The chronometer was originally devised for maritime purposes and perfected by John Harrison, who solved the longitude problem with his marine chronometers. It might not seem like a big deal, but creating a clock in the 1700s capable of maintaining accurate time in a vessel - constantly pitching and rolling and exposed to changing temperatures and humidity - was no easy task. Accuracy was of the essence for the simple reason that for every 15 degrees that a ship travels eastward, the local time moves one hour ahead; moving west 15 degrees, the time moves back an hour. If sailors could establish the accurate time of Greenwich (GMT) on board, they could measure local time by observing the Sun and calculate the difference to see how far they had moved east or west. here