THE MOON. 477 



We restrict ourselves here to numerical data referring 

 to the Earth's figure and motions : all that refers to its 

 physical constitution is deferred until the concluding terres- 

 trial portion of the Cosmos. 



THE MOON OF THE EARTH. 



The mean distance of the Moon from the Earth is 207,200 

 geographical miles ; the period of sidereal revolution is 27d. 

 7h. 43' ll"-5; the excentricity of her orbit, 0-0548442; her 

 diameter is 1816 geographical miles, nearly one-fourth of the 

 Earth's diameter; her material contents -^ those of the 

 Earth ; the mass of the Moon is, according to Lindeman, 

 sT-Ya (according to Peters and Schidloffsky, ^) of the mass 

 of the Earth, her density 0*619, therefore nearly three-fifths 

 of the density of the Earth. The Moon has no perceptible 

 flattening, but an extremely slight prolongation on the side 

 towards the Earth, estimated theoretically. The rotation of 

 the Moon upon its axis is completed exactly in the same time 

 in which it revolves round the Earth, and this is probably 

 the case with all other secondary planets. 



The sunlight reflected from the Moon is in all zones more 

 feeble than the sunlight which is reflected by a white cloud in 

 the daytime. When in determining geographical longitudes 

 it is often necessary to take the distance of the Moon from 

 the Sun, it is not unfrequently difficult to distinguish the 

 Moon between the more intensely luminous masses of cloud. 

 Upon mountain-heights, which lie between 12,791 and 17,057 

 feet above the level of the sea, and where in the clear moun- 

 tain air only feathery cirri are to be seen in the sky, 1 found 

 the detection of the Moon's disc was much more easy, because 

 the cirrus reflects less sunlight on account of its loose tex- 

 ture, and the moonlight is less weakened by its passage 

 through the rarer strata of air. The relative degree of in- 



