VENUS. 473 



Laplace." Ten years ago the density of Mercury was taken 

 as nearly three times greater than the density of the Earth- 

 as 2-56 or 2-94, when the Earth = 1-00. 



VENUS. 



The mean distance of this planet from the Sun, expressed 

 in fractional parts of the Earth's distance from the Sun, ^. e. 

 60 million geographical miles, is 0*7233317. The period of 

 its sidereal, or true revolution, is 224 days, 16h. 49m. 7s. 

 No principal planet comes so near the Earth as Venus. She 

 can approach the Earth to within a distance of 21 million 

 miles; but can also recede from it to a distance of 144 million 

 miles. This is the reason of the great variability of her 

 apparent diameter, which by no means alone determines the 

 degree of brilliancy. 13 The excentricity of the orbit of Venus 



13 " 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 con^ 

 junction and the greatest digression near the latter, and near 

 the distance of 40 from the Sun, or from the place of the 

 inferior conjunction. On the average, Venus appears with 

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

 in which case her apparent diameter (which in the inferior 

 conjunction can increase to 66") is only 40", and the greatest 

 breadth of her illuminated phase measures scarcely 10". The 

 degree of proximity to the Earth then gives the small lumi- 

 nous crescent such an intense light, that it throws shadows in 

 the absence of the Sun." Littrow, Theoretische Astronomic, 

 1834, Th. ii. p. 68. Whether Copernicus predicted the 

 necessity of a future discovery of the phases of Venus, as is 

 asserted in Smith's Optics, sec, 1050, and repeatedly in many 

 other works, has recently 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 



