🇵🇹 Abraham Zacuto

Abraham bar Samuel bar Abraham Zacuto was born around 1452 in Salamanca, in the kingdom of Castile, into a Sephardic Jewish family. He was educated in the Jewish tradition and also in the sciences, and he is thought to have studied and taught astronomy in connection with the University of Salamanca, later teaching at other centres as well. He was at once a rabbi, an astronomer, a mathematician, and a historian, a combination characteristic of the learned Jewish world of medieval Iberia.

Zacuto's central scientific work was a set of astronomical tables. Written originally in Hebrew, his Ha-hibbur ha-gadol, the Great Composition, contained dozens of detailed tables charting the positions of the sun, the moon, and the five known planets, calculated for the meridian of Salamanca. The tables let an astronomer or navigator determine celestial positions for any date, a perpetual almanac.

The work reached its famous form through translation. Zacuto's Portuguese disciple, the cosmographer Joseph Vizinho, translated the tables into Latin and Castilian under the title Almanach Perpetuum, the Perpetual Almanac, which was printed in Leiria, Portugal, in 1496, one of the earliest scientific books printed in the country. In this form Zacuto's astronomy passed directly into the hands of navigators.

Its importance for navigation was decisive. Earlier sailors had relied on the Pole Star to estimate latitude, but the Pole Star sinks toward the horizon and finally disappears as a ship crosses into the southern hemisphere, exactly where Portuguese explorers were heading. Zacuto's tables provided accurate values for the sun's declination, allowing navigators to determine their latitude from the height of the noon sun instead. Combined with the new metal nautical astrolabe, this gave Portuguese pilots a reliable way to fix their position anywhere on the globe.

When the Catholic Monarchs of Spain expelled the Jews under the Alhambra Decree of 1492, Zacuto fled to Portugal. There King John II welcomed him to court, and he was appointed Royal Astronomer and chronicler. He advised the Portuguese crown on questions of navigation and exploration, and tradition holds that he was consulted in the preparations for Vasco da Gama's expedition to India. Da Gama's pilots carried Zacuto's tables and the astrolabe on the voyage of 1497 to 1498 that opened the sea route to India, and Christopher Columbus is said to have owned and used an annotated copy of the almanac as well, reportedly using it to predict a lunar eclipse and overawe islanders in Jamaica.

Zacuto's refuge in Portugal did not last. In 1496 and 1497 King Manuel I ordered the forced conversion of Portugal's Jews and forbade them to leave. Zacuto was among the few who managed to escape. He fled first to Tunis in North Africa, where he wrote a major chronicle of Jewish history and tradition, the Sefer Yuchasin, the Book of Lineages, and later moved on toward the eastern Mediterranean, dying around 1515, by most accounts in Damascus or Jerusalem.

It is worth pausing on the irony of his life. Zacuto's tables made possible the very voyages that built Portuguese wealth and power, and the Portuguese crown honoured him for them, yet the same crown that employed him also ordered the persecution that drove him into permanent exile. He was indispensable and disposable at once, a pattern that recurs across the history of Iberia's Jewish scholars in this period. His expertise was welcomed; his identity was not.

His instruments and tables outlived the persecutions that drove him. The improved astrolabe associated with him and his solar declination tables remained in use by Spanish and Portuguese navigators for generations, the practical foundation of celestial navigation in the Age of Discovery. Long after his death, pilots who had never heard his name were still steering by his numbers.

Abraham Zacuto is remembered today as the scholar who gave the great ocean voyages their astronomy. A lunar crater bears his name, his story is told as part of both the history of science and the history of the Sephardic diaspora, and he stands as a reminder that the Portuguese discoveries rested not only on ships and courage but on the quiet, exact work of a refugee mathematician with a book of tables. The map of the early modern world was drawn, in part, with his arithmetic, the work of a scholar who never commanded a ship yet shaped every voyage.