SEQUEL TO THE EPOCH OF HIPPARCHUS. 241 



neously appreciated ; namely, the Trepidation of 

 the fixed stars. Arzachel conceived that a uniform 

 Precession of the equinoctial points would not ac- 

 count for the apparent changes of position of the 

 stars, and that for this purpose, it was necessary 

 to conceive two circles of about 8 degrees radius 

 described round the equinoctial points of the im- 

 moveable sphere, and to suppose the first points of 

 Aries and Libra to describe the circumferences of 

 these circles in about 800 years. This would pro- 

 duce, at one time a progression, and at another 

 a regression, of the apparent equinoxes, and would 

 moreover change the latitudes 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 unpro- 

 gressive character of Arabian science has been 

 pointed out recently by M. Sedillot". It appears 

 that Mohammed-Aboul Wefa-al-Bouzdjani, an Ara- 

 bian astronomer of the tenth century, who resided 

 at Cairo, and observed at Bagdad in 975, dis- 

 covered a third inequality of the moon, in addition 

 to the two expounded by Ptolemy, the Equation 

 of the Center, and the Evection. This third inequa- 

 lity, the Variation, is usually supposed to have 

 been discovered by Tycho Brahe, six centuries 



11 Sedillot, Nouvelles Rech. sur 1'Hist. de 1'Astron. chez lea 

 Arabes. Nouvean Journal Asiatique. 1836. 



VOL. I. R 



