THE MOON. 141 



mentioned by Christian Mayer, "William Herschel,* and 

 Harding, also remains exceedingly mysterious. It is not 

 probable that at so great a distance the reflected light of the 

 Earth should produce an ash-colored illumination upon Ve- 

 nus as upon our Moon. Hitherto there has been no flatten- 

 ing observed in the disks of the two inferior planets, Mercu- 

 ry and Venus. 



The Earth. 



The mean distance of the Earth from the Sun is 12,032 

 times greater than the diameter of the Earth ; therefore, 

 82,728,000 geographical miles, uncertain as to about 

 360,000 miles (-5-^0). The period of the sidereal revolution 

 of the Earth round the Sun is 365d. 6h. 9' 10"-7496. The 

 eccentricity of the Earth's orbit amounts to , 01679226 ; its 

 mass is 3-57^3-51- ; its density in relation to water, 544. Bes- 

 sel's investigation of ten measurements of degrees gave for 

 the flattening of the Earth 29 9Y5 3- The length of a geo- 

 graphical mile, sixty of which are contained in one equato- 

 rial degree, 951,807 toises, and the equatorial and polar di- 

 ameters, 6875*6 and 6852 - 4 geographical miles. [Cosmos, 

 vol. i., p. 65, note.) "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 con- 

 cluding terrestrial 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' 11"5; the eccentricity of her orbit, 0-0548442 ; her 

 diameter is 1816 geographical miles, nearly one fourth of 

 the Earth's diameter; her material contents j\ those of the 

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

 TT ? Tir (according to Peters and SchidlofTsky, -g' T ) of the mass 

 of the Earth ; her density, 0619, therefore nearly three fifths 

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

 flattening, but an extremely slight prolongation on the side 

 toward 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 



o 



Philos. Transact., 1795, vol. lxxxvi., p. 214. 



