VENUS

In Astronomy, one of the principal planets, md 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 >rih being unity; whence her true distance is about 68 millions of miles. Her slden nl revolution is performed in oTdOJ mean solar days; and her synodical period, or the interval between her succe ive conjunctions with the sun, is 083-92U mean solar days. The inclination of the orbit to the ecliptic was 3° 23' •286" at the commencement il century, and is subject to a decrease of about i" in a year. The eccentricity of the orbit is IKH386074, half the major axis being unity; and the longitude of the nsrtg node at the commencement of the present century.1-54' 129", having a motion westward, when referred Ml the fixed stars, amounting lo 17-6" in a year. The mean apparent diameter of Venus, or her apparent;er when at her mean distance, is 16-9"; but it is sab great variations, being only 9'0" at the time of her junction, and at the time of the inferior con-.11.sometimes so much as 01-2". Comparing this with iicr mean distance, her true diameter is 1175, that of the being unity; and is consequently about 7700 English Her volume is therefore -927 of that of the earth. Her ma , as compared with that of the sun, is •OOOOOOtti;;-'. 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 lo her various jRisitions relatively to the sun and earth, she changes her phases like the moon, appearing when she first emerges from a hue the luminous sun's rays,crescent,and becomes the horns visible of which in the morning are turned as m ay 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 nppears as a full orb, the illuminated Ride being turned towaids the earth. These appearances, on account of the irradiation, are not appreciable by the naked eye, but they become visible in an ordinary telescoiie. Venus is supposed to revolve about an axis; and the time of rotation is stated, from observations made by.Srhroeter, to be 23 h. 21 m. 7-2 sec, the axis of rotation bei:ig 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 Hetschel remarks, that Venus, "although il attains occasionally the considerable apparent diameter of 61", which is larger than that of any other planet is yet Ihe 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 sec 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 hive 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." (" Astronomv," Cabinet Cyclap , 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 [s. 1313]