256 A Short History of Astronomy [CH. X. 



observations of the moon as early as 1 683, and by means 

 of them effected some improvement in the tables. In 

 1676 he had already noted defects in the existing tables 

 of Jupiter and Saturn, and ultimately satisfied himself of 

 the existence of certain irregularities in the motion of these 

 two planets, suspected long ago by Horrocks (chapter vin., 

 156); these irregularities he attributed correctly to the 

 perturbations of the two planets by one another, though 

 he was not mathematician enough to work out the theory ; 

 from observation, however, he was able to estimate the 

 irregularities in question with fair accuracy and to improve 

 the planetary tables by making allowance for them. But 

 neither the lunar nor the planetary tables were ever com- 

 pleted in a form which Halley thought satisfactory. By 

 1719 they were printed, but kept back from publication, 

 in hopes that subsequent improvements might be effected. 

 After his appointment as Astronomer Royal in succession 

 to Flamsteed (1720) he devoted special attention to getting 

 fresh observations for this purpose, but he found the 

 Observatory almost bare of instruments, those used by 

 Flamsteed having been his private property, and having 

 been removed as such by his heirs or creditors. Although 

 Halley procured some instruments, and made with them 

 a number of observations, chiefly of the moon, the age (63) 

 at which he entered upon his office prevented him from 

 initiating much, or from carrying out his duties with great 

 energy, and the observations taken were in consequence 

 only of secondary importance, while the tables for the 

 improvement of which they were specially designed were 

 only finally published in 1752, ten years after the death 

 of their author. Although they thus appeared many years 

 after the time at which they were virtually prepared and 

 owed little to the progress of science during the interval, 

 they at once became and for some time remained the 

 standard tables for both the lunar and planetary motions 

 (cf. 226, and chapter XL, 247). 



205. Halley's remarkable versatility in scientific work is 

 further illustrated by the labour which he expended in 

 editing the writings of the great Greek geometer Apollonius 

 (chapter n., 38) and the star catalogue of Ptolemy 

 (chapter n., 50). He was also one of the first of modern 



