Longitude by lunar distances involves using the relationship of the moon with stars as a giant clock. Measuring the Moon's position relative to stars lets time be read. The observation would be the same anyplace on earth at the same time. Thus taking this observation would determine the time at a standard meridian. Comparing it with local time will give the navigator longitude. Noting that the moon moves past the stars faster than the planets, astronomers proposed a way to find longitude by lunar distances in the early 16th century. The invention of the Hadley quadrant in 1731 made these accurate measurements possible at sea. Another requirement was accurate tables to predict where the moon was with respect to nearby stars or the sun at a standard meridian. One of the reasons for creating the Greenwich Observatory was to record the moon's location. There Tobias Mayer and Neville Maskelyne developed useful tables and a method to calculate longitude at sea. The purpose of the tables was to give the Greenwich Mean Time (GMT) for small angles between the moon and a few stars every three hours. The navigator, in computing his observed data, found the GMT that matched the observations. That time, then, was compared with the local time, often found with a noon sun sighting, and carried on a watch that did not need long-term accuracy as did a marine chronometer. Every hour of difference between the local time of the sighting and the GMT of the predicted time counted as 15° of longitude. The method generally could not compete with the precision provided by a chronometer, yet was popular in the 19th century before chronometers became relatively inexpensive. But it took a good navigator about four hours to complete the calculations. Happily, those calculations were simplified with a new method provided by America's most famous navigator, Nathaniel Bowditch, in 1802.