Ibn al-Ḥadib’s Tables for Finding True Syzygy

Abstract

Isaac ben Solomon Ibn al-Ḥadib (or al-Aḥdab) emigrated from Castile to Sicily no later than 1396. In astronomy, his most important work, written in Hebrew, is The paved way ( Oraḥ selula), a set of tables for the motions of the Sun and the Moon. Here, we focus attention on his unusual tables for finding the difference in time and the difference in longitude between mean and true syzygy, where syzygy refers to the conjunction and opposition of the Sun and the Moon. It is shown that he took into account the effect of Ptolemy’s second lunar model on the velocity of the Moon at syzygy, which was done by very few astronomers in the Middle Ages. It is also noteworthy that he took some parameters from the zij of al-Battānī and others from the Parisian Alfonsine Tables, using them inconsistently in these tables.