144 -^ Short History of Astronomy [CH. v., 112 



there was a further irregularity which vanished at opposition 

 and conjunction as well as at quadratures, but in inter- 

 mediate positions of the moon might be as great as 40'. 

 This irregularity, known as the variation, was, as has been 

 already mentioned (chapter in., 60), very possibly dis- 

 covered by Abul Wafa, though it had been entirely lost 

 subsequently. At a later stage in his career, at latest 

 during his visit to Wittenberg in 1598-9, Tycho found that 

 it was necessary to introduce a further small inequality 

 known as the annual equation, which depended on the 

 position of the earth in its path round the sun ; this, how- 

 ever, he never completely investigated. He also ascertained 

 that the inclination of the moon's orbit to the ecliptic was 

 not, as had been thought, fixed, but oscillated regularly, 

 and that the motion of the moon's nodes (chapter n., 40) 

 was also variable. 



112. Reference has already been made to the star 

 catalogue. Its construction led to a study of precession, 

 the amount of which was determined with considerable 

 accuracy; the same investigation led Tycho to reject the 

 supposed irregularity in precession which, under the name 

 of trepidation (chapter in., 58), had confused astronomy 

 for several centuries, but from this time forward rapidly lost 

 its popularity. 



The planets were always a favourite subject of study 

 with Tycho, but although he made a magnificent series of 

 observations, of immense value to his successors, he died 

 before he could construct any satisfactory theory of the 

 planetary motions. He easily discovered, however, that their 

 motions deviated considerably from those assigned by any 

 of the planetary tables, and got as far as detecting some 

 regularity in these deviations. 



