Sepher Ibronoth [astronomy and calculations of intercalation and the Jewish calender]

The Jewish calendar is lunisolar; the months are reckoned by the moon and the year by the sun. The expertise required to make the necessary adjustments mandated an interest in astronomy by religious Jews. Maimonides devotes an entire section of his Mishneh Torah to what he lists as a positive commandment “to know how to determine which is the first day of each month of the year.” Sepher Ibronoth was prepared by Beilin as a manual “to help an individual fulfill these commandments.” See A. J. Karp, From the Ends of the Earth-Judaic Treasures of the Library of Congress, (1991) pp.197-8