A HISTORY OF SCIENCE 



idea of thus determining the parallax of the planets 

 originated, or at least was developed, by Halley, and 

 from this phenomenon he thought it possible to con- 

 clude the dimensions of all the planetary orbits. As 

 we shall see further on, his views were found to be 

 correct by later astronomers. 



In 1721 Halley succeeded Flamsteed as astronomer 

 royal at the Greenwich Observatory. Although sixty- 

 four years of age at that time his activity in astronomy 

 continued unabated for another score of years. At 

 Greenwich he undertook some tedious observations 

 of the moon, and during those observations was first 

 to detect the acceleration of mean motion. He was 

 unable to explain this, however, and it remained for 

 Laplace in the closing years of the century to do so, 

 as we shall see later. 



Halley's book, the Synopsis Astronomic? Cometicce, 

 is one of the most valuable additions to astronomical 

 literature since the time of Kepler. He was first to 

 attempt the calculation of the orbit of a comet, having 

 revived the ancient opinion that comets belong to the 

 solar system, moving in eccentric orbits round the sun, 

 and his calculation of the orbit of the comet of 1682 led 

 him to predict correctly the return of that comet in 

 1758. Halley's Study of Meteors. 



Like other astronomers of his time he was greatly 

 puzzled over the well-known phenomena of shooting- 

 stars, or meteors, making many observations himself, 

 and examining carefully the observations of other 

 astronomers. In 1714 he gave his views as to the 

 origin and composition of these mysterious visitors 

 in the earth's atmosphere. As this subject will be 



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