As we have noted, when mariners advanced below the equator they could no longer see the North Star, and so had to find another way to determine their latitude. To solve this problem King John, like Prince Henry, collected experts from everywhere, and set up a commission headed by two learned Jewish astrologer-mathematicians – a Portuguese dividend from the persecutions across the border in Spain. In 1492, when the Spanish inquisitor-general Torquemada gave Jews three months to convert to Christianity or leave the country, the brilliant Abraham Zacuto left the University of Salamanca and was welcomed to Portugal by King John II. Zacuto’s disciple at Salamanca, Joseph Vizinho, had already accepted the King’s invitation ten years before, and in 1485 had been sent out on a voyage to develop and apply the technique of determining latitude by the height of the sun at midday. He was to accomplish this by recording the declination of the sun along the whole Guinea coast. The most advanced work for finding position at sea by the declination of the sun, as would be necessary in sailing below the equator, was the Almanach Perpetuum which Zacuto had written in Hebrew nearly twenty years before. After Vizinho translated these tables into Latin, they guided Portuguese discoverers for a half-century.

“The Portuguese Discoverers”, from “The Discoverers”, Daniel J. Boorstin, The National Board for the Celebration of Portuguese Discoveries, Lisbon, 1987