^{The Almanach Perpetuum. Printed in Leiria, 1496. In the Portuguese National Library, Lisbon.}

On February 25, 1496, in the printing shop of the D’Ortas family in Leiria, Portugal, the first ever printing of the Almanach Perpetuum, was completed. This was the last book printed in a Hebrew printing shop in the Iberian Peninsula. It was the only incunabula printed in an Iberian Hebrew printing shop in a language other than Hebrew. It was the first scientific book printed in Portugal. It was by far the most influential Portuguese incunabula in any language or any genre. On December 5, 1496, a mere ten months later, the Portuguese monarch D. Manuel I decreed the expulsion of all Jews from Portugal - the native community plus a massive number of Spanish Jewish refugees in the aftermath of the Expulsion in 1492. This effectively triggered the immediate confiscation of Jewish ritual objects and Hebrew books and the systematic dismantling of Jewish institutions. Organized Jewish life in the Iberian Peninsula would soon cease to exist.

The Almanach Perpetuum is a rendition, with versions in both Latin and Spanish, of Chibbur HaGadol, the Hebrew language mathematics and astronomy masterpiece of R. Abraham Zacuto (1452–c. 1515), written in Salamanca, Spain, between 1473 and 1478. The bulk of the book is a set of astronomical tables providing information necessary to perform various types of astronomical calculations. R. Zacuto, who himself fled to Portugal in 1492, is identified in the colophon as the “astronomer of the highly serene King Manuel, King of Portugal”. The original text in Hebrew was translated into Latin and then into Spanish by Master José Vizinho, a Jewish mathematician, astronomer and medical doctor, who the colophon identifies as a disciple of R. Zacuto.

^{First page of the Almanach Perpetuum. Printed in Leiria, 1496. In the Portuguese National Library, Lisbon.}

The Almanach Perpetuum addresses a key scientific issue created by the geopolitical situation at the time. The conquest of Constantinople by the Ottoman Empire in 1453 disrupted the European land trade routes with Asia. As such, the quest for alternative maritime routes was of the greatest strategic importance. Yet, the limitations of the existing navigation techniques, which were designed for sailing along the coast or along known routes, and were based on polar observations, placed serious limitations. They did not work for high seas navigation, or for unknown routes, or in places close to the equator where polar observations were not possible, all characteristics of the new maritime voyages. R. Zacuto’s work revolutionized ocean navigation by providing in this work the first tables allowing the use of the position of the sun for location guidance. At the same time, and given the obvious difficulties in measuring the position of the sun by direct observation, R. Zacuto developed a new metal nautical astrolabe to generate indirect measurements.

The Almanach Perpetuum and the new astrolabe constituted the global positioning system of the Iberian navigators. In Spain, Christopher Columbus from his 1492 voyage on used these tables extensively. Master Vizinho had brought them to his attention when he visited Lisbon a few years prior. A personal copy of the tables belonging to Columbus with his handwritten notes is extant in the Colombiana Library in Seville, Spain. In Portugal, the tables and astrolabe were extensively used by the navigators, first in experimental exploits often under the direct guidance of Master Vizinho, and then, after 1497, directly by Vasco da Gama. Until the middle 1500s, these tables were the basic resource for location guidance when sailing to the Americas or to India.