SEQUEL TO THE EPOCH OF HIPPAECHUS. 179 



Hgree with Delambre, in doubting or extenuating the claim of Alba- 

 tegnius to this discovery, on the ground of his not having expressly 

 stated it. 



In detecting this motion, the Arabian astronomers reasoned rightly 

 from facts well observed : they were not always so fortunate. Arzachel, 

 in the llth century, found the apogee of the sun to be less advanced 

 than Albategnius had found it, by some degrees ; he inferred that it 

 had receded in the intermediate time; but we now know, from an ac- 

 quaintance with its real rate of moving, that the true inference would 

 have been, that Albategnius, whose method was less trustworthy than 

 that of Arzachel, had made an error to the amount of the difference 

 thus arising. A curious, but utterly false hypothesis was founded on 

 observations thus erroneously appreciated ; namely, the Trepidation of 

 the fixed stars. Arzachel conceived that a uniform Precession of the 

 equinoctial points would not account for the apparent changes of posi- 

 tion of the stars, and that for this purpose, it was necessary to conceive 

 two circles of about eight degrees radius described round the equinoc- 

 tial points of the immovable sphere, and to suppose the first points of 

 Aries and Libra to describe the circumference of these circles in about 

 800 years. This would produce, at one time a progression, and at 

 another a regression, of the apparent equinoxes, and would moreover 

 change the latitude of the stars. Such a motion' is entirely visionary ; 

 but the doctrine made a sect among astronomers, and was adopted in 

 the first edition of the Alphonsine Tables, though afterwards rejected. 



An important exception to the general unprogressive character of 

 Arabian science has been pointed out recently by M. Sedillot. 45 It 

 appears that Mohammed- Aboul Wefa-al-Bouzdjani, an Arabian astron- 

 omer of the tenth century, who resided at Cairo, and observed at 

 Lao-dad in 975, discovered a third inequality of the moon, in addition 

 to the two expounded by Ptolemy, the Equation of the Centre, and 

 the Evection. This third inequality, the Variation, is usually sup- 

 posed to have been discovered by Tycho Brahe, six centuries later. It- 

 is an inequality of the moon's motion, in virtue of which she moves 

 quickest when she is at new or full, and slowest at the first and third 

 quarter ; in consequence of this, from the first quarter to the full, she 

 is behind her mean place ; at the full, she does not differ from her 

 mean place ; from the full to the third quarter, she is before her true 



43 Sedillot, Nouvelles Kech. sar 1'Hist. cle 1'Astron. cliez les Arabes. Xouveau 

 Journal A*iatique. 1S36. 



