i50 HISTORY OF PHYSICAL ASTRONOMY. 



has reduced to a regularity as complete as that of the sun ; upon 

 Comets. No part of tlie Newtonian discoveries excited a more in- 

 tense interest than this. These anomalous visitants were anciently 

 gazed at with wonder and alarm ; and might still, as in former times, 

 be accused of " perplexing nations," though with very different fears 

 and questionings. The conjecture that they, too, obeyed the law of 

 universal gravitation, was to be verified by showing that they described 

 a curve such as that force would produce. Hevelius, who was a most 

 diligent observer of these objects, had, without reference to gravi- 

 tation, satisfied himself that they moved in parabolas. 34 To deter- 

 mine the elements of the parabola from observations, even Newton 

 called 35 ** problema longe difficillirnum." Newton determined the orbit 

 of the comet of 1680 by certain graphical methods. His methods 

 supposed the orbit to be a parabola, and satisfactorily represented the 

 motion in the visible part of the comet's path. But this method did 

 not apply to the possible return of the wandering star. 'Halley has 

 the glory of having first detected a periodical comet, in the case of 

 that which has since borne his name. But this great discovery was 

 not made without labor. In 1705, Halley 86 explained how the para- 

 bolic orbit of a planet may be determined from three observations ; 

 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 appearances, 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 impossible 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. 37 



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 pre- 

 diction was naturally looked forwards to, as an additional stamp of the 

 truths of the theory of gravitation. 



' Bailly, ii. 246. 3 * Principia, ed. 1. p. 494. 35 Bailly, ii. 646. 



37 The importance of Halley's labors on Comets has always been acknowledged. 

 In speaking of Halley's Synopsis Astronomicce Cometic<x, Delambre says (Ast. xviii. 

 Siecle, p. 130), " Voila bien, depuis Kepler, ce qu'on a fait de plus grand, de plus 

 ocau, de plus neuf en astronomic." Halley, in predicting the comet of 1758, says, 

 if it returns, " Hooprimum ab homine Anglo iuventum fuisse non inficiabitur sequa 

 posteritas " 



