PT. i. Astronomy and Geology compared. 31 



for these calculations, so as to enable astronomers to 

 calculate them beforehand with such perfect correct- 

 ness. Let us see what are the elements upon which 

 they are founded. The Sun and the stars may be 

 considered as stationary, and the Earth as describing 

 its annual revolution in its orbit through the void 

 between the Sun and the stars ; while this is being 

 accomplished, the Moon is describing her monthly 

 revolution round the Earth and accompanying it in 

 its progress. Now it is evident that, although the 

 positions of the Sun and the stars are in fact 

 stationary, yet they appear to alter to an observer 

 upon the Earth as it proceeds on its course ; but as 

 the Earth pursues exactly the same track in each 

 succeeding year, her relative apparent position to 

 each of the stars will be the same as it was in the 

 same day and hour in the preceding year ; and also 

 with regard to the Moon, which revolves round the 

 Earth in the same manner, her apparent distances 

 from the Sun and from the several stars will be the 

 same ; but the whole certainty of these results will 

 depend upon the perfect accuracy of this complicated 

 calculation, viz., the exact coincidence in time and 

 place of the Earth's track round the Sun, and also 

 of the Moon's revolution round the Earth. These 



