434 HISTORY OF PHYSICAL ASTRONOMY. 



Sect. 2. — Application of the Newtonian Theory to the Moon. 



The Motions of the Moon may be first spoken of, as the most ob 

 vious and the most important of the applications of the Newtonian 

 Theory. The verification of such a theory consists, as we have seen 

 in previous cases, in the construction of Tables derived from the theory, 

 and the comparison of these with observation. The advancement ot 

 astronomy w T ould alone have been a sufficient motive for this labor ; 

 but there were other reasons which urged it on with a stronger im- 

 pulse. A perfect Lunar Theory, if the theory could be perfected, 

 promised to supply a method of finding the Longitude of any place on 

 the earth's surface ; and thus the verification of a theory which pro- 

 fessed to be complete in its foundations, was identified with an object 

 of immediate practical use to navigators and geographers, and of vast 

 acknowledged value. A good method for the near discovery of the 

 longitude had been estimated by nations and princes at large sums of 

 money. The Dutch were willing to tempt Galileo to this task by the 

 offer of a chain of gold : Philip the Third of Spain had promised a re- 

 ward for this object still earlier; 1 the parliament of England, in 1714, 

 proposed a recompense of 20,000^. sterling ; the Regent Duke of Or- 

 leans, two years afterwards, offered 100,000 francs for the same pur- 

 pose. These prizes, added to the love of truth and of fame, kept this 

 object constantly before the eyes of mathematicians, during the first 

 half of the last centurv. 



If the Tables could be so constructed as to represent the moon's real 

 place in the heavens with extreme precision, as it would be seen from 

 a standard observatory, the observation of her apparent place, as seen 

 from any other point of the earth's surface, would enable the observer 

 to find his longitude from the standard point. The motions of the 

 moon had hitherto so ill agreed with the best Tables, that this method 

 failed altogether. Newton had discovered the ground of this want of 

 agreement. He had shown that the same force which produces the 

 Evection, Variation, and Annual Equation, must produce also a long 

 series of other Inequalities, of various magnitudes and cycles, which 

 perpetually drag the moon before or behind the place where she would 

 be sought by an astronomer who knew only of those principal and 

 notorious inequalities. But to calculate and apply the new inequali- 

 ties, was no slight undertaking. 



' Del. A. M. i. 39, 66. 



