How sailors found longitude before satellites: a race against the clock

Latitude is easy: measure the height of the Sun or the Pole Star. Longitude is hard, because the Earth turns one degree every four minutes. To know how far east or west you are, you need to know the time back home, and in 1700 no clock could keep it at sea.

After a fleet was wrecked on the Isles of Scilly in 1707, Britain offered up to £20,000 for a solution. A self-taught carpenter, John Harrison, answered with a watch that lost about 5 seconds on an 81-day voyage to Jamaica.