A chronometer is a navigational tool used on sailing vessels to measure accurately the time of a known fixed location in order to determine the ship’s longitude. The angle between the sun and the horizon (or if taking a reading by night the angle of the stars above the horizon) determines the latitude – a measurement taken by a sextant another navigational tool but longitude is a more difficult measurement.
To determine longitude the navigator needs to know the time difference between where they are standing and some other fixed point. In an hour the earth will rotate 15° (360° every 24 hrs) so if the time difference between you and a fixed point is 2hrs, then you know that you are 30° east or west of that point. In 1884 Greenwich, England became the official Meridian Line for all points around the Globe and GTM (Greenwich Mean Time) became the official time for the entire world.
On knowing the latitude and longitude the navigator then goes to his nautical charts and using his parallel rule can pin point his exact position.