Astronomical Curiosities: Facts and Fallacies

PREFACE

The curious facts, fallacies, and paradoxes contained 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.

September, 1909.

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 26·83, 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 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 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.4 The assumption of the sun being “a black body” seems a curious paradox; but the simple meaning of the statement is that the sun is assumed to act as a radiator as if it were a perfectly black body heated to the high temperature given above.

According to Prof. Langley, the sun’s photosphere is 5000 times brighter than the molten metal in a “Bessemer convertor.”5

Observations of the sun even with small telescopes and protected by dark glasses are very dangerous to the eyesight. Galileo blinded himself 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.

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.”6

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.”7 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.8

The discovery of sun-spots was claimed by Hariotte, in 1610, and by Galileo, Fabricius, and Scheiner, in 1611. The latter wrote 800 pages on them, and thought they were small planets revolving round the sun! This idea was also held by Tardè, who called them Astra Borbonia, and by 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 that of the earth’s surface. The most curious thing about this spot was that it appeared near a minimum of the sun-spot cycle! and was therefore rather an anomalous phenomenon. It was suggested by the late Daniel Kirkwood that this great spot was caused 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 appearance 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.9 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 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 (= 450⁄5)2, or 8,100 times as much heat as would be produced by its combustion. If the sun were formed of coal it would be consumed 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 × 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 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.”10

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 constitute the chief source of the solar heat.11

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 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 convulsions 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.”12

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: —

CHAPTER II

Mercury

As 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 —

Tycho Brahé, however, records several observations 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 appearance was that of a reddish star of the first magnitude. On November 29 I compared its brilliancy 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 1905.14

He found that the duration of visibility after sunset is about 1h 40m when seen in March, 1h 30m in April, and 1h 20m in May. He thinks that the planet is, at its brightest, “certainly much brighter than a first magnitude star.”15 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 brightness,” as it is called) of Venus was at least twice as great as that of Mercury; and Zöllner found that from a photometric point of view the surface of Mercury is comparable with that of the moon.

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 terrestrial 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 quicksilver.”16

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 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.”17

Schröter (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!18 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 one-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.”19

M. Léo Brenner thinks that he has seen the 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.20

Mercury is said to have been occulted by Venus in the year 1737.21 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 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 in the orbit of Mercury.”22 Prof. Newcomb, however, 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 2·0000001574.23

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 fé.”24

An occultation of Mercury by Venus was observed with a telescope on May 17, 1737.25

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 1940.26

From measurements of the disc of Mercury during the last transit, M. R. Jonckheere concludes that the polar diameter of the planet is greater than the equatorial! His result, which is very curious, if true, seems to be supported by the observations of other observers.27

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 1896 to 1909, Mr. John McHarg finds a period of 1·0121162 day, or 1d 0h 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.”28

CHAPTER III

Venus

Venus was naturally – owing to its brightness – 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.”29 But whatever “the star 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.30 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.31 Mr. A. Cameron, of Yarmouth, Nova Scotia, has, however, seen Venus with the naked eye three days before conjunction when the planet was only 6¼° from the sun.32 This seems a remarkable observation, and shows that the observer’s eyesight 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 on March 10, A.D. 1002.33

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 refraction 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.34

Some of the earlier observers, such as La Hire, Fontana, Cassini, and Schröter, thought they saw evidence of mountains on Venus. Schröter 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. Schröter defended himself, and was supported by Beer and Mädler, the famous lunar observers. Several modern observers seem to confirm Schröter’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 Bianchini, 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 Schröter.35