Ibn Yūnus’ Report on Early Islamic Observations for Determining the Rate of Precession of Equinoxes

The present article deals with Ibn Yūnus’ report about an astronomical parameter, so-called “precession of equinoxes”. In ancient astronomy the cause of precession was justified by different methods. The rate of precession was important for updating stars and planetary tables. Precession was discovered by Hipparchus and examined by Greek astronomers later. It apparently influenced the early generation of Muslim astronomers to repeat new observations for about three centuries. We will deal with the observational accuracy of Greek and Muslim astronomers in measuring the ecliptic longitude of Regulus, which was being employed to obtain the rate of Precession.