August 12: The Royal Astronomer

Abraham Zacuto, royal astronomer to King John II of Portugal and a rabbi in his Sephardic community, was born in Salamonica, Spain on this date in 1452. When the Jews were expelled from Spain in 1492, Zacuto emigrated to Lisbon, along with his reputation, and served as the royal astronomer until the ascension of Manuel I in 1495. (Among the projects Zacuto worked on was determining a maritime route to India that would extend the trading power of the growing Portuguese empire; Vasco da Gama would find such a route in 1498.) When Manuel I forced Jews to convert or face death or expulsion, Zacuto fled to Tunis, and later moved to Jerusalem, where he may have died in 1515 (some sources say he died in Damascus in 1520). Zacuto’s accomplishments included development of a specialized type of astrolabe for the determination of latitude while at sea, and a famous astronomical treatise, The Great Book, written in Hebrew, with sixty-five astronomical tables charting the positions of the Sun, Moon and five known planets. His tables allowed navigators to use the Sun and the constellations as their guide, which meant accurate navigational readings wherever they were; Vasco da Gama used Zacuto’s tables and the astrolabe on his first trip to India. While in Tunisia, Zacuto wrote a history of the Jewish people that was reprinted numerous times into the 19th century. The Moon’s crater Zagut is named for Zacuto.

“There was a flourishing Jewish community in Tunis at that time, under the leadership of the pious and energetic Rabbi Shimon Duran. Rabbi Abraham Zacuto was welcomed with open arms. During the next few years of peace, Rabbi Abraham Zacuto wrote his famous Sefer Hayuhasin, a chronological history of the Jews from the Creation of the world to his day. For a long time it was one of the few sources of post-Biblical Jewish history, covering the period from the Babylonian exile to the Middle Ages.” --Nissan Mindel