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The Babylonians, believing the year was 360 days, divided the circle into 360 degrees. Later, the Phoenicians sailed to destinations where they had noted the elevation of the north sky over the horizon in degrees. Still today, we use that to define a latitude; a parallel line where the north star is observed at the same altitude. For example, at latitude 35 north, the North Star is seen 35 degrees over the horizon. Finding a longitude, or a line crossing the latitude at right angle, in order to get a two-dimensional position, is another matter; it requires to measure an hour angle with an arbitrary place of reference, that has been in the past for different seafarers, Rome, Paris and even the Canary Islands from Christopher Columbus. But, today, it is internationally accepted as the Royal Observatory of Greenwich, in England. However, there weren't instrument capable to measure that time difference before at the end of the 18th century, John Harrison invented the marine chronometer. Before that, navigators only estimated the longitude from dead-reckoning position (estimated distance covered). To understand the importance of an accurate clockwork, you have to know that if 360 degrees is 24 hours of time, a minute of an hour is a distance of 15 nautical miles (roughly 28 km) at the equator. An error of say, five minutes, would put you at such an uncertain distance that it would be pointless to do the calculation. There was a previous method to the chronometer, though. We knew that the moons of Jupiter could be used as a celestial clock. It was only a matter of noting when the moons were passing behind the planet at the place of reference, to use it elsewhere as a clock. Incidentally, it is by computing such a table of Jupiter moons occlusion that the Danish astronomer Roemer discovered that the time changed from winter to summer and therefore assumed that it took a greater time for light to reach us and calculated - for the first time - the actual speed of the light. The only problem with Jupiter's moons is that it had to be observed through a telescope and that was not possible on the moving platform that a ship represented. The technique was only used when ashore. If you are interesting in navigation, I recommend to read the book, "Longitude" by Dava Sobel. A short but very well written story of the search for establishing the longitude. EDITED: I forgot to write the answer ... :-) ... A marine chronometer is an accurate timepiece; accurate enough to serve at finding a useful longitude.