234 THE GREEK ASTRONOMY. 



Sect, 7. Conclusion of the History of Greek 

 Astronomy. 



I MIGHT now proceed to give an account of 

 Ptolemy's other great step, the determination of 

 the Planetary Orbits ; but as this, though in itself 

 very curious, would not illustrate any point beyond 

 those already noticed, I shall refer to it very 

 briefly. The planets all move in ellipses about 

 the sun, as the moon moves about the earth; 

 and as the sun apparently moves about the earth. 

 They will therefore each have an Elliptic Inequality 

 or Equation of the center, for the same reason 

 that the sun and moon have such inequalities. 

 And this inequality may be represented, in the 

 cases of the planets, just as in the other two, by 

 means of an eccentric ; the epicycle, it will be re- 

 collected, had already been used in order to repre- 

 sent the more obvious changes of the planetary 

 motions. To determine the amount of the Eccen- 

 tricities and the places of the Apogees of the 

 planetary orbits, was the task which Ptolemy un- 

 dertook; Hipparchus, as we have seen, having 

 been destitute of the observations which such 

 a process required. The determination of the 

 Eccentricities in these cases involved some pecu- 

 liarities which might not at first sight occur to the 

 reader. The elliptical motion of the planets takes 

 place about the sun; but Ptolemy considered their 

 movements as altogether independent of the sun. 





