32 Astronomy and Geology compared. PT. i. 



data being given, and the position of the Sun and 

 stars being assumed as fixed, the apparent distance 

 of the Moon from different stars will be calculated 

 with certainty. The calculations, however, are 

 necessarily very complicated, as they involve the 

 two motions of the Earth round the Sun, and of 

 the Moon round the Earth, and when we see in the 

 Nautical Almanac that these calculations are made 

 to decimals of a second, we may form a conception 

 both of the labour required and of the thorough 

 knowledge of both these motions to calculate them 

 beforehand with such minuteness. 



A similar illustration of the uniformity in all 

 the motions of the heavenly bodies composing our 

 planetary system, and of the correctness with which 

 astronomers are enabled to calculate them, may be 

 furnished by the observations of the satellites of 

 Jupiter. This planet, as may be seen in the fore- 

 going table, has a mean distance from the centre of 

 the Sun of 475,692,000 miles ; its distance from the 

 Earth varies in proportion to the position of each 

 planet in their respective orbits. When the Sun 

 is exactly between Jupiter and the Earth, their 

 distance then is the whole distance of the Earth 

 from the Sun, added to the whole distance between 



