822) as a star of the 8th magnitude, having been mistaken for a star by Lalande on May 8 and 10, 1795 ; and the present writer has 1 Copernicus, vol. ii. p. 64.

URANUS AND NEPTUNE 95

found Harding's 8th magnitude stars quite easy objects with a binocular field- glass having object- glasses of two inches diameter, and a power of about six diameters.

SUPPOSED PLANET BEYOND NEPTUNE. — The possible existence of a planet beyond Neptune has been frequently suggested. From considera- tions on the aphelia of certain comets, Prof. Forces in 1880 computed the probable position of such a body. He thought this hypothetical planet would be considerably larger than Jupiter, and probably revolve round the sun at a distance of about 100 times the earth's mean distance from the sun. The place indicated was between R.A. llh 24m and 12h 12m, and declination 0° 0' to 6° 0' north. With a view to its discovery, the late Dr. Roberts took a series of eighteen photographs covering the region indicated. The result of an examination of these photographs showed, Dr. Roberts says, that " no planet of greater brightness than a star of the 1 5th magnitude exists on the sky area herein indicated." Prof. W. H. Pickering has recently revived the question, and has arrived at the following results : Mean distance of the planet from the sun, 51'0 (earth's mean distance = 1); period of revolution, 373J years ; mass about twice the earth's mass ; probable position for 1909 about R.A. 7h 47m, north declination 21°, or about 5° south-east of the star * Geminorum. The supposed planet would be faint, its brightness

96 ASTRONOMICAL CURIOSITIES

being from 11 J to 13J, according to the " albedo" (or reflecting power) it may have.1

Prof. Forbes has again attacked the question of a possible ultra-Neptunian planet, and from a consideration of the comets of 1556, 1843 I, 1880 I, and 1882 II, finds a mean distance of 105*4, with an inclination of the orbit of 52° to the plane of the ecliptic. This high inclination implies that "during the greatest part of its revolution it is beyond the zodiac," and this, Mr. W. T. Lynn thinks, "may partly account for its not having hitherto been found by observation.2

From a consideration of the approximately circular shape of the orbits of all the large planets of the solar system, Dr. See suggests the existence of three planets outside Neptune, with approxi- mate distances from the sun of 42, 56, and 72 respectively (earth's distance = 1), and recommends a photographic search for them. He says, "To suppose the planetary system to terminate with an orbit so round as that of Neptune is as absurd as to suppose that Jupiter's system terminates with the orbit of the fourth satellite." 3

According to Grant, even twenty years before the discovery of Neptune the error of Prof. Adams' first approximation amounted to little more than 10°.4

1 Knowledge, May, 1909.

2 Journal, British Astronomical Association, January, 1909, p. 132.

3 Ast. Nach., No. 4308. 4 History of Physical Astronomy, p, 204.

CHAPTER XI

Comets

WE learn from Pliny that comets were classified in ancient times, according to their peculiar forms, into twelve classes, of which the principal were : Pogonias, bearded ; Lampadias, torch-like ; Xiphias, sword- like ; Pitheus, tun-like ; Acontias, javelin-like ; Ceratias, horn-like ; Disceus, quoit-like ; and Hip- pias, horse-mane-like.1

Of the numerous comets mentioned in astrono- mical records, comparatively few have been visible to the naked eye. Before the invention of the tele- scope (1610) only those which were so visible could, of course, be recorded. These number about 400. Of the 400 observed since then, some 70 or 80 only have been visible by unaided vision ; and most of these now recorded could never have been seen without a telescope. During the last century, out of 300 comets discovered, only 13 were very visible to the naked eye. Hence, when we read in the newspapers that a comet has been discovered the

1 Smyth's Celestial Cycle, pp. 210, 211.

H

98 ASTRONOMICAL CURIOSITIES

chances are greatly against it becoming visible to the naked eye.1

Although comparatively few comets can be seen without a telescope, they are sometimes bright enough to be visible in daylight ! Such were those of B.C. 43, A.D. 1106, 1402, 1532, 1577, 1744, 1843, and the " great September comet " of 1882.

If we except the great comet of 1861, through the tail of which the earth is supposed to have passed, the comet which came nearest to the earth was that of 1770, known as Lexell's, which approached us within two millions of miles, moving nearly in the plane of the ecliptic. It produced, however, no effect on the tides, nor on the moon's motion, which shows that its mass must have been very small. It was computed by Laplace that if its mass had equalled that of the earth, the length of our year would have been shortened by 2 hours 47 minutes, and as there was no perceptible change Laplace concluded that the comet's mass did not exceed ^otn of tne earth's mass. This is the comet which passed so near to Jupiter that its period was reduced to 5J years. Owing to another near approach in 1779 it became invisible from the earth, and is now lost.2 Its identity with the recently discovered eighth satellite of Jupiter has been suggested by Mr. George Forbes (see under " Jupiter "). At the near approach of Lexell's comet to the earth in 1770, Messier, " the comet ferret,

1 Poor, The Solar System,?. 274. « Celestial Cycle, p. 246.

COMETS 99

found that its head had an apparent diameter of 2J°, or nearly five times that of the moon !

Another case^of near approach to the earth was that of Biela's comet at its appearance in 1805. On the evening of December 9 of that year, the comet approached the earth within 3,380,000 miles.1

The comet of A.D. 1106 is stated to have been seen in daylight close to the sun. This was on February 4 of that year. On February 10 it had a tail of 60° in length, according to Gaubil.2

The comet of 1577 seems to have been one of the brightest on record. According to Tycho Brahe, it was visible in broad daylight. He describes the head as " round, bright, and of a yellowish light," with a curved tail of a reddish colour.3

The comet of 1652 was observed for about three weeks only, and Hevelius and Comiers state that it was equal to the moon in apparent size ! This would indicate a near approach to the earth. An orbit computed by Halley shows that the least distance was about 12 millions of miles, and the diameter of the comet's head rather less than 110,000 miles, or about 14 times the earth's diameter.

According to Mr. Denning, " most of the periodical comets at perihelion are outside the earth's orbit, and hence it follows that they escape

1 Nature, October 2, 1879. 2 Ibid., May 6, 1880.

3 Ibid., February 19, 1880.

100 ASTRONOMICAL CURIOSITIES

observation unless the earth is on the same side of the sun as the comet." l

It was computed by M. Faye that the volume of the famous Donati's comet (1858) was about 500 times that of the sun ! On the other hand, he calculated that its mass (or quantity of matter it contained) was only a fraction of the earth's mass. This shows how almost inconceivably tenuous the material forming the comet must have been — much more rarefied, indeed, than the most perfect vacuum which can be produced in an air-pump. This tenuity is shown by the fact that stars were seen through the tail "as if the tail did not exist." A mist of a few hundred yards in thick- ness is sufficient to hide the stars from our view, while a thickness of thousands of miles of comet- ary matter does not suffice even to dim their brilliancy !

At the time of the appearance of the great comet of 1843, it was doubtful whether the comet had transited the sun's disc. But it is now known, from careful calculations by Prof. Hubbard, that a transit really took place between llh 28m and 12h 29m on February 27, 1843, and might have been observed in the southern hemisphere. The dis- tance of this remarkable comet from the sun at its perihelion passage was less than that of any known comet. A little before 10 p.m. on Feb- ruary 27, the comet passed within 81,500 miles of 1 Nature, September 30, 1897.

COMETS 101

the sun's surface with the enormous velocity of 348 miles a second ! It remained less than 2£ hours north of the ecliptic, passing from the ascending to the descending node of its orbit in 2h 13m*4.1 The great comet of 1882 transited the sun's disc on Sunday, September 17, of that year, the ingress taking place at 4h 50m 58s, Cape mean time. When on the sun the comet was absolutely invisible, showing that there was nothing solid about it. It was visible near the sun with the naked eye a little before the transit took place.2 This great comet was found by several computers to have been travelling in an elliptic orbit with a period of about eight centuries. Morrison found 712 years ; Frisby, 794 ; Fabritius, 823 ; and Kreutz, 843 years.3

The great southern comet of 1887 may be de- scribed as a comet without a head ! The popular idea of a comet is a star with a tail. But in this case there was no head visible — to the naked eye at least. Dr. Thome of the Cordoba Observatory — its discoverer — describes it as " a beautiful object — a narrow, straight, sharply denned, graceful tail, over 40° long, shining with a soft starry light against a dark sky, beginning apparently without a head, and gradually widening and fading as it extended upwards." 4

1 Nature, August 5, 1875.

2 IMd., October 12, 1882, and Copernicus, vol. iii. p. 85.

3 Nature, May 8, 1881. 4 Ibid, June 16, 1887.

102 ASTRONOMICAL CURIOSITIES

The great southern comet of 1901 had five tails on May 6 of that year. Two were fairly bright, and the remaining three rather faint. Mr. Gale saw a number of faint stars through the tails. The light of these seem to have been "undim- med." Mr. Cobham noticed that the stars Rigel and /5 Eridani shone through one of the faint tails, and " showed no perceptible difference." *

Prof. W. H. Pickering says that " the head of a comet, as far as our present knowledge is con- cerned, seems therefore to be merely a meteor swarm containing so much gaseous material that when electrified by its approach to the sun it will be rendered luminous" (Harvard Annual, vol. xxxii. part ii. p. 295) "... if the meteors and their atmospheres are sufficiently widely separated f rom one another, the comet may be brilliant and yet transparent at the same time."

In the case of Swift's comet of 1892 some periodical differences of appearance were due, according to Prof. W. H. Pickering, to a rotation of the comet round an axis passing longitudinally through the tail, and he estimated the period of rotation at about 94 to 97 hours. He computed that in this comet the repulsive force exerted by the sun on the comet's tail was " about 39' 5 times the gravitational force." 2

The comet known as 19026 approached the

1 Journal, B.A.A., December 13, 1901.

2 Nature, September 20, 1900.

UNIVERSITY

COMETS 103

planet Mercury within two millions of miles on November 29 of that year. Prof. O. C. Wendell, of Harvard Observatory, made some observations on the transparency of this comet. He found with the aid of a photometer and the 15-inch telescope of the observatory that in the case of two faint stars over which the comet passed on October 14, 1902, the absorption of light by the comet was insensible, and possibly did not exceed one or two hundredths of a magnitude,1 an amount quite imperceptible to the naked eye, and shows con- clusively how almost inconceivably rarefied the substance of this comet must be.

The comet known as Morehouse (1908c) showed some curious and wonderful changes. Mr. Borelly found that five tails are visible on a photographic plate taken on October 3, 1908, and the trail of an occulted star indicates a slight absorption effect. According to M. L. Rabourdin, great changes took place from day to day, and even during the course of an hour ! Similar changes were recorded by G. M. Gauthier; and Prof. Barnard, who photographed the comet on 30 nights from September 2 to October 13, states that the photo- graphs of September 30 "are unique, whilst the transformation which took place between the taking of these and the taking of the next one on October 1 was very wonderful." • The spectrum

1 Ast. Nach., No. 3868, and Nature, March 12, 1903.

2 Nature, November 13, 1908.

10* ASTRONOMICAL CURIOSITIES

showed the lines of cyanogen instead of the hydrocarbon spectrum shown by most comets.

Prof. Barnard has suggested that all the phenomena of comets' tails cannot be explained by a repulsive force from the sun. Short tails issuing from the comet's nucleus at considerable angles with the main tail point to eruptive action in the comet itself. The rapid changes and dis- tortions frequently observed in the tails of some comets suggest motion through a resisting medium ; and the sudden increase of light also occasionally observed points in the same direction.1

It was computed by Olbers that if a comet having a mass of j^^h of the earth's mass — which would form a globe of about 520 miles in diameter and of the density of granite — collided with the earth, with a velocity of 40 miles a second, our globe would be shattered into fragments.2 But that any comet has a solid nucleus of this size seems very doubtful; and we may further say that the collision of the earth with any comet is highly improbable.

It seems to be a common idea that harvests are affected by comets, and even " comet wines " are sometimes spoken of. But we know that the earth receives practically no heat from the brightest comet. Even in the case of the brilliant comet of 1811, one of the finest on record, it was

1 Nature, December 7, 1905. 2 Celestial Cycle, p. 259.

COMETS 105

found that " all the efforts to concentrate its rays did not produce the slightest effect on the blackened bulb of the most sensitive thermometer." Arago found that the year 1808, in which several comets were visible, was a cold year, "and 1831, in which there was no comet, enjoyed a much higher temperature than 1819, when there were three comets, one of which was very brilliant." 1 We may, therefore, safely conclude that even a large comet has no effect whatever on the weather.

From calculations on the orbit of Halley's comet, the next return of which is due in 1910, Messrs. Cowell and Crommelin find that the identity of the comet shown on the Bayeux Tapestry with Halley's comet is now " fully established." They find that the date of perihelion passage was March 25, 1066, which differs by only 4 days from the date found by Hind. The im- posing aspect of the comet in 1066 described in European chronicles of that time is confirmed by the Chinese Annals. In the latter records the brightness is compared to that of Venus, and even with that of the moon ! The comparison with the moon was probably an exaggeration, but the comet doubtless made a very brilliant show. In the Bayeux Tapestry the inscription on the wall behind the spectators reads: "isti mirant stella." Now, this is bad Latin, and Mr. W. T. 1 Celestial Cycle, p. 260.

106 ASTRONOMICAL CURIOSITIES

Lynn has made the interesting suggestion that some of the letters are hidden by the buildings in front and that the real sentence is " isti mirantur stellam." 1 The present writer has examined the copy of the Bayeux Tapestry which is in the Dublin Museum, and thinks that Mr. Lynn's suggestion seems very plausible. But the last letter of stellam is apparently hidden by the comet's tail, which does not seem very probable !

The conditions under which the comet will appear in 1910 are not unlike those of 1066 and 1145. " In each year the comet was discovered as a morning star, then lost in the sun's rays ; on its emergence it was near the earth and moved with great rapidity, finally becoming stationary in the neighbourhood of Hydra, where it was lost to view." 2 In 1910 it will probably be an evening star before March 17, and after May 11, making a near approach to the earth about May 12. About this time its apparent motion in the sky will be very rapid. As, however, periodical comets — such as Halley's — seem to become fainter at each return, great expectations with reference to its appearance in 1910 should not be indulged in.

The appearance of Halley's comet in A.D. 1222 is thus described by Pingre— a great authority on comets — (quoting from an ancient writer) — " In autumn, that is to say in the months of

> Journal, B.A.A., April, 1907.

2 Monthly Notices, R.A.S., March, 1908.

COMETS 107

August andSeptember,'a star of the first magnitude was seen, very red, and accompanied by a great tail which extended towards the top of the sky in the form of a cone extremely pointed. It appeared to be very near the earth. It was observed (at first ?) near the place of the setting sun in the month of December."

With reference to its appearance in the year 1456, when it was of " vivid brightness," and had a tail of 60° in length, Admiral Smyth says,1 " To its malign influence were imputed the rapid successes of Mahomet II., which then threatened all Christendom. The general alarm was greatly aggravated by the conduct of Pope Callixtus III., who, though otherwise a man of abilities, was a poor astronomer; for that pontiff daily ordered the church bells to be rung at noon-tide, extra Ave-Marias to be repeated, and a special protest and excommunication was composed, exorcising equally the Devil, the Turks, and the comet." With reference to this story, Mr. G. F. Chambers points out2 that it is probably based on a pas- sage in Platina's Vitce Pontificum. But in this passage there is no mention made of excommuni- cation or exorcism, so that the story, which has long been current, is probably mythical. In con- firmation of this view, the Rev. W. F. Rigge has shown conclusively 3 that no bull was ever issued

1 Celestial Cycle, p. 231.

2 Journal, B.A.A., July, 1908.

3 Popular Agronomy, October, 1908.

108 ASTRONOMICAL CURIOSITIES

by Pope Callixtus III. containing a reference to any comet. The story would therefore seem to be absolutely without foundation, and should be consigned to the limbo of all such baseless myths. With reference to the appearance of Halley's comet, at his last return in 1835, Sir John Herschel, who observed it at the Cape of Good Hope, says —

"Among the innumerable stars of all magni- tudes, from the ninth downwards, which at various times were seen through it, and some extremely near to the nucleus (though not exactly on it) there never appeared the least ground for presuming any extinction of their light in traver- sing it. Very minute stars indeed, on entering its brightest portions, were obliterated, as they would have been by an equal illumination of the field of view ; but stars which before their entry appeared bright enough to bear that degree of illumination, were in no case, so far as I could judge, affected to a greater extent than they would have been by so much lamp-light artificially introduced."1

It is computed by Prof. J. ^Holetschak that, early in October, 1909, Halley's comet should have the brightness of a star of about 14^ magnitude.3 It should then — if not detected before — be dis- coverable with some of the large telescopes now available.

According to the computations of Messrs. Cowell and Crommelin, the comet should enter Pisces

> Cape O&a., p. 401. 2 Nature, July 2, 1908.

COMETS 109

from Aries in January, 1910. "Travelling west- ward towards the star y Piscium until the begin- ning of May, and then turning eastward again, it will travel back through the constellations Cetus, Orion, Monoceres, Hydra, and Sextans." From this it seems that observers in the southern hemisphere will have a better view of the comet than those in northern latitudes. The computed brightness varies from 1 on January 2, 1910, to 1112 on May 10. But the actual brightness of a comet does not always agree with theory. It is sometimes brighter than calculation would indicate.

According to Prof. O. C. Wendell, Halley's comet will, on May 12, 1910, approach the earth's orbit within 4*6 millions of miles ; and he thinks that possibly the earth may " encounter some meteors," which are presumably connected with the comet. He has computed the " radiant point " of these meteors (that is, the point from which they appear to come), and finds its position to be R.A. 22h 42m-9, Decl. N. 1° 18'. This point lies a little south-west of the star /3 Piscium.

According to Dr. Smart, the comet will, on June 2, " cross the Equator thirteen degrees south of Regulus, and will then move slowly in the direction of </> Leonis. The comet will be at its descending node on the ecliptic in the morning of May 16, and the earth will pass through the node on the comet's orbit about two and a half

110 ASTRONOMICAL CURIOSITIES

days later. The comet's orbit at the node is about 13 million miles within that of the earth. Matter repelled from the comet's nucleus by the sun with a velocity of about 216,000 miles per hour, would just meet the earth when crossing the comet's orbit plane. Matter expelled with a velocity of 80,000 miles per hour, as in the case of Comet Morehouse, would require seven days for the journey. Cometary matter is said to have acquired greater velocities than this, for (according to Webb, who quotes Chacornac) Comet II., 1862, shot luminous matter towards the sun, with a velocity of nearly 2200 miles per second. It is therefore possible that matter thrown off by the comet at the node may enter our atmosphere, in which case we must hope that cyanogen, which so often appears in cometary spectra, may not be inconveniently in evidence." 1

Cyanogen is, of course, a poisonous gas, but cometary matter is so rarefied that injurious effects on the earth need not be feared.

If we can believe the accounts which have been handed down to us, some very wonderful comets were visible in ancient times. The following may be mentioned : —

B.C. 165. The sun is said to have been "seen for several hours in the night." If this was a comet it must have been one of extraordinary brilliancy.2

1 Journal, B.A.A., January 20, 1909, pp. 123-4.

2 Chambers' Hanrtbooli of Astronomy, Catalogue of Cometa.

COMETS 111

B.C. 146. " After the death of Demetrius, king of Syria, the father of Demetrius and Antiochus, a little before the war in Achaia, there appeared a comet as large as the sun. Its disc was first red, and like fire, -spreading sufficient light to dissipate the darkness of night; after a little while its size diminished, its brilliancy became weakened, and at length it entirely disappeared." l

B.C. 134. It is recorded that at the birth of Mithridates a great comet appeared which "occupied the fourth part of the sky, and its brilliancy was superior to that of the sun." (?) 2

B.C. 75. A comet is described as equal in size to the moon, and giving as much light as the sun on a cloudy day. (!) 3

A.D. 531. In this year a great comet was observed in Europe and China. It is described as " a very large and fearful comet," and was visible in the west for three weeks. Hind thinks that this was an appearance of Halley's comet,4 and this has been confirmed by Mr. Crommelin.

A.D. 813, August 4. A comet is said to have appeared on this date, of which the following curious description is given: "It resembled two moons joined together ; they separated, and having taken different forms, at length appeared like a man without a head." (!) 5

1 Seneca, quoted by Chambers, Handbook, voL i. p. 554 (Fourth Edition).

2 Ibid. 3 Hid. * Ibid,, p. 534. 3 Ibid.

ASTRONOMICAL CURIOSITIES

A.D. 893. A great comet is said to have appeared in this year with a tail 100° in length, which afterwards increased to 200° ! l

A.D. 1402. A comet appeared in February of this year, which was visible in' daylight for eight days. " On Palm Sunday, March 19, its size was prodigious." Another comet, visible in the day- time, was seen from June to September of the same year.

When the orbit of the comet known as 1889 V was computed, it was found that it had passed through Jupiter's system in 1886 (July 18-21). The calculations show that it must have passed within a distance of 112,300 miles of the planet itself — or less than half the moon's distance from the earth — and " its centre may possibly have grazed the surface of Jupiter." 2

Sir John Herschel thought that the great comet of 1861 was by far the brightest comet he had ever seen, those of 1811 and 1858 (Donati's) not excepted.3 Prof. Kreutz found its period of revo- lution round the sun to be about 409 years, with the plane of the orbit nearly at right angles to the plane of the ecliptic.

On November 9, 1795, Sir William Herschel saw the comet of that year pass centrally over

1 Ma-tuoan-lin, quoted by Chambers, Handbook, p. 570.

2 Astronomy and Astrophysics, 1893, p. 798.

3 TJte Observatory, October, 1898.

COMETS 113

a small double star of the llth and 12th magnitudes, and the fainter of the two com- ponents remained distinctly visible during the comet's transit over the star. This comet' was an appearance of the comet now known as Encke's.1 Struve saw a star of the 10th magni- tude through nearly the brightest part of Encke's comet on November 7, 1828, but the star's light was not dimmed by the comet.

Sir John Herschel saw a cluster of stars of the 16th or 17th magnitude through Biela's comet, although the interposed cometary matter must have been at least 50,000 miles in thickness.2

Bessel found that on September 29, 1835, a star of the 10th magnitude shone with undimmed lustre through the tail of Halley's comet within 8 seconds of arc of the central point of the head. At Dorpat (Russia) Struve saw the same star " in conjunction only 2""2 from the brightest point of the comet. The star remained continuously visible, and its light was not perceptibly diminished whilst the nucleus of the comet seemed to be almost extinguished before the radiance of the small star of the 9th or 10th magnitude." 3

Webb says —

" Donati saw a 7 mg. star enlarged so as to show a sensible disc, when the nucleus of comet III.,

1 Grant's History of Physical Astronomy, p. 293.

2 Ibid., p. 294.

3 Humboldt's Cosmos, vol. i. pp. 89, 90 (Otte's translation).

I

114 ASTRONOMICAL CURIOSITIES

1860, passed very near it. Stars are said to have started, or become tremulous, during occulta- tions by comets. Birmingham observed the comet of Encke illuminated by a star over which it passed, August 23, 1868; and Klein, in 1861, remarked an exceptional twinkling in 5 mg. stars involved in the tail." l

The comet of 1729 had the greatest perihelion distance of any known comet ; 2 that is, when nearest to the sun, it did not approach the central luminary ;within four times the earth's distance from the sun I

Barnard's comet, 1889 I., although it never became visible to the naked eye, was visible with a telescope from September 2, 1888, to August 18, 1890, or 715 days — the longest period of visibility of any comet on record. When last seen it was 65 times the earth's distance from the sun, or about 580 millions of miles, 3 or beyond the orbit of Jupiter !

Messier, who was called " the comet ferret," discovered " all his comets with a small 2-foot telescope of 2J inches aperture, magnifying 5 times, and with a field of 4°." 4

It is a very curious fact that Sir William Herschel, " during all his star-gaugings and sweeps for nebulae, never discovered a comet ; " 5 that is

1 Celestial Objects, vol. i. p. 211, footnote.

2 Denning, Telescopic Work far Starlight Evenings, p. 248. » Ibid., p. 248.

4 Ibid., p. 250. * Ibid., p. 231.

COMETS 115

an object which was afterwards proved to be a comet. Possibly, however, some of his nebulae which are now missing, may have been really comets.

Sir William Herschel found the diameter of the head of the great comet of 1811 to be 127,000 miles. The surrounding envelope he estimated to be at least 643,000 miles, or about three-fourths of the sun's diameter.

On a drawing of the tails of the great comet of 1744 given in a little book printed in Berlin in that year, no less than 12 tails are shown ! These vary in length and brightness. A copy of this drawing is given in Copernicus.1 The observations were made by " einen geschichten Frauenzimmer," who Dr. Dreyer identifies with Christian Kirch, or one of her two sisters, daughters of the famous Gottfried and Maria Margaretta Kirch (Idem, p. 107). Dr. Dreyer thinks that the drawing " seems to have been carefully made, and not to be a mere rough sketch as I had at first supposed " (Idem, p. 185).

The tails of some comets were of immense length. That of the comet of 1769 had an absolute length of 38 millions of miles. That of 1680, 96 million of miles, or more than the sun's distance from the earth. According to Sir William Herschel, the tail of the great comet of 1811 was over 100 millions of miles in length. That of the 1 Vol. iii. p. 106.

116 ASTRONOMICAL CURIOSITIES

great comet of 1843 — one of the finest in history — is supposed to have reached a length of 150 millions of miles ! l

In width the tails of comets were in some cases enormous. According to Sir William Herschel, the tail of the comet of 1811 had a diameter of 15 millions of miles I Its volume was, therefore, far greater than that of the sun ! l

According to Hevelius the comet of 1652 was of such a magnitude that it " resembled the moon when half full ; only it shone with a pale and dismal light." 2

Halley's comet at its next appearance will be examined with the spectroscope for the first time in its history. At its last return in 1835, the spectroscope had not been invented.

For the great comet of 1811, Arago computed a period of 3065 years ; and Eiicke found a period of 8800 years for the great comet of 1680.3

The variation in the orbital velocity of some comets is enormous. The velocity of the comet of 1680 when passing round the sun (perihelion) was about 212 miles a second ! Whereas at its greatest distance from the sun (aphelion) the velocity is reduced to about 10 feet a second !

1 Grant's History of Physical Astronomy, p. 298.

8 Ibid., p. 305.

3 Humboldt's Cosmos, vol. i. p. 95.

CHAPTER XII

Meteors

MR. DENNING thinks that the meteor shower of the month of May, known as the Aquarids, is probably connected with Halley's comet. The meteors should be looked for after 1 a.m. during the first week in May, and may possibly show an enhanced display in May, 1910, when Halley's comet will be near the sun and earth.1

On November 29, 1905, Sir David Gill observed a fireball with an apparent diameter equal to that of the moon, which remained visible for 5 minutes and disappeared in a hazy sky. Observed from another place, Mr. Fuller found that the meteor was visible 2 hours later! Sir David Gill stated that he does not know of any similar phenomenon. 2

Mr. Denning finds that swiftly moving meteors become visible at a greater height above the earth's surface than the slower ones. Thus, for the Leonids and Perseicls, which are both swift,

1 Nature, April 30, 1908.

2 Bulletin, AsL Soc. de France, Mny, 190G.

118 ASTRONOMICAL CURIOSITIES

it has been found that the Leonids appear at an average height of 84 miles, and disappear at a height of 56 miles ; and the Perseids at 80 and 54 miles respectively. " On the other hand, the mean height of the very slow meteors average about 65 miles at the beginning and 38 miles at the end of their appearance." 2

During the night of July 21-22, 1896, Mr. William Brooks, the well-known, astronomer, and director of the Smith Observatory at Geneva (New York), saw a round dark body pass slowly across the moon's bright disc, the moon being nearly full at the time. The apparent diameter of the object was about one minute of arc, and the duration of the transit 3 or 4 seconds, the direction of motion being from east to west. On August 22 of the same year, Mr Gathman (an American observer) saw a meteor crossing the sun's disc, the transit lasting about 8 seconds.2

A meteor which appeared in Italy on July 7, 1892, was shown by Prof, von Niessl to have had an ascending path towards the latter end of its course ! The length of its path was computed to be 683 miles. When first seen, its height above the earth was about 42 miles, and when it dis- appeared its height had increased to about 98 miles, showing that its motion was directed upwards ! 3

1 Nature, November 24, 1904. - Ibid., September 10, 189C.

3 Ibi<l, June 29, 18IKJ.

METEORS 119

In the case of the fall of meteoric stones, which occasionally occur, it has sometimes been noticed that the sound caused by the explosion of the meteorite, or its passage through the air, is heard before the meteorite is seen to fall. This has been explained by the fact that owing to the resistance of the air to a body moving at first with a high velocity its speed is so reduced that it strikes the earth with a velocity less than that of sound. Hence the sound reaches the earth before the body strikes the ground.1

The largest meteoric stone preserved in a museum is that known as the Anighita, which weighs 36^ tons, and was found at 'Cape York in Greenland. It was brought to the American Museum of Natural History by Commander R. E. Peary, the Arctic explorer.

The second largest known is that of Bacubirito in Mexico, the weight of which is estimated at 27J tons.

The third largest is that known as the Willianiette, which was found in 1902 near the town of that name in Western Oregon (U.S.A.). It is composed of metallic nickel-iron, and weighs about 13^ tons. It is now in the American Museum of Natural History.

A large meteorite was actually seen, from the deck of the steamer African Prince, to fall into the Atlantic Ocean, on October 7, 1906! The captain 1 Journal, B.A.A., May 22, 1903.

120 ASTRONOMICAL CURIOSITIES

of the vessel, Captain Anderson, describes it as having a train of light resembling " an immense broad electric-coloured band, gradually turning to orange, and then to the colour of molten metal. When the meteor came into the denser atmosphere close to the earth, it appeared, as nearly as is possible to describe it, like a molten mass of metal being poured out. It entered the water with a hissing noise close to the ship." l This was a very curious and perhaps unique phenomenon, and it would seem that the vessel had a narrow escape from destruction.

In Central Arizona (U.S.A.) there is a hill called Coon Butte, or Coon Mountain. This so-called " mountain " rises to a height of only 130 to 160 feet above the surrounding plain, and has on its top a crater of 530 to 560 feet deep ; the bottom of the crater — which is dry — being thus 400 feet below the level of the surrounding country. This so-called " crater " is almost circular and nearly three-quarters of a mile in diameter. It has been suggested that this " crater " was formed by the fall of an enormous iron meteorite, or small asteroid. The " crater " has been carefully examined by a geologist and a physicist. From the evidence and facts found, the geologist (Mr. Barringer) states that " they do not leave, in my mind, a scintilla of doubt that this moun- tain and its crater were produced by the impact 1 Nature, December 13, 1906, p. 159.

METEORS 121

of a huge meteorite or small asteroid." The physicist (Mr. Tilglmiann) says that he "is justified, under due reserve as to subsequently developed facts, in announcing that the formation at this locality is due to the impact of a meteor of enormous and unprecedented size." There are numerous masses of meteoric iron in the vicinity of the " crater." The so-called Canyon Diabolo meteorite was found in a canyon of that name about 2J miles from the Coon Mountain. The investigators estimate that the great meteoric fall took place " not more than 5000 years ago, perhaps much less." Cedar trees about 700 years old are now growing 011 the rim of the mountain. From the results of artillery experiments, Mr. Gilbert finds that " a spherical projectile striking solid limestone with a velocity of 1800 feet a second will penetrate to a depth of something less than two diameters,' and from this Mr. L. Fletcher concludes "that a meteorite of large size would not be prevented by the earth's atmosphere from having a penetration effect sufficient for the production of such a crater." *

The meteoric origin of this remarkable " crater " is strongly favoured by Mr. G. P. Merrill, Head Curator of Geology, U.S. National Museum.

The Canyon Diabolo meteorite above referred to was found to contain diamonds ! some black, others transparent. So some have said that " the 1 Nature, September 13, 190G.

ASTRONOMICAL CURIOSITIES

diamond is a gift from Heaven," conveyed to earth in meteoric showers.1 But diamond- bearing meteorites would seem to be rather a freak of nature. It does not follow that all diamonds had their origin in meteoric stones. The mineral known as periodot is frequently found in meteoric stones, but it is also a constituent of terrestrial rocks.

In the year 1882 it was stated by Dr. Halm and Dr. Weinhand that they had found fossil sponges, corals, and crinoids in meteoric stones ! Dr. Wein- hand thought he had actually determined three genera ! 2 But this startling result was flatly contradicted by Carl Vogt, who stated that the supposed fossils are merely crystalline conforma- tions.3

Some meteorites contain a large quantity of occluded gases, hydrogen, helium, and carbon oxides. It is stated that Dr. Odling once " lighted up the theatre of .the Royal Institution with gas brought down from interstellar space by meteorites " 1 4

On February 10, 1896, a large meteorite burst over Madrid with a loud report. The concussion was so great that many windows in the city were broken, and some partitions in houses were shaken down ! 5

1 Nature, October 12, 1905, p. 596.

2 Knowledge, January 13, 1882. 3 Ibid., January 20, 1882, 4 Popular Astronomy, June- July, 1908, p. 345.

4 The Observatory, March, 189C, p. 135.

METEORS

A very brilliant meteor or fireball was seen in daylight on June 9, 1900, at 2h 55ra p.m. from various places in Surrey, Sussex, and near London. Calculations showed that " the meteor began 59 miles in height over a point 10 miles east of Valognes, near Cherbourg, France. Meteor ended 23 miles in height, over Calais, France. Length of path 175 miles. Radiant point, 280°, 12°." l

It was decided some years ago " in the American Supreme Court that a meteorite, though a stone fallen from heaven, belongs to the owner of the freehold interest in the land 011 which it falls, and not to the tenant." 2

With reference to the fall of meteoric matter on the earth, Mr. Proctor says, " It is calculated by Dr. Kleiber of St. Petersburg!! that 4250 Ibs. of meteoric dust fall on the earth every hour — that is, 59 tons a day, and more than 11,435 tons a year. I believe this to be considerably short of the truth. It sounds like a large annual growth, and the downfall of such an enormous mass of meteoric matter seems suggestive of some degree of danger. But in reality, Dr. Kleiber' s estimate gives only about 25 millions of pounds annually, which is less than 2 ounces annually to each square mile of the earth's surface," 3 a quantity which is, of course, quite insignificant.

1 The Observatory, February, 1900, pp. 106-7. • Knowledge, March, 1893, p. 51. 3 llnil, July 3, 1835, p. 11.

124 ASTRONOMICAL CURIOSITIES

According to Humboldt, Chladiii states that a Franciscan monk was killed by the fall of an aerolite at Milan in the year 1660.1 Humboldt also mentions the death by meteoric stones of a monk at Crema on September 4, 1511, and two Swedish sailors on board ship in 1674.2

It is a curious fact that, according to Olbers, " no fossil meteoric stones " have ever been dis- covered.3 Considering the number which are supposed to have fallen to the earth in the course of ages, this fact seems a remarkable one.

On May 10, 1879, a shower of meteorites fell at Eitherville, Iowa (U.S.A.). Some of the fragments found weighed 437, 170, 92^, 28, 10J, 4 and 2 Ibs. in weight. In the following year (1880) when the prairie grass had been consumed by a fire, about " 5000 pieces were found from the size of a pin to a pound in weight." 4

According to Prof. Silvestria of Catania, a shower of meteoric dust mixed with rain fell on the night of March 29, 1880. The dust contained a large proportion of iron in the metallic state. In size the particles varied from a tenth to a hundredth of a millimetre.5

It is sometimes stated that the average mass of a " shooting star " is only a few grains. But from

1 Cosmos, vol. i. p. 108 (Otte's translation).

2 Ibid., vol. i. p. 124.

3 Ibid., vol. i. p. 119, footnote.

4 Copernicus, vol. i. p. 72.

5 Ibid.

METEORS 125

comparisons with an electric arc light, Prof. W. II. Pickering concludes that a meteor as bright as a third magnitude star, composed of iron or stone, would probably have a diameter of 6 or 7 inches. An average bright fireball would perhaps measure 5 or 6 feet in diameter.1

In the Book of Joshua we are told "that the LORD cast down great stones from heaven upon them unto Azekah, and they died " (Joshua x. 11). In the latter portion of the verse " hailstones " are mentioned, but as the original Hebrew word means stones in general (not hailstones), it seems very probable that the stones referred to were aerolites.2

The stone mentioned in the Acts of the Apostles, from which was found " the image which fell down from Jupiter " (Acts xix. 35), was evidently a meteoric stone.2

The famous stone in the Caaba at Mecca, is probably a stone of meteoric origin.3

I

" Stones from Heaven ! Can you wonder,

You who scrutinize the Earth, At the love and veneration

They received before the birth Of our scientific methods ?

II

" Stones from Heaven ! we can handle

Fragments fallen from realms of Space ;

1 Astrophysical Journal, June, 1909, pp. 378-9.

2 Knowledge, July, 1909, p. 264.

126 ASTRONOMICAL CURIOSITIES

Oh ! the marvel and the mystery,

Could we understand their place In the scheme of things created !

Ill

" Stones from Heaven ! With a mighty

Comet whirling formed they part ? Fell they from their lofty station

Like a brilliant fiery dart, Hurl'd from starry fields of Night?" »

1 Quoted by Miss Irene E. T. Warner in Knowledge, July, 19CW, p. 261.

CHAPTER XIII

The Zodiacal Light and Gegenschein

ACCORDING to Gruson and Brugsch, the Zodiacal Light was known in ancient times, and was even worshipped by the Egyptians. Strabo does not mention it; but Diodorus Siculus seems to refer to it (B.C. 373), and he probably obtained his information from some Greek writers before his time, possibly from Zenophon, who lived in the sixth century B.C.1 Coming to the Christian era, it was noticed by Nicephorus, about 410 B.C. In the Koran, it is called the " false Aurora " ; and it is supposed to be referred to in the " Rubaiyat " of Omar Khayyam, the Persian astronomical poet, in the second stanza of that poem (Edward Fitzgerald's translation) —

" Dreaming when Dawn's Left Hand was in the Sky,2 I heard a voice within the Tavern cry, Awake, my Little ones, and fill the Cup, Before Life's Liquor in its Cup be dry."

It was observed by Cassini in 1668,* and by

1 The Observatory, November, 1900.

2 Or, " Before the phantom of false morning died " (4th edition) ; The Observatory, September, 1905, p. 356.

3 The Observatory, July, 1896, p. 274.

128 ASTRONOMICAL CURIOSITIES

Hooke in 1705. A short description of its appear- ance will be found in; Childrey's Britannia Baconica (1661), p. 183.

The finest displays of this curious light seem to occur between the middle of January and the middle of February. In February, 1856, Secchi found it brighter than he had ever seen it before. It was yellowish towards the axis of the cone, and it seemed to be brighter than the Milky Way in Cygnus. He described it as " un graiide spectacle." In the middle of February, 1866, Mr. Lassell, during his last residence in Malta, saw a remarkable display of the Zodiacal Light. He found it at least twice as bright as the brightest part of the Milky Way, and much brighter than he had previously seen it. He found that the character of its light differed considerably from that of the Milky Way. It was of a much redder hue than the Galaxy. In 1874 very remarkable displays were seen in the neighbourhood of London in January and February of that year ; and in 1875 on January 24, 25, and 30. On January 24 it was noticed that the "light" was distinctly reddish and much excelled in brightness any portion of the Milky Way.

Humboldt, who observed it from Andes (at a height of 13,000 to 15,000 feet), from Venezuela and from Cumana, tells us that he has seen the Zodiacal Light equal in brightness to the Milky Way in Sagittarius.

THE ZODIACAL LIGHT 129

As probably many people have never seen the " light," a caution may be given to those who care to look for it. It is defined by the Rev. George Jones, Chaplain to the " United States' Japan Expedition" (1853-55), as "a brightness that appears in the western sky after sunset, and in the east before sunrise ; following nearly or quite the line of the ecliptic in the heavens, and stretching upwards to various elevations according to the season of the year." From the description some might suppose that the light is visible immediately after sunset. But this is not so ; it never appears until twilight is over and " the night has fully set in."

The " light " is usually seen after sunset or before sunrise. But attempts have recently been made by Prof. Simon Newcomb to observe it north of the sun. To avoid the effects of twilight the sun must be only slightly more than 18° below the horizon (that is, a little before or after the longest day). This condition limits the place of observation to latitudes not much south of 46°; and to reduce atmospheric absorption the observing station should be as high as possible above the level of the sea. Prof. Newcomb, observing from the Brienzer Rothorn in Switzerland (latitude 46° 47' N., longitude 8° 3' E.), succeeded in tracing the "light" to a distance of 35° north of the sun. It would seem, therefore, that the Zodiacal Light envelops the sun on all

K

130 ASTRONOMICAL CURIOSITIES

sides, but has a greater extension in the direction of the ecliptic.1 From observations at the Lick Observatory, Mr. E. A. Fath found an extension of 46° north of the sun.2

From observations of the "light" made by Prof. Barnard at the Yerkes Observatory during the summer of 1906, he finds that it extends to at least 65° north of the sun, a considerably higher value than that found by Prof. Newcomb.3 The difference may perhaps be explained by actual variation of the meteoric matter producing the light. Prof. J. H. Poynting thinks that possibly the Zodiacal Light is due to the "dust of long dead comets." 4

From careful observations of the "light," Mr. Gavin J. Burns finds that its luminosity is " some 40 or 50 per cent, brighter than the background of the sky. Prof. Newcomb has made a precisely similar remark about the luminosity of the Milky Way, viz. that it is surprisingly small." This agrees with my own observations during many years. It is only on the finest and clearest nights that the Milky Way forms a conspicuous object in the night sky. And this only in the country. The lights of a city almost entirely obliterate it. Mr. Burns finds that the Zodiacal Light

1 Journal, B.A.A., January 24, 1906.

- Ast. Soc. of the Pacific, December, 1908, p. 280.

3 Nature, November 1, 1906.

4 Ibid., November 22, 1906, p. 93.

THE ZODIACAL LIGHT 131

appears " to be of a yellowish tint ; or if we call it white, then the Milky Way is comparatively of a bluish tint." During my residence in the Punjab the Zodiacal Light seemed to me constantly visible in the evening sky in the spring months. In the west of Ireland I have seen it nearly as bright as the brightest portions of the Milky Way visible in this country (February 20, 1890). The " meteoric theory " of the " light " seems to be the one now generally accepted by astronomers, and in this opinion I fully concur.

From observations made in Jamaica in the years 1899 and 1901, Mr. Maxwell Hall arrived at the conclusion that " the Zodiacal Light is caused by reflection of sunlight from masses of meteoric matter still contained in the invariable plane, which may be considered the original plane of the solar system." l According to Humboldt, Cassini believed that the Zodiacal Light "consisted of innumerably small planetary bodies revolving round the sun." 2

THE GEGENSCHEIN, or COUNTER-GLOW. — This is a faint patch of light seen opposite the sun's place in the sky, that is on the meridian at mid- night. It is usually elliptical in shape, with its longer axis lying nearly in the plane of the ecliptic. It seems to have been first detected by Brorsen (the discoverer of the short-period comet

1 Nature, August 30, 1906.

- Cosmos, vol. i. p. 131, footnote.

132 ASTRONOMICAL CURIOSITIES

of 1846) about the middle of the nineteenth century. But it is not easy to see, for the famous Heis of Munster, who had very keen eyesight, did not succeed in seeing it for several years after Brorsen's announcement.1 It was afterwards independently discovered by Backhouse, and Barnard.

Prof. Barnard's earlier observations seemed to show that the Gegenschein does not lie exactly opposite to the sun, but very nearly so. He found its longitude is within one degree of 180°, and its latitude about 1°'3 north of the ecliptic.2 But from subsequent observations he came to the conclusion that the differences in longitude and apparent latitude are due to atmospheric absorp- tion, and that the object really lies in the ecliptic and exactly opposite to the sun.3

Barnard finds that the Gegenschein is not so faint as is generally supposed. He says " it is best seen by averted vision, the face being turned 60° or 70° to the right or left, and the eyes alone turned towards it." It is invisible in June and December, while in September it is round, with a diameter of 20°, and very distinct. No satis- factory theory has yet been advanced to account for this curious phenomenon. Prof. Arthur Searle of Harvard attributes it to a number of

1 Nature, December 16, 1875.

2 Ibid., July 23, 1891.

3 Bulletin, Ast. Soc. de France, April, 1903.

THE ZODIACAL LIGHT 133

asteroids too small to be seen individually. When in " opposition " to the sun these would be fully illuminated and nearest to the earth. Its distance from the earth probably exceeds that of the moon. Dr. Johnson Stoney thinks that the Gegenschein may possibly be due to a "tail" of hydrogen and helium gases repelled from the earth by solar action ; this " tail " being visible to us by reflected sunlight.

It was observed under favourable circumstances in January and February, 1903, by the French astronomer, M. F. Quenisset. He found that it was better seen when the atmosphere was less clear, contrary to his experience of the Zodiacal Light. Prof. Barnard's experience confirms this. M. Quenisset notes that — as in the case of the Zodiacal Light — the southern border of the Gegenschein is sharper than the northern. He found that its brightness is less than that of the Milky Way between Betelgeuse and y Geminorum ; and thinks that it is merely a strengthening of the Zodiacal Light.1

A meteoritic theory of the Gegenschein has been advanced by Prof. F. R. Moulton, which explains it by light reflected from a swarm of meteorites revolving round the sun at a distance of 930,240 miles outside the earth's orbit.

Both the Zodiacal Light and Gegenschein were observed by Herr Leo Brenner on the evening of 1 Bulletin, Ast. Soc. de France, April, 1903.

134 ASTRONOMICAL CURIOSITIES

March 4, 1896. He found the Zodiacal Light on this evening to be "perhaps eight times brighter than the Milky Way in Perseus." The " Gegen- schein distinctly visible as a round, bright, cloud- like nebula below Leo (Virgo), and about twice the brightness of the Milky Way in Monoceros between Canis Major and Canis Minor." *

Humboldt thought that the fluctuations in the brilliancy of the Zodiacal Light were probably due to a real variation in the intensity of the phenomenon rather than to the elevated position of the observer.2 He says that he was " astonished in the tropical climates of South America, to observe the variable intensity of the light."

1 The Observatory, May, 1896. The italics are Brenner's.

2 Cosmos, vol. iv. p. 563.

CHAPTER XIV

The Stars

PLINY says that Hipparchus " ventured to count the stars, a work arduous even for the Deity." But this was quite a mis- taken idea. Those visible to the naked eye are comparatively few in number, and the enumera- tion of those visible in an opera-glass — which of course far exceed those which can be seen by un- aided vision — is a matter of no great difficulty. Those visible in a small telescope of 2| inches aperture have all been observed and catalogued ; and even those shown on photographs taken with large telescopes can be easily counted. The pre- sent writer has made an attempt in this direction, and taking an average of a large number of counts in various parts of the sky, as shown on stellar photographs, he finds a total of about 64 millions for the whole sky in both hemispheres.1 Prob- ably the total number will not exceed 100 millions. But this is a comparatively small

1 For details of this enumeration, see Astronomical Essays, p. 222.

136 ASTRONOMICAL CURIOSITIES

number, even when compared with the human population of our little globe.

With reference to the charts made by photo- graphy in the International scheme commenced some years ago, it has now been estimated that the charts will probably contain a total of about 9,854,000 stars down to about the 14th magnitude (13-7). The "catalogue plates" (taken with a shorter exposure) will, it is expected, include about 2,676,500 stars down to 11J magnitude. These numbers may, however, be somewhat increased when the work has been completed.1 If this estimate proves to be correct, the number of stars visible down to the 14th magnitude will be con- siderably less than former estimates have made it.

Prof. E. C. Pickering estimates that the total number of stars visible on photographs down to the 16th magnitude (about the faintest visible in the great Lick telescope) will be about 50 millions.2 In the present writer's enumeration, above referred to, many stars fainter than the 16th magnitude were included.

Admiral Smyth says, with reference to Sir William Herschel — perhaps the greatest observer that ever lived — "As to Sir William himself, he could unhesitatingly call every star down to the 6th magnitude, by its name, letter, or number." 3

1 Nature, June 11, 1908.

2 Popular Astronomy, vol. U (190G), p. 510.

3 Bedford Catalogue, p. 532.

THE STARS 137

This shows great powers of observation, and a wonderful memory.

On a photographic plate of the Pleiades taken with the Bruce telescope and an exposure of 6 hours, Prof. Bailey of Harvard has counted " 3972 stars within an area 2° square, having Alcyone at its centre." l This would give a total of about 41 millions for the whole sky, if of the same richness.

With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less that 110' square a total of 4621 stars. He thinks, " That all of these stars belong to the Pleiades group is not at all probable. The great majority of them probably lie at immense distances beyond the group, and simply appear in it by projection." l He adds, " It has been found, however, by very careful measurements made during the last 75 years at the Konigsbergh and Yale Observatories, that of the sixty-nine brighter stars, including those down to the 9th magnitude, only eight show any certain movement with reference to Alcyone. Since Alcyone has a proper motion or drift of 6" per century, this means that all the brightest stars except the eight mentioned are drifting with Alycone and so form a true cluster, at approximately the same distance from the earth. Six of the eight stars which show relative drift are moving in the opposite direction to the 1 Popular Astronomy, Yol. 15 (1907), p. 194.

138 ASTRONOMICAL CURIOSITIES

movement of Alycone, and at nearly the same rate, so that their motion is only apparent. They are really stationary, while Alycone and the rest of the cluster are moving past them." l This tends to show that the faint stars are really behind the cluster, and are unconnected with it.

It is a popular idea with some people that the Pole Star is the nearest of all the stars to the celestial pole. But photographs show that there are many faint stars nearer to the pole than the Pole Star. The Pole Star is at present at a distance of 1° 13' from the real pole of the heavens, but it is slowly approaching it. The minimum distance will be reached in the year 2104. Prom photographs taken by M. Flammarion at the Juvisy Observatory, he finds that there are at least 128 stars nearer to the pole than the Pole Star ! The nearest star to the pole was, in the year 1902, a small star of about 12J magni- tude, which was distant about 4 minutes of arc from the pole.2 The estimated magnitude shows that the Pole Star is nearly 10,000 times brighter than this faint star !

It has been found that Sirius is bright enough to cast a shadow under favourable conditions. On March 22, 1903, the distinguished French astronomer Touchet succeeded in photographing

1 Popular Astronomy, vol. 15 (1907), p. 195.

2 Bulletin, Ast. Soc, de France, February, 1903.

THE STARS 139

the shadow of a brooch cast by this brilliant star. The exposure was lh 5m.

Martinus Hortensius seems to have been the first to see stars in daylight, perhaps early in the seventeenth century. He mentions the fact in a letter to Gassendi dated October 12, 1636, but does not give the date of his observation. Schickard saw Arcturus in broad daylight early in 1632. Morin saw the same bright star half an hour after sunset in March, 1635.

Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and trans- parent. " Many more stars were visible at a glance, and the familiar stars appeared more brilliant. ... In the great bright cloud of the Milky Way, between /? and y Cygni, one could count easily sixteen or seventeen stars, besides the bright ones TJ and x> while at Northfield it is difficult to distinctly w see eight or nine with the naked eye." Some nebulae and star fields were photographed with good results by the aid of a 2J-inch Darlot lens and 3 hours' exposure.2

Prof. Barnard has taken some good stellar photo- graphs with a lens of only 1J inches in diameter,

1 Here x i3 probably 17 Cygni, x being the famous variable near it.

Popular Astronomy, vol. 13 (1904), p. 509.

HO ASTRONOMICAL CURIOSITIES

and 4 or 5 inches focus belonging to an ordinary " magic lantern " ! He says that these " photo- graphs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means." l

With reference to the rising and setting of the stars due to the earth's rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, "I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it ! " 2

Admiral Smyth says, " I have been struck with the beautiful blue tint of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity." This bluish colour of small stars agrees with the conclusion arrived at by Prof. Pickering in recent years, that the majority of faint stars in the Milky Way have spectra of the Sirian type and, like that brilliant star, are of a bluish white colour. Sir William Herschel saw many stars of a redder tinge than other observers have noticed. Admiral Smyth

1 Astrophy steal Journal, December, 1895.

2 The Observatory, July, 1895, p. 290.

THE STARS 141

says, "This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both." l

The ancient astronomers do not mention any coloured stars except white and red. Among the latter they only speak of Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds Alphard (a Hydrse).

Sir William Herschel remarked that no decidedly green or blue star "has ever been noticed un- associated with a companion brighter than itself." An exception to Herschel's rule seems to be found in the case of the star ft Librae, which Admiral Smyth called " pale emerald." Mr. George Kiiott observed it on May 19, 1852, as " beautiful pale green " (3*7 inches achromatic, power 80), and on May 9, 1872, as "fine pale green" (5*5 inches achromatic, power 65).

The motion of stars in the line of sight, as shown by the spectroscope — should theoretically alter their brightness in the course of time ; those ap- proaching the earth becoming gradually brighter, while those receding should become fainter. But the distance of the stars is so enormous that even with very high velocities the change would not become perceptible for ages. Prof. Oudemans found that to change the brightness of a star by only one-tenth of a magnitude — a quantity barely ? Celestial Cycle, p. 302.

142 ASTRONOMICAL CURIOSITIES

perceptible to the eye — a number of years would be necessary, which is represented by the formula

5916 years parallax X motion

for a star approaching the earth, and for a reced- ing star

6195 years p X m

This is in geographical miles, 1 geographical mile being equal to 4*61 English miles.

Reducing the above to English miles, and taking an average for both approaching and receding stars, we have

27,660 years p X m

where p = parallax in seconds '\ of arc, and m = radial velocity in English miles per second.

Prof. Oudemans found that the only star which could have changed in brightness by one-tenth of a magnitude since the time of Hipparchus is Aldebaran. This is taking its parallax as 0"*52. But assuming the more reliable parallax 0"*12 found by Dr. Elkin, this period is 4J times longer. For Procyon, the period would be 5500 years.1 The above calculation shows how absurd it is to suppose that any star could have gained or lost in brightness by motion in the line of sight during historical times. The " secular variation " of stars 1 Nature, December 13, 1894.

THE STARS 143

is quite another thing. This is due to physical changes in the stars themselves.

The famous astronomer Halley, the second Astronomer Royal at Greenwich, says (Phil. Trans., 1796), " Supposing the number of 1st magnitude stars to be 13, at twice the distance from the sun there may be placed four times as many, or 52 ; which with the same allowance would nearly represent the star we find to be of the 2nd magnitude. So 9 x 13, or 117, for those at three times the distance; and at ten times the distance 100 X 13, or 1300 stars ; of which distance may probably diminish the light of any of the stars of the 1st magnitude to that of the 6th, it being but the hundredth part of what, at their present distance, they appear with." This agrees with the now generally accepted " light ratio " of 2'512 for each magnitude, which makes a first magnitude star 100 times the light of a 6th magnitude.

On the 4th of March, 1796,1 the famous French astronomer Lalande observed on the meridian a star of small 6th magnitude, the exact position of which he determined. On the 15th of the same month he again observed the star, and the places found for 1800 refer to numbers 16292-3 of the reduced catalogue. In the observation of March 4 he attached the curious remark, " Etoile singu- liere " (the observation of March 15 is without 1 Hietoire Celeste, p, 211.

144 ASTRONOMICAL CURIOSITIES

note). This remark of Lalande has puzzled observers who failed to find any peculiarity about the star. Indeed, " the remark is a strange one for the observer of so many thousands of stars to attach unless there was really something singular in the star's aspect at the time." On the evening of April 18, 1887, the star was examined by the present writer, and the following is the record in his observing book, " Lalande's etoile singuliere (16292-3) about half a magnitude less than rj Cancri. With the binocular I see two streams of small stars branching out from it, north preceding like the tails of comet." This may perhaps have something to do with Lalande's curious remark.

The star numbered 1647 in Baily's Flamsteed Catalogue is now known to have been an obser- vation of the planet Uranus.1

Prof. Pickering states that the fainter stars photographed with the 8-inch telescope at Cam- bridge (U.S.A.) are invisible to the eye in the 15-inch telescope.2

Sir Norman Lockyer finds that the lines of sulphur are present in the spectrum of the bright star Rigel (/? Orionis). 3

About 8J° south of the bright star Regulus (a Leonis) is a faint nebula (H I, 4 Sextaiitis). On or near this spot the Capuchin monk De Rheita fancied he saw, in the year 1643, a

1 Nature, October, 1887. - Ibid., August 29, 1889.

3 Science Abstracts, February 25, 1908, pp. 82, 83.

THE STARS 145

group of stars representing the napkin of S. Veronica — "sudarium Veronicas sive faciem Domini maxima similitudina in astris expressum." And he gave a picture of the napkin and star group. But all subsequent observers have failed to find any trace of the star group referred to by De Rheita ! l

The Bible story of the star of the Magi is also told in connection with the birth of the sun-gods Osiris, Horns, Mithra, Serapis, etc.2 The present writer has also heard it suggested that the phenomenon may have been an apparition of Halley's comet! But as this famous comet is known to have appeared in the year B.C. 11, and as the date of the Nativity was probably not earlier than B.C. 5, the hypothesis seems for this (and other reasons) to be inadmissible. It has also been suggested that the phenomenon might have been an appearance of Tycho Brahe's temporary star of 1572, known as the " Pilgrim star " ; but there seems to be no real foundation for such an hypothesis. There is no reason to think that " temporary " or new stars ever appear a second time.

Admiral Smyth has well said, " It checks one's pride to recollect that if our sun with the whole system of planets, asteroids, and moons, and comets were to be removed from the spectator

1 Bedford Catalogue, pp. 227-8,

2 Knowledge, February 1, 1888.

146 ASTRONOMICAL CURIOSITIES

to the distance of the nearest fixed star, not one of them would be visible, except the sun, which would then appear but as a star of perhaps the 2nd magnitude. Nay, more, were the whole system of which our globe forms an insignificant member, with its central luminary, suddenly annihilated, no effect would be produced on those unconnected and remote bodies; and the only annunciation of such a catastrophe in the Sidereal "Times" would be that a small star once seen in a distant quarter of the sky had ceased to shine." l

Prof. George C. Comstock finds that the average parallax of 67 selected stars ranging in brightness between the 9th and the 12th magnitude, is of the value of 0"*0051.2 This gives a distance repre- senting a journey for light of about 639 years !

Mr. Henry Norris Russell thinks that nearly all the bright stars in the constellation of Orion are practically at the same distance from the earth. His reasons for this opinion are : (1) the stars are similar in their spectra and proper motions, (2) their proper motions are small, which suggests a small parallax, and therefore a great distance from the earth. Mr. Russell thinks that the average parallax of these stars may perhaps be 0"-005, which gives a distance of about 650 " light years." 3

1 Celestial Cycle, p. 280. * Popular Astronomy, February, 1904. 3 Ibid., vol. 15 (1907), p. 444.

THE STARS 147

According to Sir Norman Lockyer's classifica- tion of the stars, the order of increasing tempera- ture is represented by the following, beginning with those in the earliest stage of stellar evo- lution : — Nebulae, Antares, Aldebaran, Polaris, a Cygni, Rigel, c Tauri, ft Crucis. Then we have the hottest stars represented by e Puppis, y Argus, and Alnitam (c Orionis). Decreasing temperature is represented by (in order), Achernar, Algol, Markab, Sirius, Procyon, Arcturus, 19 Piscium, and the "Dark Stars."1 But other astronomers do not agree with this classification. Antares and Aldebaran are by some authorities considered to be cooling suns.

According to Ritter's views of the Constitution of the Celestial Bodies, if we "divide the stars into three classes according to age corresponding to these three stages of development, we shall assign to the first class, A, those stars still in the nebular phase of development ; to the second class, B, those in the transient stage of greatest brilliancy ; and to the class C, those stars which have already entered into the long period of slow extinction. It should be noted in this classifica- tion that we refer to relative and not absolute age, since a star of slight mass passes through the