A Short History of Astronomy 



[CH. IX. 



pole B from the centre of the earth o, then the actual 

 surface of the earth extends at the equator beyond this 

 circle as far as A, where, according to Newton, a\ is about 

 -2^ of o B or o A, and according to modern estimates, based 

 on actual measurement of the earth as well as upon theory 

 (chapter x,, 221), it is about ^-g- of o A. Both Newton's 

 fraction and the modern one are so small that the resulting 

 flattening cannot be made sensible in a figure; in fig. 72 



FIG. 72. The spheroidal form of the earth. 



the length #A is made, for. the sake of distinctness, nearly 

 30 times as great as it should be. 



Newton discovered also in a similar way the flattening 

 of Jupiter, which, owing to its more rapid rotation, is 

 considerably more flattened than the earth ; this was also 

 detected telescopically by Domenico Cassini four years 

 after the publication of the Principia. 



1 88. The discovery of the form of the earth led to 

 an explanation of the precession of the equinoxes, a 

 phenomenon which had been discovered i, 800 years before 



