Short answers, pulled from the story.
Abraham Zacuto was a Sephardic Jewish astronomer, astrologer, mathematician, rabbi, and historian born in Salamanca on the 12th of August 1452. He is best known for his astronomical tables and a copper astrolabe that Vasco da Gama and Christopher Columbus used during the age of oceanic exploration, and for serving as Royal Astronomer to King John II of Portugal.
The Almanach perpetuum is the Latin title for Zacuto's principal astronomical work, published in Leiria, Portugal in 1496 by Samuel d'Ortas. It supplied the first accurate table of solar declination, allowing navigators to use the sun rather than the Pole Star to fix their position, which solved a critical problem for ships sailing toward the equator where the Pole Star disappears below the horizon.
Vasco da Gama and his crew underwent a direct briefing by Zacuto before their fleet departed in 1496, learning to use the new instruments he had developed. Already in 1497, da Gama used Zacuto's tables and the metal nautical astrolabe on his maiden voyage to India, and Portuguese ships continued to rely on them for subsequent voyages to Brazil and India.
On one of Columbus's voyages, when he was threatened by hostile natives, Columbus consulted Zacuto's astronomical tables and noted that an eclipse had been predicted for that day. He threatened to extinguish the Sun and Moon if the confrontation did not end. The eclipse arrived on schedule and Columbus and his crew were spared.
King Manuel I of Portugal enacted forced conversions of Jews and prohibited their departure, wanting them to remain as nominal Christians for foreign policy reasons. Zacuto was one of the few who escaped, fleeing first to Tunis and later moving to Jerusalem. He probably died in 1515 in Jerusalem, though some reports place his death in Damascus in 1520.
Sefer yuhasin is a history of the Jewish people written by Abraham Zacuto in Tunisia in 1504, spanning from the Creation of the World to the year 1500. It was widely reprinted, appearing in Cracow in 1581, Amsterdam in 1717, and Konigsberg in 1857, with a complete uncensored edition published by Herschell Filipowski in London in 1857.