By the time Christopher Columbus sailed westward in 1492, navigators were already using hefty volumes containing astronomical tables to guide them across unknown seas. These books often included detailed instructions for manipulating navigational instruments and for computing geographical positions from celestial observations.
Columbus himself probably carried copies of two invaluable books. The "perpetual almanac" prepared by Abraham Zacuto contained more than 300 pages of astronomical tables that had already contributed to such navigational feats as Vasco da Gama's famous expedition from Portugal around the tip of Africa to India. The second volume, called the Ephemerides, had been produced by the prominent German astronomer and mathematician Johannes Müller, who went by the Latin name Regiomontanus.
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