A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT) and the time at the current location found from observations of celestial bodies.

What does a chronometer measure?

a timepiece or timing device with a special mechanism for ensuring and adjusting its accuracy, for use in determining longitude at sea or for any purpose where very exact measurement of time is required.

How was longitude determined?

In principle, you could tell your longitude by observing the angle between the Moon and a particular star then consulting an almanac, which catalogued the time at Greenwich based on the position of a range of celestial objects. And then your longitude can be determined by comparing Greenwich time with your local time.