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Encyclopedia Britannica - Main :: VAN-VIR |
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VENUS , in astronomy, the second of the major planets in the order of distance from the sun, and moving next within the orbit of the earth. Its symbol is ?. At inferior conjunction it approaches nearer to the earth than any other major planet, but in that position it is practically invisible. Its apparent motion may be described as an oscillation from one side of the sun to the other, the complete period of which is 1.6 years, and the greatest elongation about 450 on each side of the sun. When east of the latter it appears as the " evening star " in the west after sunset, while near western elongation it is seen as the " morning star " before sunrise. In these aspects it was known to the ancients as "Earrepos, Hesperus, and `Ewv4opos or 4'our4 opos, Phosphorus. The eccentricity of its orbit is smaller than that of any other planet except Neptune. Notwithstanding the near approach of Venus to the earth, its situation relative to the sun is unfavourable to the study of its physical constitution. Near inferior conjunction only a narrow crescent
this crescent
stantly increases. When it appears as a half-moon it is at a distance of more than two-thirds that of the sun, and nearly double the distance of Mars in opposition. The difficulty of reaching any conclusion on the subject of its constitution is heightened by the seeming absence of any well-marked features on the visible part of its brilliant surface. In the telescope it presents much the appearance of burnished silver, without spot or blemish. It is true that observers have from time to time thought they could detect slight variations of shade indicating an axial rotation. As far back as 1667 G. D. Cassini thought he saw a bright spot near the southern horn, observationsof yenous. of which gave a period of about 23 hours. In 1726 of Ven Francesco Bianchini (16621729), a papal chamberlain, made similar observations from which he inferred a period of more than 24 days. It was shown, however, that the observations of Bianchini could be reconciled with those of Cassini by supposing that, as he observed the planet night after night, it had made one rotation and a little more. J. H. Schroeter also found a revolution of less than 24 hours. But Sir W. Herschel, as in the case of Mercury, was never able to detect any changes from which a period of rotation could be determined. During the years 1888189o, G. Schiaparelli made an exhaustive study of the whole subject, the results of which were summed up in five brief notes, read to the Lombardian Academy of Sciences during the year 1800. His general conclusion was that Venus always presents the same face to the sun, as the moon does to the earth. The same result has been reached by the observations at the Lowell Observatory. The inference that the axial rotation is at least much slower than that of the earth is strengthened by the measures of different diameters of the planet made while it was in transit across the disk of the sun in 1874 and 1882. These show no measurable ellipticity of the disk, but they are not sufficiently accurate to lead to any more precise conclusion thanthat just stated. Still, the difficulty and uncertainty attending all observations hitherto made upon the disk are such that no conclusion respecting the time of rotation can be regarded as established. Against the view of Schiaparelli is to be set the great improbability that a body
Other observations than those we have cited show that Venus is surrounded by an atmosphere so filled with clouds that it is doubtful whether any view of the solid body
diameter had entered upon the sun the outline outside the disk of the sun began to be marked by broken portions of an arc of light. This did not begin at the point A (fig. 1) farthest from the sun, as it should have done if dunSufi wholly to refraction, but immediately at the sun itself, as sf''lown in the cut at the point B. Portions of this arc were formed one by one at various other points of the dotted outline, and when the planet was about three-fourths upon the sun the are- was completed. But there was no strengthening of the line at the middle point, as there should have been if due to refraction. Yet refraction must have played some part in the phenomenon, because otherwise no illumination could have been visible under the circumstances. The most satisfactory explanation seems to be that of H. N. Russell, whose conclusion is that the atmosphere is so permeated with fine particles of vapour up to its outer limit as to be only translucent without being fully transparent. Thus what is seen is the irregular reflection of the light at an extremely small angle from the particles of vapour. The question whether Venus has a satellite has always interested astronomers. During the 17th and 18th centuries Cassini at Paris and James Short (17101768) in England, as well as other observers during the same period, saw an ssateiNiupF-'sea. e object which had the appearance of a satellite. But as no such object has been seen by the most careful search with the best instruments of recent
century. Venus passes the seen against the sun only fofollowing list
1518 June 2. 1526 June I. 1631 December 1639 December 1761 June 6. The first of these transits actually seen was that of 1639, which was imperfectly observed by Jeremiah Horrox (1619-1641) shortly before sunset. Special
interest
parallax
parallax
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