254 ^ Short History of Astronomy [CH. x. 



the resemblance between the paths described by the 

 comets of 1531, 1607, and 1682, and by the approximate 

 equality in the intervals between their respective appear- 

 ances and that of a fourth comet seen in 1456, he was 

 shrewd enough to conjecture that the three later comets, 

 if not all four, were really different appearances of the same 

 comet, which revolved round the sun* in an elongated 

 ellipse in a period of about 75 or 76 years. He explained 

 the difference between the 76 years which separate the 

 appearances of the comet in 1531 and 1607, and the slightly- 

 shorter period which elapsed between 1607 and 1682, as 

 probably due to the perturbations caused by planets near 

 which the comet had passed ; and finally predicted the 

 probable reappearance of the same comet (which now 

 deservedly bears his name) about 76 years after its last 

 appearance, i.e. about 1758, though he was again aware 

 that planetary perturbation might alter the time of its 

 appearance ; and the actual appearance of the comet about 

 the predicted time (chapter XL, 231) marked an important 

 era in the progress of our knowledge of these extremely 

 troublesome and erratic bodies. 



201. In 1693 Halley read before the Royal Society a 

 paper in which he called attention to the difficulty of 

 reconciling certain ancient eclipses with the known motion 

 of the moon, and referred to the possibility of some slight 

 increase in the moon's average rate of motion round the 

 earth. 



This irregularity, now known as the secular acceleration 

 of the moon's mean motion, was subsequently more 

 definitely established as a fact of observation ; and the 

 difficulties met with in explaining it as a result of gravitation 

 have rendered it one of the most interesting of the 

 moon's numerous irregularities (cf. chapter XL, 240, and 

 chapter XHI., 287). 



202. Halley also rendered good service to astronomy 

 by calling attention to the importance of the expected 

 transits of Venus across the sun in 1761 and 1769 as a 

 means of ascertaining the distance of the sun. The 

 method had been suggested rather vaguely by Kepler, and 

 more definitely by James Gregory in his Optics published 

 in 1663. The idea was first suggested to Halley by 



