INDUCTIVE EPOCH OF HIPPARCHUS. 147 



its apogee precedes the summer solstice by 24-£ degrees nearly, the 

 zodiac containing 360." 



The exactness of the Solar Tables, or Canon, which was founded on 

 these data, was manifested, not only by the coincidence of the sun's 

 calculated place with such observations as the Greek astronomers of 

 this period were able to make (which were indeed very rude), but by 

 its enabling them to calculate solar and lunar eclipses ; phenomena 

 which are a very precise and severe trial of the accuracy of such tables, 

 inasmuch as a very minute change in the apparent place of the sun or 

 moon would completely alter the obvious features of the eclipse. Though 

 the tables of this period were by no means perfect, they bore with 

 tolerable credit this trying and perpetually recurring test ; and thus 

 proved the soundness of the theory on which the tables were calculated. 



The Moon's Eccentric. — The moon's motions have many irregulari- 

 ties ; but when the hypothesis of an Eccentric or an Epicycle had suf- 

 ficed in the case of the sun, it was natural to try to explain, in the 

 same way, the motions of the moon ; and it was shown by Hipparchus 

 that such hypotheses would account for the more obvious anomalies. 

 It is not very easy to describe the several ways in which these hypoth- 

 eses were applied, for it is, in truth, very difficult to explain in words 

 even the mere facts of the moon's motion. If she were to leave a vis- 

 ible bright line behind her in the heavens wherever she moved, the 

 path thus exhibited would be of an extremely complex nature ; the 

 circle of each revolution slipping away from the preceding, and the 

 traces of successive revolutions forming a sort of band of net-work run- 

 ning round the middle of the sky. 2 In each revolution, the motion in 

 longitude is affected by an anomaly of the same nature as the sun's 

 anomaly already spoken of; but besides this, the path of the moon 

 deviates from the ecliptic to the north and to the south of the ecliptic, 

 and thus she has a motion in latitude. This motion in latitude would 

 be sufficiently known if we knew the period of its restoration, that is, 

 the time which the moon occupies in moving from any latitude till 

 she is restored to the same latitude ; as, for instance, from the ecliptic 

 on one side of the heavens to the ecliptic on the same side of the 

 heavens again. But it is found that the period of the restoration of 

 the latitude is not the same as the period of the restoration of the 

 longitude, that is, as the period of the moon's revolution among the 



* The reader will find an attempt to make the nature of this path generally intel- 

 ligible in the Companion to the British Almanac for 1814. 



