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As long ago as 1514, people had ideas that the Moon's movement could be used to measure longitude at sea (how far east or west your position is). In fact, the Royal Observatory in Greenwich was founded to put this theory to the test, collecting the astronomical data needed to help sailors find their position.
This culminated in the publication of the first Nautical Almanac, which, combined with navigational tools like a sextant, could be used to determine a ship's longitude with significant accuracy - a method sailors still use to this day.
