The problem of measuring longitude plagued navigators, shippers, and the sovereigns who funded expeditions from the very first Atlantic crossings onward. Without using the horizon as a reference, the only way to establish the ship's position was to determine its geographic coordinates. While latitude was easy to measure, longitude remained a dilemma until the eighteenth century, when John Harrison's chronometer allowed a precise measurement of the time difference between two locations. As an alternative to mechanical clocks, and before their introduction on ships, the most common method used was the lunar distance method. In its revolution around the Earth, the Moon travels across the sky by about 13° a day, so it appears as the fastest-moving heavenly body. As we can predict the Moon's position relative to the stars, we can use its movement as an indicator of time. To perform the calculation, we need three elements: first, the positions of the stars that are near the Moon's path across the sky; second, tables that accurately predict lunar movements; third, adequate measuring instruments. By measuring the Moon's distance from a given star or planet and comparing it with the distance tabulated for a reference location, we can determine the time difference that allows the calculation of longitude. The first to apply the lunar distance method was Amerigo Vespucci (1454-1512), who, on August 23, 1500, used the conjunction of the Moon with Mars to determine longitude along the coast of Venezuela. Later, Petrus Apianus (1495-1552) illustrated the principles of the method in his Cosmography of 1524, and other astronomers sought to impose it as the means of resolving the controversial problem of longitude. In practice, however, the lunar distance method could not be applied effectively until 1767, when the astronomer Nevil Maskelyne (1732-1811) began the annual publication of the Nautical Almanac, which tabulated the position of the Moon at different times.