244 HISTORY OF PHYSICAL ASTRONOMY. 



of that which has since borne his name. But this 

 great discovery was not made without labour. In 

 1705, Halley 35 explained how the parabolic orbit 

 of a planet may be determined from three obser- 

 vations; and, joining example to precept, himself 

 calculated the positions and orbits of twenty-four 

 comets. He found, as the reward of this industry, 

 that the comets of 1607, and of 1531, had the 

 same orbit as that of 1682. And here the intervals 

 are also nearly the same, namely, about seventy- 

 five years. Are the three comets then identical? 

 In looking back into the history of such appear- 

 ances, he found comets recorded in 1456, in 1380, 

 and in 1305 ; the intervals are still the same, 

 seventy-five or seventy-six years. It was impos- 

 sible now to doubt that they were the periods of 

 a revolving body ; that the comet was a planet ; its 

 orbit a long ellipse, not a parabola (o). 



But if this were so, the comet must reappear in 

 1758 or 1759. Halley predicted that it would do 

 so; and the fulfilment of this prediction was na- 

 turally looked forwards to, as an additional stamp 

 of the truths of the theory of gravitation. 



But in all this, the comet had been supposed to 

 be affected only by the attraction of the sun. The 

 planets must disturb its motion as they disturb each 

 other. How would this disturbance affect the time 

 and circumstances of its reappearance ? Halley had 

 proposed, but not attempted to solve, this question. 

 35 Bailly, ii. 646. 



