H ALLEY'S COMET 



The " Principia " appeared in 1687. So far as the conclusions there 

 reached apply to the subject before us, the labor of adapting the theo- 

 retical results to numerical form and applying them to the practical 

 problem of orbit computation, fell to Edmund Halley. 



Halley, a contemporary and friend of Newton, occupies a very 

 prominent place in the history of astronomical and physical science. 

 Among other important discoveries and researches, may be mentioned 

 the long inequality of Jupiter and Saturn, the proper motions of the 

 stars, the secular acceleration of the moon's motion, method of deter- 

 mining the solar parallax by observations of transits of Venus, re- 

 searches on terrestrial magnetism, and his epoch-making work on the 



motions of comets. 



Halley's Comet. 



But perhaps Halley's most important work was the part which he 

 took in the publication of the " Principia." It was largely through 

 Halley's influence that Newton was persuaded to prepare this great 

 work for publication, and entirely at his expense that the printing was 

 finally done. 



Among the contemporaries of Halley, we find many astronomers 

 whose names have survived in history. We need only mention Flam- 

 steed, the first astronomer royal, Cassini, Hevelius, Koemer and 

 Huyghens. In his sixty-fourth year, Halley succeeded Flamsteed as 

 astronomer royal and, at once, boldly set about the task of observing 

 the moon through a complete revolution of the nodes, viz., a period of 



