This clock, called Harrison's H5 Chronometer, was one of the first clocks ever invented that could keep time on a moving ship.
Did you know that Columbus could find his latitude, but had NO WAY to find his longitude? None of the early explorers you study in school could do this. In fact, a way to find longitude was not invented until the late 1700's, and was considered one of the most challenging questions of the 16 and 17 hundreds. Several governments even offered large cash prizes for anyone who could figure out a way to calculate longitude from a moving ship.
The problem is that when you move east and west, the stars do not change position, but the TIME they rise and set changes. This is why you have to switch your watch back 3 hours when you fly west from New York to California, but not when you fly south from New York to Florida. Navigators in the old days did not have clocks that could keep accurate time on a moving ship. So they could not keep accurate track of the changes in the rising, passing and setting of the sun. Thus they had no way to figure out their longitude.
Can you guess which country invented a way to figure out longitude? Hint: The Prime Meridian goes through that country's capitol as a result! Click here to reveal the answer.
Want to try your hand at using time to figure out your longitude? Try this game.