How did John Harrison solve the longitude problem?
Stimulated by a Parliamentary award of £20,000 for a method of finding a ship’s longitude anywhere on Earth to an accuracy of half a degree, Harrison spent nearly all his life perfecting a marine chronometer to solve the longitude determination problem. He reasoned that an extremely accurate clock set to the correct time when at the Greenwich meridian could be carried on a ship and its reading compared with local time, determined astronomically, to determine the ship’s longitude at any place. Since the Earth revolves 360° in 24 hours, or 15° per hour, the time difference in hours multiplied by 15 is the ship’s longitude in degrees. Harrison’s chronometer Number Four belatedly won the prize in 1773, long after a successful five-month sea trial in 1761. Carried across the Atlantic from England to Jamaica and back, Harrison’s clock was found to be five seconds slow, corresponding to an error of only 1.
