The Life and Legacy Of Astronomer Abraham Zacuto

A perk  of joining  Jewish heritage tours in Portugal is learning about the influencial Jews who made their mark in the country’s history. One such individual is Abraham Zacuto.

Abraham Zacuto

Abraham Zacuto (Hebrew: אברהם זכות‎)  was a Portuguese astronomer, astrologer, mathematician, rabbi and historian who served as Royal Astronomer in the 15th century to King John II of Portugal. The crater Zagut on the Moon is named after him.
Zacuto was born in Salamanca, Spain in August 12 1452. He may have studied and taught astronomy at the University of Salamanca. He later was a teacher of astronomy at the universities of Zaragoza and then Cartagena. He was versed in Jewish Law, and was rabbi of his community.

With the expulsion of the Jews from Spain in 1492, Zacuto took refuge in Lisbon, Portugal. Already famous in academic circles, he was invited to court and nominated Royal Astronomer and Historian by King John II of Portugal, a position which he held until the early reign of Manuel I. He was consulted by the king on the possibility of a sea route to India, a project which he supported and encouraged.

 Zacuto would be one of the few who managed to flee Portugal during the forced conversions and prohibitions of departure that Manuel I enacted nin order to keep the Jews in Portugal as nominal Christians for foreign policy reasons. He fled first to Tunis and later moved to Jerusalem. He died probably in 1515 in Jerusalem, however, other reports indicate his final home was in Damascus and the death occurred in 1520.

In a similar vein to other giants of the Jewish faith, such as Saadia, Maimonides and the Vilna Gaon, he followed the extremely old Jewish custom (believed to have begun in Babylon) of being buried as close to Jerusalem as possible. Zacuto had established his wish to make his death pilgrimage at a Passover gathering.

Zacuto’s Contributions

Zacuto developed a new type of astrolabe specialized for practical determination of latitude while at sea. This was in contrast to earlier multipurpose devices intended for use ashore. Abraham Zacuto's principal claim to fame is the great astronomical treatise written while he was in Salamanca. The treatise is named Ha-ḥibbur ha-gadol (Hebrew: החיבור הגדול‎) ("The Great Book") and was written from 1470 to 1478. It was composed of 65 detailed astronomical tables (ephemerides), with radix set in year 1473 and the meridian at Salamanca, charting the positions of the Sun, Moon and five planets.

Almanach Perpetuum

The calculations were based on the Alfonsine Tables and the works of earlier astronomers (notably of the 14th-century Majorcan school). Zacuto set out the data in a simple "almanac" format, with the positions of a planet easily interpolated between entries, making it quite easy to use.

Already in 1497, Vasco da Gama took Zacuto's tables and the astrolabe with him on the maiden trip to India. It would to be used by Portuguese ships thereafter to reach far destinations such as Brazil. Vasco da Gama and his crew underwent thorough briefing and preparation by Zacuto, in addition to learning to use the new instruments which he had developed for their trip before setting on the voyage to India.

Prior to that, Zacuto had again improved on the existing astronomical tables, mostly those prepared under King Alphonso X of Castille. Christopher Columbus had used Zacuto's tables as well. There is a story telling that on one of his voyages, when attacked by the natives, Columbus noted that Zacuto had predicted an eclipse for that day, and used this information to threaten the natives and convince them that he could extinguish the Sun and Moon and deprive them of all light. Zacuto's work saved the Columbus‘  life and that of his crew.

In 1504, while in Tunisia, Abraham Zacuto wrote a history of the Jewish people, Sefer yuḥasin (Hebrew: ספר יוחסין‎), since the Creation of the World until 1500, and several other astronomical/astrological treatises. The History was greatly respected and was reprinted in Cracow in 1581, at Amsterdam in 1717, and at Königsberg in 1857. A complete uncensored edition was published by Filipowski in London at 1857.

Annotations in Hebrew to chapter five of Sefer Yuhasin, were published by Yoel Lieberman in 2001 in a masters thesis called "A Record of Medieval Sages In Sefer Yuhasin of Rabbi Abraham Zacut". The book was translated into English and published in 2005 by the Zacuto foundation founded by Dr. Vladimir Rozenblit, a 20th generation direct descendant of Zacuto.

Salamanca, Spain

For more lessons on historical Jewish figures in Portugal, feel free to join one of the many kosher tours that will happily guide you around the sights and sounds of Portugal.