Episode notes
For centuries sailors could find latitude from the noon sun or Polaris, but longitude required knowing the exact time at a reference point like London while at sea, and an error of just four seconds could put a ship more than a mile off course. Galileo proposed timing the eclipses of Jupiter's moons, which proved impossible from a rolling deck. In 1714 Britain's Longitude Act offered £20,000 for a solution. Pendulum clocks failed on ships, both from the motion and because gravity varies by about 0.2% with latitude, while spring clocks drifted as temperature changed their elasticity. The answer came from John Harrison, a working class Yorkshire carpenter who approached the Board of Longitude in 1730 and spent 31 years on his timekeepers, from lignum vitae wooden clocks to the counter oscillating sea clocks H1 and H2 and the bimetallic strip in H3.
In 1761 Harrison abandoned heavy mechanisms for H4, a five inch watch with a fast beating balance that performed brilliantly on a voyage to Jamaica and won him the prize. Pierre Le Roy, Thomas Earnshaw, and John Arnold then developed the detached spring detent escapement, which needed no oil, and Charles-Édouard Guillaume later won a Nobel Prize for elinvar, an alloy whose stiffness ignored temperature. Observatories rated each chronometer for 30 to 50 days so navigators could correct for its steady error. The Royal Navy made them standard by 1825. In World War II Hamilton mass produced its Model 21 and 22, while Wempe and A. Lange & Söhne built the Einheitschronometer with interchangeable parts, a design the Soviets copied in Moscow as the MX-6.
- Harrison invented caged roller bearings to cut friction in his metal clocks.
- Detent escapements used jewel bearings of sapphire, ruby, and diamond at the pivots.
- A time ball dropped daily at 1 p.m. at the Royal Observatory in Greenwich so ships on the Thames could set their chronometers.
- The 1815 wreck of the East Indiaman Arniston off South Africa killed 372 people after a longitude miscalculation.
- More than 58,000 of the unified German and Soviet design were built, the most produced mechanical marine chronometer in history.