VENUS, the most beautiful star in the heavens, known by the names of the morning and evening star, likewise keeps near the sun, though she recedes from him almost double the distance of Mer cury. She is never seen in the eastern quarter of the heavens when the sun is in the western ; but al ways seems to at tend him in the evening, or to give no tice of his approach in the morning. The planet Venus presents the same pheno mena with Mercury : but her different phoses are much more sensible, her os cillations wider, and of longer duration. Her greatest distance from the sun varies from 45° to nearly 48°, and the mean du ration of a complete oscillation is 584 days. Venus has been sometimes seen moving across the sun's disc in the form of a round black spot, with an apparent diameter of about 59v. A few days after this has been observed, Venus is seen in the morning, west of the sun, in the form of a fine crescent, with the convexity turned toward the sun. She moves gra dually westward with a retarded motion, and the crescent becomes more full. In about ten weeks she has moved 46° west of the sun, and is now a semicircle, and her diameter is 26.' She is now station ary. She t@ren moves eastward, with a motion gradually accelerated, and over takes the sun about n months after hav ing been seen on his disc. Some time after she, is seen in the evening, east of the sun, round, but very small. She moves• eastward, and increases in diameter, but loses of her roundness, till she gets about 46° east of the sun, when she is again a semicircle. She now moves westwar4, increasing in diameter, but becoming a crescent like the waning moon ; and, at last, after a period of nearly 584 days, comes again into conjunction with the tun with an apparent diameter of 59". She does not move exactly in the plane of the ecliptic, but deviates from it several degrees. Like Mercury, she sometimes crosses the sun's disc. The duration of these transits, as observed from different parts of the earth's surface, are very dif ferent: this is owing to the parallax of Venus, in consequence of which different observers refer to different parts of the sun's disc, and see her describe different chords on that disc. In the transit which happened in 1769, the difference of its duration, as observed at Otaheite and at Wardhuys in Lapland, amourted to 23 minutes, 10 seconds. This difference
gives us the parallax of Venus, and of course her distance from the earth during a conjunction. The knowledge of this parallax enables us, by a method to be afterwards described, to ascertain that of the sun, and consequently to discover its distance from the earth. The great vari ations of the apparent diameter of Venus demonstrate that her distance from the earth is exceedingly variable. It is largest when the planet passes over the surface of the sun. Her mean apparent diameter is 58".
Venus, as we have already observed, is occasionally seen in the disc of the sun, in form of a dark round spot. This hap pens when the earth is about her nodes at the time of her inferior conjunction. These appearances, called transits, hap pen but very seldom. During the last century there were two transits, one in June, 1761, and the other in 1769: no other will occur till the writers and most of the present readers of this Dictionary shall be no more, viz. in 1874. 'Excepting such transits as these, Venus exhibits the same r,ppearanees to us regularly every eight years ; her conjunctions, elonga tions, and times of rising and setting being very nearly the same, on the same days, as before. From the transit of Venus in 1761 was deduced the sun's parallax, and of course his distance from 'the earth with very great accuracy. See Philosophical Transactions, vol. li. and lii. On the day of the transit, when the sun was nearly at his greatest distance from the earth, the parallax was found to,be 8" there fore, at his mean distance it. will be 8" 65m.. Whence, by logarithms, we have 10;000, &c.-5.622 (sine of 8" 65")--e: 4.376=23882.84, the number of semi diameters of the earth contained in its dis tance from the sun. This last number, multiplied by 3985, the number of miles in the earth's semi-diameter,gives 95,173 122 miles for the mean distance of the earth from the sun. This being ebtained, we easily, by calculation, find the dis tances of all the other planets. Other ob servers made the parallax somewhat dif ferent, but it was generally admitted that this distance is somewhere between 95 and 96 millions of miles.