442 HISTORY OF PHYSICAL ASTRONOMY. 



Attempts have been made, from the time of the solution cf the Prob- 

 lem of three bodies to the present, to give the greatest possible accu- 

 racy to the Tables of the Sun, by considering the effect of the various 

 perturbations to which the earth is subject. Thus, in 1756, Euler cal- 

 culated the effect of the attractions of the planets on the earth (the 

 prize-question of the French Academy of Sciences), and Clairaut soon 

 after. Lacaille, making use of these results, and of his own numerous 

 observations, published Tables of the Sun. In 1786, Delambre 26 un- 

 dertook to verify and improve these tables, by comparing them with 

 314 observations made by Maskelyne, at Greenwich, in 1775 and 1784, 

 and in some of the intermediate years. He corrected most of the ele- 

 ments ; but lie could not remove the uncertainty which occurred re- 

 specting the amount of the inequality produced by the reaction of the 

 moon. He admitted also, in pursuance of Clairaut's theory, a second 

 term of this inequality depending on the moon's latitude ; but irreso- 

 lutely, and half disposed to reject it on the authority of the observa- 

 tions. Succeeding researches of mathematicians have shown, that this 

 term is not admissible as a result of mechanical principles. Delambre's 

 Tables, thus improved, were exact to seven or eight seconds ; 57 which 

 was thought, and truly, a very close coincidence for the time. But 

 astronomers were far from resting content with this. In 1806, the 

 French Board of Longitude published Delambre's improved Solar Ta- 

 bles; and in the Connaissance des Terns for 1816, Burckhardt gave 

 the results of a comparison of Delambre's Tables with a great number 

 of Maskelyne's observations ; far greater than the number on which 

 they were founded. 28 It appeared that the epoch, the perigee, and the 

 eccentricity, required sensible alterations, and that the mass of Venus 

 ought to be reduced about one-ninth, and that of the Moon to be sen- 

 sibly diminished. In 1827, Professor Airy 29 compared Delambre's 

 tables with 2000 Greenwich observations, made with the new transit- 

 instrument at Cambridge, and deduced from this comparison the cor- 

 rection of the elements. These in general agreed closely with Eurck- 

 hardt's, excepting that a diminution of Mars appeared necessary. Some 

 discordances, however, led Professor Airy to suspect the existence of 

 an inequality which had escaped the sagacity of Laplace and Burck- 

 hardt. And, a few weeks after this suspicion had been expressed, the 

 Bame mathematician announced to the Roval Societv that he had de- 



25 Voiron, Hist. p. 315. " Montucla, iv. 42. 



= 3 Airy, faport, p. 150. =' PJiil. Trans. IS'JS. 



