VENUS, in astronomy, the second planet In order of distance from the sun. The mean dis tance of Venus from the sun is 67,200,000 miles; its orbit has an eccentricity of .00686 and the plane of its orbit is inclined to the plane of the ecliptic at an angle of 3° 23' 35". It completes one revolution about the sun in 224.7 days and as its orbit lies wholly within the orbit of the earth the planet shows phases as seen by us, being seen full when it is at its greatest dis tance beyond the sun, half full when farthest to the left or right of that body and new when between the sun and the earth. During its motion around its orbit its distance from us varies enormously and hence also does its ap parent size. When at its greatest distance, of 161,200,000 miles, its apparent diameter is but 9".5, while at its nearest approach it is but 24,500,000 miles away and its apparent diameter then is 65"2. It is only when near the former position that the illuminated hemisphere is al most entirely turned toward the earth; as the planet draws nearer, this hemisphere is steadily replaced by the dark one, so that near the time of closest approach we see the planet only as an exceedingly narrow crescent. This is why, notwithstanding the fact that Venus draws nearer the earth than do any of the other planets, we nevertheless view it under much less favorable conditions than occur when an outer planet reaches its position of closest ap proach.
The planet Venus tnore closely resembles the earth than does any other body in our solar system. Its diameter is but 7,700 miles, its sur face gravity is 0.8 of that on the earth and it is surrounded by an atmosphere of considerable density. This last is made evident by the fact that as Venus approaches the crescent phase the horns extend far beyond the 180° points; when very near new, a thin, bright ring due to its atmosphere has, in fact, several times been seen completely encircling the darkened ball. It is another evidence of an atmosphere that the brilliance of the disc is greatest at the limb and falls off very rapidly as the terminator is approached; twilight effects, also, have been observed along the terminator for more than a century. As a result of the presence of this
atmosphere, our knowledge of the surface markings of Venus is practically nothing. Many drawings showing shaded areas, white areas or white spots have been made from a century ago until now, but whether a part of these markings may be continental or oceanic areas dimly seen through the overlying atmos phere, or whether all are merely transtent cloud forms we do not lcnow. This uncertainty also prevents any certain answer being given to the unportant and interesting question of the time of rotation of the planet. The earlier ob servers, and many of the modern ones, be lieved that this was between 23 and 24 hours; the observations of others led to a rotation period equal in length to the planet's year, thus indicating that Venus keeps always the same face turned toward the sun. A different line of attack is furnished by the utilization of the spectroscope to determine the velocities of dif ferent portions of the planet's disc toward or away from the earth. (See SPECTROSCOPY). But the observations are very difficult and thus far the results from different observatories are contradictory.
In the course of its motion about its orbit Venus sometimes passes between the earth and the sun and is seen crossing the bright disc of the latter as a round, black spot surrounded by a thin, atmospheric ring. These occurrences are known as Transits of Venus; when the transit is central, the passage occupies about eight hours. The earliest observed transit occurred in 1639; four have since occurred, in the years 1761, 1769, 1874 and 1882. Extensive observa tions were made upon the last two for the pur pose of determining the solar parallax but the processes then employed have since been super seded by more accurate methods. (See PARAL tAx). The next two transits of Venus will occur as follows: 2004, June 8, 9 A.M., Greenwich M. T. Dura tion 6 hours 2 minutes.
2012, June 6, 1 A.M., Greenwich M. T. Dura tion 6 hours 20 minutes.
Ewe Doourme, Director, Flotver Astronomical Observatory; University of Pennsylvania.