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Longitude: The Great Sea Mystery

51:28 recording · EN · 2 speakers

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Brief overview

Longitude was solved slowly, by clocks, star tables and agreement, not by one man alone.

  1. Longitude is a question of time, not distanceKnowing the time difference between Greenwich and your ship gives your east-west position directly, because the Earth turns 360 degrees in 24 hours.
  2. Harrison's H4 lost five seconds in 81 daysThe 1762 Jamaica trial was far more accurate than the strictest test the Board of Longitude had set, yet the board still withheld the full prize.
  3. No single inventor solved longitudeAckermans calls it a gradual, messy process: chronometers and lunar tables needed each other, and the answer only held once nations agreed on a common zero line.
Executive Summary AI
  • On 22 October 1707 near the Isles of Scilly, Sir Cloudsley Shovel's fleet, unable to fix its position after days without sun or stars, was driven onto the rocks, HMS Association, Eagle, Romney and Firebrand sinking in the greatest maritime disaster of the age, claiming nearly 2,000 lives.1:09
  • Dr Emily Ackermans, curator of time at Royal Museums Greenwich, explains that longitude is really a time problem: the Earth spins 360 degrees in 24 hours, so a ship exactly three hours from Greenwich is 45 degrees east or west of it, and before the 18th century no clock could keep that reference time at sea.8:06
  • The Royal Observatory Greenwich was founded in 1675 under King Charles II, where John Flamsteed spent more than 40 years compiling star data for the lunar distance method, which took a hundred years before the nautical almanac let sailors actually use it.14:12
  • The Longitude Act of 1714 offered up to £20,000, around 3.5 million today, and Lincolnshire clockmaker John Harrison answered it with H1 to H4, the five-inch, three-pound watch that lost just five seconds over 81 days sailing to Jamaica in 1762, yet the board paid him only £1,500 and he was finally rewarded by Parliament in 1773 at the age of 80.18:01
  • Greenwich was voted the prime meridian by an international conference of 26 nations in 1884, the globe was divided into hour-wide time zones, and today satellites recalculate a container ship's position every second in the Thames estuary.43:27
Key Quote
“Battles may have been won at sea, but longitude remains unconquered.”
— John Hopkins5:36
Key Quote
“The problem with longitude is you need a measurement of time because you're measuring the rotation of the earth.”
— Dr Emily Ackermans8:06
Key Quote
“you're using the heavens as a giant clock. So the stars form like the dial of the clock and the moon moves relatively quickly against this background.”
— Dr Emily Ackermans15:08