Astronomical Curiosities: Facts and Fallacies

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 - Nature, October 30, 1902.] 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 - Charles Lane Poor, The Solar System, p. 170.]

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 - Smyth, Celestial Cycle, p. 60.]

An occultation of Mercury by Venus was observed with a telescope on May 17, 1737.[25 - Denning, Telescopic Work for Starlight Evenings, p. 225.]

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 - The Observatory, 1894, p. 395.]

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 - Ast. Nach. 4333, quoted in Nature, July 1, 1909, p. 20.]

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 - English Mechanic, July 23, 1909.]

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 - Nature, December 22, 1892.] 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 - Celestial Objects, vol. i. p. 52, footnote.] 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 - Ibid., p. 54.] 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 - Astronomy and Astrophysics, 1892, p. 618.] 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 - Nature, August 7, 1879.]

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 - The World of Space, p. 56.]

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 - Nature, September 15, 1892.]

A white spot near the planet’s south pole was seen on several occasions by H. C. Russell in May and June, 1876.[36 - Observatory, 1880, p. 574.]

Photographs of Venus taken on March 18 and April 29, 1908, by M. Quénisset at the Observatory of Juvissy, France, show a white polar spot. The 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 confidence 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), Schröter (1788-93), Mädler (1836), De Vico (1840?) Trouvelot (1871-79), Flammarion, Léo 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 observations 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, inasmuch as the absorption of its immense atmosphere certainly prevents any detail on its surface from being perceived.”[37 - Knowledge, November 1, 1897, pp. 260, 261.]

The eminent Swedish physicist Arrhenius thinks, however, that the dense atmosphere and clouds of Venus are in favour of a rapid rotation on its axis.[38 - Worlds in the Making, p. 61.] 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.”[39 - Ibid., p. 48.]

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.[40 - Nature, June 1, 1876.] The same effect was also noted by Webb.[41 - Cel. Objects, vol. i. p. 66 (5th Edition).] 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 observation made by Andreas Mayer, Professor of Mathematics at Griefswald 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 Hahn 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.”

Schröter of Lilienthal – the famous observer of the moon – saw the horns of the crescent of Venus extended many degrees beyond the semicircle 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 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 Göttingen, 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.”[42 - Celestial Objects, vol. i. p. 65 (5th Edition).]

Captain Noble’s observation was rather unique. He found that the dark side was “always distinctly and positively darker than the background upon which it is projected.”

The “light” was also seen by Lyman in America in 1867, and by Safarik at Prague. In 1871 the whole disc of Venus was seen by Professor Winnecke.[43 - Ast. Nach. No. 1863.] On the other hand, Winnecke stated that he only saw it twice in 24 years; and the great observers Dawes and Mädler never saw it at all![44 - Nature, June 1, 1876.]

Various attempts have been made to explain the visibility – at times – of the “dark side” of Venus. The following may be mentioned[45 - Ibid., June 8, 1876.]: – (1) Reflected earth-light, analogous to the dark side of the crescent moon. This explanation was advocated by Harding, Schröter, 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) Luminosity 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 projection on a somewhat lighter background, such as the zodiacal light, or an extended solar envelope.[46 - Nature, October 17, 1895.]

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 Zöllner, 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 phosphorescent, and therefore more easily visible by the increased solar activity.”[47 - Ibid., July 27, 1905.] This seems a very plausible hypothesis.

On the whole the occasional illumination of the 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 improbable.

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 Rœdickœr 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 χ4 Orionis, and the “star” χ3 Orionis. A supposed observation of a satellite made by Horrebow on January 3, 1768, was undoubtedly θ Libræ. 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 6 inches aperture, a star of the 8th or even the 9th magnitude can be well seen when close to Venus.[48 - Nature, October 6, 1887.]

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 Durchmusterung. 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 remains unexplained. It might possibly have been a nova, or temporary star.[49 - Ast. Nach., No. 4106.]

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.[50 - Copernicus, vol. ii. p. 168.]

Humboldt speaks of the supposed satellite of Venus as among “the astronomical myths of an uncritical age.”[51 - Cosmos, vol. iv. p. 476, footnote.]

An occultation of Venus by the moon is mentioned in the Chinese Annals as having occurred on March 19, 361 A.D., and Tycho Brahé observed another on May 23, 1587.[52 - Denning, Telescopic Work for Starlight Evenings, p. 153.]

A close conjunction of Venus and Regulus (α Leonis) is recorded by the Arabian astronomer, Ibn 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.[53 - Ibid., p. 154.]

Seen from Venus, the maximum apparent distance between the earth and moon would vary from about 5′ to 31′.[54 - Nature, July 13, 1876.]

It is related by Arago that Buonaparte, when going to the Luxembourg in Paris, where the Directory were giving a fête 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.[55 - 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.