HALLEY'S COMET 495 



came by its name. Speaking generally, all great comets of 

 comparatively recent years bear, popularly, the name of their 

 discoverer, but Halley's comet is an exception to this rule. 

 Edmund Halley was Astronomer-Royal from 1720 to 1742, and 

 was a friend and contemporary of the great Isaac Newton, 

 who died in 1727. Newton had propounded the theory that 

 comets were members of the solar system and as such circled 

 round the sun. Halley, whose admiration for Newton almost 

 amounted to worship, was attracted by and determined to 

 investigate this theory (it is said that Newton was not fond 

 of "doing sums"; he had a mind above such trivialities), so 

 Halley applied it to a comet which appeared in 1682. The 

 result of his labours in this direction — labours which entailed 

 patient and prolonged research and investigation of records 

 of comets — was to prove the correctness of Newton's theory r 

 at least so far as this particular comet was concerned. Halley 

 obtained an elliptic orbit for this rover, and predicted its return 

 in 1758 or 1759. This was perhaps the boldest step ever 

 taken by an astronomer in the matter of prophecy. Whether 

 the knowledge in his case that in all human probability he 

 could not live to see his prediction fulfilled or falsified — he 

 would have been well over 100 in 1758 — stimulated his courage 

 to this unusual extent would be idle to speculate, and, in 

 view of what happened, to the last degree ungenerous to 

 suggest. 



The predicted return of this comet was anxiously awaited, 

 and when, on Christmas Day, 1758, it reappeared and was 

 recognised, Halley's boldness was vindicated ; since then the 

 visitor has always been associated with his name and known as 

 Halley's comet. 



Halley found that the period of this comet — that is, the 

 time occupied in making the circuit or round trip from the sun 

 to the utmost limits of the solar system beyond Neptune and 

 back again — was not constant, but varied from 75 to j6 years, 

 and he identified this comet with others which had appeared 

 in the years 1456, 1531 and 1607. Since his time further 

 researches have been made and much has been learnt as to 

 the causes of these variations. The period appears to vary 

 from 74 to 79 years, with an average of about 76! years ; the 

 variation is ascribed to perturbation due to the attraction of 

 the planets. Exhaustive investigations and examinations of 



