Longitude measures the angular distance of a point from the reference meridian. Today, by convention, we refer to the meridian passing through the Greenwich Observatory, in England. In the age of the great geographic discoveries, however, the zero meridian passed through the Canary Islands, formerly known as the Fortunate Islands, which, in Ptolemy's time, marked the western boundary of the known world. For centuries, measuring longitude was an intractable problem, and this caused tremendous difficulties particularly in marine navigation. After the discovery of the New World, the development of an effective method for determining longitude became an ever more urgent need. Amerigo Vespucci (1454-1512) obtained good results by observing lunar eclipses, but frequent measurement errors often caused huge human and material losses. Maritime powers promised prizes and incentives to whoever could find a practical solution. The cosmographer Reiner Gemma Frisius (1508-1555) was the first to propose the use of portable clocks that could show the time of the port of departure on board, which navigators could then compare with the time calculated on the ship using astronomical instruments. However, the mechanical clocks of that period were not accurate enough to ensure a good result. Galileo Galilei (1564-1642) improved their efficiency by applying the pendulum. He believed he could solve the longitude problem by measuring the periods and eclipses of Jupiter's satellites. The solution was not found until the eighteenth century, when John Harrison built a precision chronometer that made it possible to determine the exact time difference between the port of departure and the ship's position. Given that every hour represents one-twenty-fourth of 360°, or 15°, mariners could simply multiply the length of a degree at a given parallel, typically expressed in miles or leagues, by the number of degrees corresponding to the time difference.