Work
Zacuto developed a new type of astrolabe, specialized for practical determination of latitude while at sea, 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, in Hebrew, with the title Ha-ḥibbur ha-gadol (Hebrew: החיבור הגדול) ("The Great Book"), begun around 1470 and completed in 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. The calculations were based on the Alfonsine Tables and the works of earlier astronomers (notably of the 14th C. 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.
The first Castilian translation was undertaken in 1481 by Juan de Salaya. Zacuto's Portuguese disciple Joseph Vizinus (Mestre José Vizinho, the much-valued physician and advisor of John II of Portugal) adapted it into a Latin translation, under the title Tabulae tabularum Celestium motuum sive Almanach perpetuum ("Book of Tables on the celestial motions or the Perpetual Almanac"), immediately along with a new Castilian translation, and arranged for its publication in 1496 by Abraão de Ortas in Leiria, Portugal. (one of the first books published in Portugal with a movable type printing press).
Zacuto's Almanach perpetuum (or Biur Luhoth) helped immediately revolutionize ocean navigation. Prior to the Almanach, navigators seeking to determine their position in the high seas had to correct for "compass error" (the deviation of the magnetic north from the true north) by recourse to the quadrant and the Pole Star. But this proved less useful as they approached the equator and the Pole Star began to disappear into the horizon. Zacuto's Almanach supplied the first accurate table of solar declination, allowing navigators to use the sun instead. As the quadrant could not be used to look directly at the sun, Portuguese navigators began using the astrolabe on board (an old land-based instrument to measure the height of the sun indirectly). Zacuto's tables in conjunction with the new metal nautical astrolabe allowed navigators to take accurate readings anywhere. Already in 1497, Vasco da Gama took Zacuto's tables and the astrolabe with him on the maiden trip to India. It would continue to be used by Portuguese ships thereafter to reach far destinations such as Brazil and India.
Abraham Zacuto might have an uncredited appearance in Luís de Camões's 1572 epic poem, The Lusiad, as the unnamed "old man of Restelo beach", a Cassandra-like character that surges forward just before Vasco da Gama's departure to chide the vanity of fame and warn of the travails that await him (Canto IV, v.94-111). This may be Camões' poetic interpretation of an alleged meeting (reported in Gaspar Correia) between Vasco da Gama and the older Abraham Zacuto at a monastery by Belém beach, just before his fleet's departure, in which Zacuto reportedly gave Gama some final navigational tips and warned him of dangers to avoid.
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, while a complete edition was published by Filipowski in London at 1857. Annotations in Hebrew to chapter five of "Sefer Hayuhasin", were published by Yoel Lieberman in 2001 in a masters thesis called "A Record of Medieval Sages In Sefer Yuchasin 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.
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