The quest for longitude was the most crucial problem in the art of navigation, often determining the success of voyages and the survival of seamen. In 1530 the cosmographer Reiner Gemma Frisius (1508-1555) proposed a method based on the difference between the time at the port of departure, retained aboard ships by mechanical clocks, and the local time measured with astronomical instruments. Each hour of difference equalled fifteen degrees of longitude, and each degree of longitude was equivalent to a continuously varying distance from the Pole to the Equator.