This paper presents an analysis of the systematic astronomical observations performed
by Muh. y¯ı al-D¯ın al-Maghrib¯ı (d. 1283 AD) at theMaragha observatory (northwestern
Iran, ca. 1260–1320 AD) between 1262 and 1274 AD. In a treatise entitled Talkh¯ıs.
al-majist.¯ı (Compendium of the Almagest), preserved in a unique copy at Leiden,
Universiteitsbibliotheek (Or. 110), Muh. y¯ı al-D¯ın explains his observations and measurements
of the Sun, the Moon, the superior planets, and eight reference stars. His
measurements of the meridian altitudes of the Sun, the superior planets, and the eight
bright stars were made using the mural quadrant of the observatory, and the times
of their meridian transit using a water clock. The mean absolute error in the meridian
altitudes of the Sun is~3.1, of the superior planets ~4.6, and of the eight fixed
stars~6.2. The clepsydras used by Muh. y¯ı al-D¯ın could apparently fix time intervals
with a precision of±5 min. His estimation of the magnitudes of three lunar eclipses
observed in Maragha in 1262, 1270, and 1274 AD is in close agreement with modern
data.