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XXIII. 



HIPPAECHUS AKD THE PEECESSION OE THE EQUINOXES. 

 By KEV. maxwell H. CLOSE, M.A. 



[EeadMAY 13, 1901.] 



We assume, in agreement with, the general belief, that it was the 

 admirable Hipparchus who first discovered what we now designate as 

 the Precession of the Equinoxes. This has, indeed, been questioned 

 by a few writers. But the elaborate discussion of this matter by 

 Th. Henri Martin^ seems to have thoroughly vindicated the claims of 

 Hipparchus in this respect. 



The immediate, prime particular of his great discovery was the 

 increase of the longitudes of certain fixed stars dtuing the period 

 between the observations of those stars made by Timocharis and 

 Aristyllus, on the one hand, and himself, on the other. The longitudes 

 had increased by nearly two degrees. 



This increase of the longitudes of the fixed stars might conceivably 

 be produced in one of three different ways ; viz. — 



(1) By the eastward progression of the stars themselves. 



(2) By the westward retrogression of the equinoctical points, from 



one of which the longitudes are measured. 



(3) By both these movements existing together, each producing 



its own share of the whole precession. 



This last may be dismissed at once for its unnecessary complexity ; 

 especially as we do not know of more than two writers^ who have 

 seriously entertained it. The choice, then, lies between the first and 

 the second ; that is between the progression of the fixed stars, and the 

 retrogression of the equinoctial points. 



If we were concerned simply with the increase of the longitudes 

 of the stars, or the relative motion between the stars and the 

 equinoctial points, it would be of little importance which of the 

 alternatives we selected. But there is here vastly more than a 

 mere question of relative motion. The progression of the stars 

 leaves the Earth, unmoved ; but the retrogradation of the equinoctial 



1 "Mems. de I'Acad. des Inscripts., torn, viii., part 1, 1869. 



- Messahalah, a Jew patronized by the Khalif Al Mamun, fl. about a.d. 800. 

 In De Scientia Motus Orbis, or De Elementis et Orbibiis Ccelestibus. Also Fracasto- 

 rius, "Homocentrica," Sectio ii. 



