The Zij al-Shah is a compilation of astronomical tables produced under the patronage of the Sasanian king Khosrow I Anushirvan (r. 531-579 AD), synthesizing three traditions: Babylonian planetary-prediction astronomy, the mathematical geometry of Ptolemy's Almagest (translated into Persian from Greek via Syriac), and the numerical parameters of Siddhantic Indian astronomy. It constitutes the first documented attempt at a systematic synthesis of the three great astronomical traditions of antiquity. When the Umayyads and later the Abbasids conquered Persia, the Zij al-Shah was the immediately available reference astronomical text: its translation into Arabic in the 8th century under Al-Mansur and Al-Ma'mun was the starting point of classical Islamic astronomy. Al-Battani, Al-Biruni, and Abu al-Wafa built their systems on the parameters and format of the Zij al-Shah. The historical canon presents Islamic astronomy as a direct heir of Greece, ignoring that the real intermediary was Sasanian Persia. The original work in Middle Persian (Pahlavi) was lost; it is known only through summaries and references in Arabic authors.