In the early medieval period, several astronomical observatories were built in the Islamic world. The first of these was the 9th-century Baghdad observatory built during the time of the Abbasid caliph al-Ma'mun, though the most famous were the 13th-century Maragheh observatory, 15th-century Ulugh Beg Observatory.E. S. Kennedy (1962), by Aydin Sayili", Isis '53 (2): 237-239.