How Do You Find Your Longitude at Sea? Carry the Time With You
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Knowing the time at a reference place while observing local noon gives a ship its east-west position, and building a clock accurate enough to do that at sea took decades. The problem killed a great many sailors before it was solved.
Why time gives position
The Earth turns once in twenty four hours, so any point on it moves through fifteen degrees of longitude every hour. A navigator who knows the exact time at a fixed reference place, and who observes the moment of local noon by watching the sun reach its highest point, can compare the two and convert the difference directly into degrees east or west of that reference. Four minutes of difference is one degree. Latitude had been measurable for centuries from the height of the sun or a star, but longitude had no comparable method, so ships could establish how far north they were and not how far along.
What made the clock so hard
A timepiece at sea faces conditions that defeated every existing design:
- •Continuous motion, which disrupts a pendulum entirely
- •Temperature changes across climates, which alter the size of every component
- •Humidity and salt, which corrode and swell
- •Changes in gravity with latitude, which alter a pendulum's rate
- •Variations in air pressure
- •Months without any opportunity to check or reset it
How it was solved
The British government offered a large prize in 1714 for a method of determining longitude, and the solution came from John Harrison, a Yorkshire carpenter and self-taught clockmaker who worked on the problem for over forty years. His early machines were large and elaborate, using counterbalanced linked pendulums that were unaffected by the motion of a ship and bimetallic strips that compensated automatically for temperature. His decisive instrument was a watch about the size of a large pocket watch, which on a voyage to Jamaica in 1761 lost only a few seconds over months at sea. The board administering the prize resisted paying, demanding repeated demonstrations, and Harrison received the money only after an appeal to the king.
What the failures cost
The prize was offered because the consequences of not knowing were catastrophic and repeated. A fleet under Cloudesley Shovell struck the Isles of Scilly in 1707 in fog, having believed itself further west, and four ships were lost with around two thousand men, which was the immediate prompt for parliamentary action. Ships regularly sailed to a known latitude well away from their destination and then ran east or west along it until they found land, which added weeks to a voyage and concentrated traffic onto predictable routes that pirates exploited. Extended voyages meant scurvy, spoiled water and exhausted supplies. Cargo losses and insurance costs were substantial. The prize was therefore a rational investment rather than a gesture.
The rival method
A competing approach nearly succeeded and its existence explains much of the resistance Harrison faced. The lunar distance method uses the moon's motion against the background stars as a clock in the sky, since the moon moves at a known rate and its position at any reference time can be tabulated in advance. A navigator measures the angle between the moon and a star, looks up when that angle occurs at the reference place, and obtains the reference time without any timepiece. It works, requires no expensive instrument, and demands a complicated set of calculations taking a skilled navigator a substantial time for each fix. Astronomers on the prize board favoured it, and it remained in use alongside chronometers for many decades because a clock can fail and the sky cannot.
The takeaway
Every hour of difference between local noon and the time at a reference place corresponds to fifteen degrees of longitude, so carrying accurate reference time gives east-west position directly. Motion, temperature, humidity and months without checking defeated existing clocks. Harrison solved it over forty years, with a watch that lost only seconds across an Atlantic crossing in 1761, and the lunar distance method remained a rival because a clock can fail.