VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. , 32^ 



it may seem strange, that a book so excellent in its kind, should not till now 

 have been printed in its native Greek, a language so peculiarly adapted to ma- 

 thematical purposes. But this present edition may make ample amends, the 

 paper and the elegance and correctness of the print being remarkable. 



Considerations on the Change of the Latitudes of some of the principal fixed 

 Stars. By Edmund Halley, R S. Sec. N'' 355, p. 736. 



Having of late had occasion to examine the quantity of the precession of the 

 equinoctial points, I took the pains to compare the declinations of the fixed 

 stars delivered by Ptolomy, in the 3d chapter of the 7th book of his Almag. as 

 observed by Timocharis and Aristyllus, near 300 years before Christ, and by 

 Hipparchus about 170 years after them, that is about 130 years before Christ, 

 with what we now find : and by the result of a great many calculations, I con- 

 cluded that the fixed stars in 1800 years were advanced somewhat more than 

 25 degrees in longitude, or that the precession is somewhat more than 50" per 

 ann. But that with so much uncertainty, by reason of the imperfect observa- 

 tions of the ancients, that I have chosen in my tables to adhere to the even 

 proportion of 5 minutes in 6 years, which from other principles we are assured 

 is very near the truth. But while I was on this inquiry, I was surprised to find 

 the latitudes of three of the principal stars in Heaven directly to contradict the 

 supposed greater obliquity of the ecliptic, which seems confirmed by the 

 latitudes of most of the rest : they being set down in the old catalogue, as if 

 the plane of the earth's orbit had changed its situation, among the fixed stars, 

 about 20' since the time of Hipparchus. Particularly all the stars in Gemini 

 are set down, those to the northward of the ecliptic, with so much less latitude 

 than we find, and those to the southward with so much more southerly lati- 

 tude. Yet the three stars Palilicium or the Bull's Eye, Sirius, and Arcturus 

 contradict this rule directly : for by it Palilicium, being in the days of Hip- 

 parchus in about 10° of Taurus, ought to be about 15 min. more southerly than 

 at present; and Sirius, being then in about 15^ of Gemini, ought to be 20' 

 more southerly than now ; yet on the contrary Ptolomy places the first 20' and 

 the other 22' more northerly in latitude than we now find them. Nor are these 

 errors of transcription, but are proved to be right by their declinations ^et down 

 by Ptolomy, as observed by Timocharis, Hipparchus and himself, which show 

 that those latitudes are the same as these authors intended. As to Arcturus, he 

 is too near the equinoctial colure, to argue from him concerning the change of 

 the obliquity of the ecliptic ; but Ptolomy gives him 33' more north latitude 

 than he has now ; and that greater latitude is likewise confirmed by the de- 

 clinations delivered by the said observers. So then all these three stars are 



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