VENUS

In Astronomy, one of the principal planets, the second in the order of distance from the sun, and the most brilliant of all the planetary bodies. From her alternate appearance in the morning and evening, Venus was called by the Greeks Hesperus and Phosphorus, the evening and morning star; sometimes also she is called the shepherd's star. The distance of Venus from the sun is -7233316, that of the earth being unity; whence her true distance is about 68 millions of miles. Her sidereal revolution is performed in 224-7007869 mean solar days; and her synodical period, or the interval between her succe ive conjunctions with the sun, is 583-920 mean solar days. The inclination of the orbit to the ecliptic was 3° 23' 28-6" at the commencement of the present century, and is subject to a decrease of about 0-04.55" in a year. The eccentricity of the orbit is -00686074, half th(^ major axis being unity; and the longitude of the ascending node at the commencement of the present century was 74° 54' 12-9", having a motion westward, when referred to the fixed stars, amounting to 17-6" in a year. The mean apparent diameter of Venus, or her apparent diameter when at her mean distance, is 16-9"; but it is subject to great variations, being only 96" at the time of her superior conjunction, and at the time of the inferior conjunction sometimes so much as 61-2". Comparing this with her mean distance, her true diameter is •975, that of the earth being unity; and is consequently about 7700 English miles. Her volume is therefore "927 of that of the earth. Her ma , as compared with that of the sun, is -0000024638. Venus, being an inferior planet, is never seen in opposition to the sun. Her greatest elongation, or angular distance from the sun, varies from 45° to 47° 12'. According to her various positions relatively to the sun and earth, she changes her phases like the moon, appearing when she first emerges from the sun's rays and becomes visible in the morning as a fine luminous crescent, the horns of which are turned away from the sun. As the planet recedes from the sun, the breadth of the crescent increases: at the greatest elongation it becomes a semicircle; after this the breadth of the disk goes on increasing till the planet arrives at its superior conjunction, where it appears as a full orb, the illuminated side being turned towards the earth. These appearances, on account of the irradiation, are not appreciable by the naked eye, but they become visible in an ordinary telescope. Venus is supposed to revolve about an axis; and the time of rotation is stated, from observations made by Schroeter, to be 23 h. 21 m. 7'2 sec, the axis of rotation being inclined to the ecliptic in an angle of about 75°. There is, however, considerable doubt as to the accuracy of these conclusions. On this subject Sir John Herschel remarks, that Venus, " although it attains occasionally the considerable apparent diameter of 61", which is larger than that of any other planet, is yet the most difficult of them all to define with telescopes. The intense lustre of its illuminated part dazzles the sight, and exaggerates every imperfection of the telescope; yet we see clearly that its surface is not mottled over with permanent spots like the moon: we perceive in it neither mountains nor shadows, but a uniform brightne , in which we may, indeed, fancy obscurer portions, but can seldom or never rest fully satisfied of the fact. It is from some observations of this kind that both Venus and Mercury have been concluded to revolve on their axes in about the same time as the earth. The most natural conclusion, from the very rare appearance and want of permanence in the spots, is, that we do not see, as in the moon, the real surface of these planets, but only their atmospheres, much loaded with clouds, and which may serve to mitigate the otherwise intense glare of their sunshine." (" Astronomy," Cabinet Cyclopisdia, p. 279.) Venus is sometimes seen to pa over the disc of the sun, presenting a phenomenon analogous to that of a solar eclipse by the moon. This phenomenon, which is called a transit of Venus, can only happen when the planet is at its inferior conjunction, and, at the same time, very near one of its nodes. • It is therefore of rare occurrence; but very important, inasmuch as it affords the best means astronomers po e of determining the sun's parallax, and consequently the dimensions of the planetary system. {See Planet.) Delambre [s. 1302]