CHAPTER V.

VENUS.

66

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Period, &c.-Phases resemble those of Mercury.-Most favourably placed for observation once in 8 years.-Observations by Lihou.-By Lacerda.-Daylight observations.-Its brilliancy.-Its Spots and Axial Rotation.-Suspected mountains and atmosphere.-Its ashy light."-Phase irregularities.-Suspected Satellite.-Alleged Observations of it.-The Mass of Venus.-Ancient observations.-Galileo's anagram announcing his discovery of its Phases.--Venus useful for nautical observations.-Tables of Venus.

NEX

EXT 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 67,190,000 miles. The eccentricity of the orbit of Venus amounting

to only 0.007, the extremes of distance are only 67,652,000 miles and 66,728,000 miles. This eccentricity is very small. No other planet, major or minor, has an eccentricity SO small. The apparent diameter of Venus varies between 9.5" in Superior and 65.2" in Inferior Con

junction. At its greatest Elongation its

its apparent

diameter is about 25". A

Fig. 48.

numerous series of careful observations enabled Main to determine that the planet's diameter (reduced to mean distance) is

Figs. 49-50 are copied, with an unimportant variation, from Pl. xlii of Schröter's Selenotopographische Fragmente.

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, obtained 16.944", with a probable error of +0-08". Tennant in 1874 (during the Transit) obtained, as the mean of 68 measures, 16.9036" (reduced) with a probable error of 0.0016" only". The real diameter corresponding to this latter evaluation is about 7500 miles, or, roundly, Venus is a planet almost as large as the Earth. The compression must be small, but Tennant thinks he found traces thereof. Great difficulty must ever remain in clearly detecting it, because the planet's diameter in Superior Conjunction is so small.

Venus exhibits phases precisely identical in character with those of Mercury.

Fig. 49.

own

Though under the most favourable circumstances Venus is never farther removed from the Sun than 47° 15', and is therefore always more or less under the influence of twilight, yet it is difficult to scrutinise this planet for a reason additional to that which obtains with Mercury, namely, its extreme brilliancy. This is such as to render the planet not unfrequently visible in full daylight and capable of casting a sensible shadow at night. This happened in January 1870, and indeed occurs every 8 years, when the planet is at or near its greatest North latitude and about

(Schröter.)

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 of the entire disc is illuminated; b Month. Not., vol. xxxv. p. 347. May 1875.

but this fraction transmits more light than do phases of greater extent, because the latter occur at greater distances from the Earth. A lesser maximum 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 the phases of the Moon when 11d and 17d old. Figs. 50-1 are selected from some drawings by Lihou taken in the winter of 1885-6 with a refractor of 44 inches aperture.

He makes the following remarks on what he saw :

Nov. 10, 1885.-" With a telescope of about 4 inches aperture armed with a magnifying power of 100 I was able to distinguish a grey spot in the northern hemisphere. Spots on Venus being very difficult to see with small instruments, this observation merits attention."

Dec. 8, 1885.-"Sky very pure. The light of Venus is so bright as to fatigue the eye, but by making use of a coloured glass I am able to see the limbs sharply defined."

Dec. 16, 1885.-"Sky very pure. The image of Venus is extremely sharp, and the limbs well defined; the northern cusp is sharply pointed, whilst the southern is slightly truncated."

Dec. 23, 1885.-"The northern cusp of Venus is sharply pointed, and the southern cusp slightly truncated."

L'Astronomie, vol. v. p. 148, April 1886.

Figs. 52-5 are intended to represent some drawings of Venus made in 1884 by M. Lacerda of Lisbon. Respecting these he writes as follows:-

"Sept. 8, 1884.-The crescent of Venus appears sensibly more narrow towards the North Pole than towards the South Pole. With a magnifying power of 250 I cannot distinguish the Southern spots, which, however, were very visible with a magnifying power of 160. I notice that the Northern hemisphere is brighter than the rest of the planet. A very obscure and elongated spot is visible near the North Pole."

"Sept. 9, 1884.-There is a very bright thread of light concentric with the Eastern limb of the planet; perhaps some high clouds lying along a maritime shore of Venus. Two large spots are also visible on the crescent; the one, oblong, stretched parallel to the bright spot; the other, almost round, and much smaller, to the North of the first. The Southern horn is always longer than the Northern one. The elongated spot which hollows out the planet near the North Pole continues to be very visible.

M. Lacerda says that on the following morning, Sept. 10, he was unable to distinguish any spots.

His next observation is dated

"Oct. 8, 1884.-The 2 dark spots have sensibly shifted their positions towards the North. They disclose also a slight movement towards the West. The terminator which seemed shrunk up towards the North Pole is to-day almost perfect; but the Southern horn continues to appear longer and more pointed than the Northern one. The lustre of the planet seems uniform. The dark spot which cut into the crescent near the North Pole is not visible."

"Oct. 13, 1884.-There is a great depression near the Southern horn. 2 spots are visible on the planet; one to the South; the other, smaller, and to the North; and a third was suspected under the equator, near the illuminated limb and concentric with it. The Northern horn is truncated."

M. Lacerda concludes his observations by remarking that the most favourable time for observing Venus is between an hour before sunrise and 1 hour after sunrise. He adds that he was never able to see any spots when the planet was in the west, at or near the time of sunset".

Observations of Venus in the daytime were made at a very early period; the following are the dates of a few instances: 398 A.D., 984, 1008, 1014, 1077, 1280, 1363, 1715, 1750. "Bouvard has related to me," says Arago, "that General Buonaparte, 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

L'Astronomie, vol. iii. p. 462, Dec. 1884.

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