CHAPTER V.

VENUS.

Period, &c.-Phases resemble those of Mercury.-Most favourably placed for observations once in eight years.-Daylight observations-Its brilliancy.-Its Spots and Axial Rotation.-Suspected mountains and atmosphere.-Its ashy light"Phase irregularities.-Suspected Satellite.-Observations on it.-The Mass of Venus.-Ancient observations.-Galileo's anagram.-Venus useful for nautical observations.-Tables of Venus.

TEXT in order of distance from the Sun, after Mercury, is Venus; which revolves round the Sun in 224d 16h 49m 8, at a mean distance of 66,131,000 miles. The eccentricity of the orbit of Venus amounting to only 0.006, the extremes of distance are only Fig. 23. 66,585,000 and 65,677,000 miles, a difference less than is the case with any of the other major planets, and greater only than Europa amongst the minor ones. The apparent diameter of Venus varies between 9.7" in superior and 66.5" in inferior conjunction. At its greatest elongation its apparent diameter is about 25". A numerous series of careful observations have enabled Main to determine that

the planet's diameter reduced to mean distance is 17.55", subject to a correction of -0.5" for the effects of irradiation. Stone, from an elaborate discussion of a large series of Greenwich

observations, has obtained 16.944", with a probable error of ± 0.08" only. The real diameter corresponding to this latter evaluation is 7510 miles, or, roundly, Venus's diameter is almost the same as the Earth's. The compression is very small, so much so that I am not aware that any attempt has ever been made to assign a value.

Venus exhibits phases precisely identical with those of Mercury. Though under the most favourable circumstances never farther removed from the Sun than 47° 15', and therefore always more or less under the influence of twilight, Venus is difficult to observe for a reason additional to that which obtains with Mercury, namely its own extreme brilliancy. This is such as to render the planet not unfrequently visible in the full daylight and capable of casting a sensible shadow at night. This last happened in Feb. 1862, and occurs every 8 years, when the planet is at or near its greatest north latitude and about 5 weeks from inferior conjunction. Its apparent diameter is then about 40", and the breadth of the illuminated part nearly 10", so that rather less than 4th of the entire disc is illuminated, but this fraction transmits more light than do phases of greater extent, because the latter correspond to greater distances from the Earth. A lesser extreme of brilliancy, due, to the same circumstances less favourably carried out, occurs on either side of the Sun at intervals of about 29 months. The planet's angular distance from the Sun on these occasions is rather less than 40° (in the superior part of its orbit); its phase therefore corresponds with that of the Moon 11 and 17 days old.

Fig. 24.

Observations of Venus in the daytime were first made at a very early period; the following are the dates of a few instances: 398, 984, 1008, 1014, 1077, 1280, 1363, 1715, 1750.

"Bouvard

has related to me," says Arago, "that General Bonaparte, upon repairing to the Luxembourg, when the Directory was about to give him a fête, was very much surprised at seeing the multitude. which was collected in the Rue de Tournon pay more attention to the region of the heavens situate above the palace than to his person or to the brilliant staff which 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 no other than Venus "."

دو

The dazzling brilliancy of this planet makes daytime preferable for observing it, but under the best of circumstances it is far too glaring to permit physical observations being conveniently made. J. D. Cassini attacked it in 1667, and some ill-defined dusky spots seen on various occasions during April, May, and June, enabled him to assign 23h 16m for its axial rotation. Bianchini, in 1726 and 1727, favoured by an Italian sky, observed spots with greater facility: thence he inferred a rotation performed in 24 days 8 hours. Sir William Herschel, desirous of arriving at some more harmonious conclusion, devoted much care to the inquiry; but he was unable to assign a precise period beyond generally believing that Bianchini's was largely in excess of the true amount. Schröter, by close attention to certain spots, deduced a period of 23h 21m 7.98, which Di Vico and his colleagues at Rome, in 1840-2, only slightly modified to 23h 21m 23'93°. We may thus be assured that the period of the axial rotation of this planet is known to within a very small fraction of the whole.

Sir W. Herschel's opinion of the spots he saw was that they were in an atmosphere, and did not belong to the solid body-an opinion wanting in analogy, and now, with reason, believed to be altogether groundless, for Di Vico found the spots just as delineated by Bianchini. The Roman observers, 6 in number, displayed great diligence in the matter, and Bianchini's drawings, with exception, were confirmed. Of the 6 observers, the

most successful were those who had most difficulty in catching very minute companions to large stars, the reason of which Webb points out to be obvious enough. A very sensitive eye, which would detect the spots more readily, would be more easily overpowered by the light of a brilliant star, so as to miss a very minute one in its neighbourhood.

Mountains probably exist on Venus, though the testimony on which the statement must rest is not quite so complete as could be desired. In August 1700 La Hire, observing the planet in the daytime near its inferior conjunction, perceived in the lower region of the crescent inequalities which could only be produced by mountains higher than those in the Moon. To the same effect Derham, writing in 1715. Schröter asserted the existence of several high mountains, in which he was confirmed by Beer and Mädler, but his details as to toises must be accepted with great reserve, amongst other reasons because it is doubtful whether his micrometers were of sufficient delicacy. Sir W. Herschel disbelieved him on some points, and attacked him in the Philosophical Transactions in 1793: his reply was published in the volume for the year but one after; it was calm and dignified, and vindicated the mountains if not the measurements. Di Vico, at Rome, in April and May 1841, appears to have noticed a surface-configuration akin to that of the Moon. A bluntness of the southern horn, referred to by Schröter, was also seen by the Roman astronomers, and often by Breen subsequently with the Northumberland telescope at Cambridge.

That Venus has an atmosphere is generally admitted; that it is of considerable density is likewise an opinion apparently well founded. During the transits of 1761 and 1769, the planet was observed by several persons to be surrounded by a faint ring of light, such as an atmosphere would account for. Schröter, too, discovered what appeared to him to be a faint crepuscular light extending beyond the cusps of the planet into the dark hemisphere. From micrometrical measures of the space over which this light was diffused he considered the horizontal refraction at the surface of the planet to amount to 30' 34", or much the same as that of C Ibid., vol. lxxxv.

b Phil. Trans., vol. lxxxiii.

the Earth's atmosphere. Sir W. Herschel confirmed the discovery as a wholed, and more recently (May 1849) Mädler was able to do the same with the mere modification of making the amount greater by 1th, or equal to 43'7'.

The existence of snow at the poles of Venus has been suspected by Webb and Phillips, but the idea awaits confirmation, though there is no prima facie reason why it should not be well founded, indeed rather the reverse is the case.

A phenomenon analogous to the lumière cendrée, or 'ashy light,' of the Moon is well attested in observations of Venus when near inferior conjunction. Many observers have noticed the entire contour of the planet to be of a dull grey hue beyond the Sunillumined crescent. Webb uses the expression the phosphorescence of the dark side: I cannot but regard this as rather an unhappy one, for phosphorescence notably conveys the idea that some inherent light is spoken of, whereas there can be little doubt that reflection is in some way or other the cause of it; the precise mode it may be difficult at present to specify. Derham noticed this appearance, and refers to it in his book; and Schröter, Sir W. Herschel, Di Vico, and Guthrie' are amongst those that have seen it.

The peculiarity about Mercury's phases already pointed out (the measured breadth being different from the calculated) obtains also with Venus. At the greatest elongations, the line terminating the illumination ought to be straight as with a half-moon, but several observers have found a discrepancy of between 3 and 8 days between the first or last appearance of the dichotomisation, according to which elongation was referred to. Thus, at the westerly elongation of August 1793 Schröter found the terminator slightly concave, and not straight till 8 days after the epoch of greatest elongation.

During the last and the preceding centuries testimony was not wanting that Venus had a satellite, but nothing has been ascertained about it in recent times, and Webb, with great pro

d Phil. Trans., vol. lxxxiii. p. 214. 1793.

The supposition of the existence of

some such phenomenon as our Aurora Borealis seems a rather poor hypothesis.

Month. Not. R.A.S., vol. xiv. p. 169.