The Book of Intercalation: On the Christian Calendar

The Book of Intercalation (Sefer ha-‘ibbur) was one of the most influential medieval works on the Jewish calendar. In it, bar Ḥiyya presents the rules of the Hebrew calendar, detailed astrological and astronomical tables, an overview of the Egyptian and Roman calendars, theories of the seven climes of the habitable earth, and a variety of dates for historical events. These topics bring together themes found throughout bar Ḥiyya’s writings, particularly his concern with study of the heavens and its practical applications. The Book of Intercalation proved very popular among readers of Hebrew as a repository of knowledge developed by Jews in the Islamic world. These excerpts discuss the Christian calendar and the determination of the date for Christmas.