 In his letter to Abu Ja‛far Yusuf Ibn Hasday (IB-coll-alawi: 77–81), Avempace blames the astronomer az-Zarqalluh for not having properly understood astronomy and attacks him for his “Treatise to refute the method employed by Ptolemy to calculate the apogee of Mercury”. This work of az-Zarkalluh is not extant. J. Samsó informs us that Ptolemy’s calculation of the apogee was wrong by approximately 30º and that az-Zarkalluh calculated the apogee in another work only with an error of 10º (Samsó 1993–1994: 676–677). Avempace, however, criticizes the method that az-Zarkalluh employed.