astronomical tables (ephemerides), with radix set for the year 1473 and with 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-century 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) translated it into Latin, under the title Tabulae tabularum Celestium motuum sive Almanach perpetuum ("Book of Tables on the celestial motions or the Perpetual Almanac"), along with a new Castilian translation, and arranged for its publication in 1496 by Samuel d'Ortas in Leiria, Portugal. (one of the first books published in Portugal with a movable type printing press). Biur luḥot Zacuto's Almanach perpetuum (or Biur luḥot) revolutionised ocean navigation. Prior to the Almanach, navigators seeking to determine their position on 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 under 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 his maiden trip to India. It continued to be used by Portuguese ships thereafter to reach far-off destinations such as Brazil and India. Vasco da Gama and his crew underwent a thorough briefing and preparation by Zacuto. They learnt how to use the new instruments which he had developed for their trip before they set off on their voyage to India in 1496. Prior to that, Zacuto had further improved existing astronomical tables, mostly those prepared under King Alphonso X of Castille. Already Columbus had used Zacuto's tables.