VENUS. . 139 



parent diameter, which by no means alone determines the 

 degree of brilliancy.* 1 The eccentricity of the orbit of Verms 

 expressed, as in all cases, in fractional parts of half the major 

 axes, is only 0-00686182. The diameter of this planet is 

 6776 geographical miles ; the mass ^orVf > ^he material 

 contents 0*957, and the density 0'94 in comparison to the 

 Earth. 



Of the transits of the two inferior planets first announced 

 by Kepler after the appearance of his Rudolphine tables, 

 that of Venus is of most importance for the theory of the 

 whole planetary system, on account of the determination of 

 the Suns parallax, and the distance of the Earth from the 

 Sun deduced from the latter. According to Encke's thor- 

 ough investigation of the transit of Venus in 1769, the Sun's 

 parallax is 8"-57116. (Berliner Jahrbuch for 1852, p. 323.) 

 A new examination of the Sun's parallax has been under- 

 taken since 1849, by command of the government of the 

 United States, at the suggestion of Professor Gerling of Mar- 

 burg. The parallax is to be obtained by means of observa- 

 tions of Venus near the eastern and western stationary points, 

 as well as by micrometer measurements of the differences in 

 the right ascension and declination of well-determined fixed 

 stars in very different latitudes and longitudes. (Schum., 

 Astr. Nachr., No. 599, p. 363, and No. 613, p. 193.) The 

 astronomical expedition, under the command of the learned 

 Lieutenant Gilliss, has proceeded to Santiago in Chili. 



The rotation of Venus was long subject to great doubt. 

 Dominique Cassini, 1669, and Jacques Cassini, 1732, found 



* " That point of the orbit of Venus in which she can appear to us 

 with the brightest light, so that she may be seen at noon even with the 

 naked eye, lies between the inferior conjunction and the greatest di- 

 gression, near the latter, and near the distance of 40 from the Sun, or 

 from the place of the inferior conjunction. On the average, Venus ap- 

 pears with the finest light when distant 40 east or west from the Sun, 

 in \yhich case her apparent diameter (which in the inferior conjunction 

 can increase to 66") is only 40", and the greatest breadth of her illu- 

 minated phase measures scarcely 10". The degree of proximity to the 

 Earth then gives the small luminous crescent such an intense light, that 

 it throws shadows in the absence of the Sun." Littrow, Tkeoretische 

 Astronomic, 1834, th. ii., p. 68. Whether Copernicus predicted the ne- 

 cessity of a future discoveiy of the phases of Venus, as is asserted in 

 Smith's Optics, sec. 1050, and repeatedly in many other works, has re- 

 cently become altogether doubtful, from Professor de Morgan's strict 

 examination of the work De Revolutionibus, as it has come down to us. 

 See the letter from Adams to the Rev. R Main, on the 7th of Sep- 

 tember, 1846, in the Report of the Royal Astronomical Society, vol. vil., 

 No. 9, p. 142. (Compare also Cosmos, vol. ii., p. 325.) 



