# Ibn Yunus and Astronomical Tables

The field in which Ibn Yunus excelled was in preparing astronomical tables. His al-Zij al-Hakimi al-kabir is aptly named (al-kabir means 'large'). This zij, or astronomical handbook with tables, was dedicated to his patron the Caliph al-Hakim. The Hakimi Zij is unusual in that it records a large number of observations, both Ibn Yunus' own and those of previous observers. This large Zij covers most areas of interest to the medieval Islamic astronomer, starting with a chapter on the Muslim, Coptic and Syrian calendars, with detailed instructions for converting a date from one to any of the others, together with tables for determining the dates of Lent and Easter in both the Syrian and Coptic calendars. The following eighty chapters include sophisticated sections on spherical astronomy and sundial theory; tables and instructions relating to the determination of solar, lunar and planetary longitudes and latitudes; discussions on solar and lunar distances; and elegant solutions to many problems, such as finding the meridian, as well as, of course, methods for the determination of the qibla (the direction of Mecca)Ibn Yunus' second major work was part of the corpus of spherical astronomical tables for time keeping used in Cairo until the nineteenth century. It is not certain how many tables in this corpus, the Kitab ghayat al-intifa' ('Very Useful Tables') were actually computed by Ibn Yunus, but the tables are generally accurately computed and all based on parameters (the value of 30;0° for the latitude of Cairo and 23;35° for the obliquity of the ecliptic) calculated by Ibn Yunus.

Ibn Yunus' influence, in the form of his methodology and parameters, as well as actual observations, can be seen in many later Islamic tables, such as al-Tusi's Ilkhani zij 250 years later, where Ibn Yunus' values for the longitudes of the sun and moon are used.