 Abu Ja‛far Yusuf ibn Hasday moved to the East and from 1121 to 1125 he worked for al-Ma’mun al-Bata’ihi, a vizier of the Fatimid Caliph al-Amir. He wrote for him commentaries on Hippocrates and was involved in building an observatory at Cairo known as the Ma’muni Observatory (Maqrizi 1913: 174–176). The observatory was demolished when the vizier fell into disgrace in 1125. Dunlop added a few names to the circle of Avempace, like Abu l-Hasan Ibn Judi, a younger friend, but Malik Ibn Wuhayb, ‛Ali Ibn al-Imam and Abu Ja‛far Ibn Hasday are the outstanding figures in his circle. See  Annex 1  for a discussion of sources on his life.