74 Mr Pearson, On some ^points in the history of Astronomy. [May 21, 



Lyra, or Vega, was last visible when setting in the evening, about 

 Feb. 1. "Ubi est hodie, quae Lyra fulsit heri?"' Employing again 

 the method of calculation indicated above, we find on that day at 

 Rome the Sun would set about 5.10 p.m., and Lyra about 5.44. 

 As the days at that time of the year are rapidly lengthening, while 

 the star would set earlier every day, it is obvious that the date as- 

 signed for the last appearance of the latter is neai"ly exact. 



He makes however a remark about Capella which seems really 

 erroneous. He says (Fasti v. 113) that she rises on May 1st, i.e. is 

 then first visible in the morning. But at the time when Ovid lived 

 she would, according to the mode of computation used in the pre- 

 vious examples, have risen about 3.0 a.m., while the Sun would not 

 have risen until after 5.0. We have a similar apparent mistake in 

 Pliny and Columella, nearly contemporaries, who flourished in the 

 latter half of the first century a.d. They fix Arcturus' rising for 

 the 23rd or 21st of February; whereas on those days the Sun 

 would set at Rome about 5.35 p.m., while the star would not 

 pass the horizon in their time before 6.30 p.m. They seem to 

 have copied from Hesiod without any thought \ 



The late Mr F. Baily, in his edition of Ancient Star Catalogues, 

 published in Vol. xiii. of the Memoirs of the Royal Ast. Society, 

 does not seem to have actually compared the positions there given 

 to any of the principal stars with those which in the present day 

 we must suppose them to have then occupied, though he refers 

 to Delambre {Hist. Ast. Anc. Vol. Ii.), who gives tabulated results 

 on this point from his own calculations. As however the present 

 rate of change in the obliquity of the ecliptic would have made 

 it in the time of Eratosthenes (230 B.C.) about 23" 43', whereas 

 that astronomer fixed it roughly at 23° 51', it is to be hoped 

 that, making allowance for inaccuracies in the MSS., such a 

 process of verification may be attempted with some prospect of 

 success; and possibly some explanation found of Ptolemy's idea, 

 that in his time (a.d. 140) the amount of annual precession was 

 only 36". It is curious that the error of 15' in the latitude of 

 Alexandria, which Delambre imputes to the Greeks, answers 

 nearly exactly to the obliquity of 23** 43\ to which we are brought 

 by its present known rate of change. 



Mr Glaisher also communicated to the Society a sketch of 

 his Table of Exponential Functions, an abstract of- which cannot be 

 conveniently given, but which will probably appear in the next 

 part of the Society's Transactions. 



1 In the time of Ovid the position of Vega must liave been about E.A. 171i. 

 29m., Dec. 38o 23' N.; that of Capella, E.A. 2h. 55m., Dec. 40o 35' N. 



