Why did finding longitude at sea take a clockmaker, not an astronomer?

Finding longitude at sea took a clockmaker because longitude is a matter of time. Latitude could be found by measuring the sun's angle at noon, but east-west position needs the time difference between a clock showing the reference time and the ship's local time. That difference gives the ship's longitude relative to the prime meridian. Without it, navigators relied on dead reckoning, whose cumulative errors frequently led to shipwrecks and great loss of life.

The catch is how precise the clock must be. For every four seconds of error in the time source, the east-west position may be off by just over one nautical mile. A simple pendulum clock was useless at sea, because of the rolling of the ship and variations in Earth's gravity of up to 0.2%. The clock also had to resist changes in temperature, pressure and humidity, resist corrosion in salt air, and work on a constantly moving ship. Newton and Huygens doubted such a clock could ever be built and favoured methods such as lunar distances.

After the Scilly naval disaster of 1707, Parliament offered rewards of up to £20,000 under the 1714 Longitude Act. John Harrison, an English joiner and clockmaker, took up the challenge. In a trial of his first sea clock, the ship's master noted that his own calculations put the ship sixty miles east of its true landfall, which Harrison had correctly predicted using the clock. He presented his first design in 1730 and worked over many years on improved ones, with the Longitude Board's support in building and testing them.

Then came a surprise. Harrison realised that a mere watch could be accurate enough, and was a far more practical marine timekeeper. In 1761 his watch H4 sailed for Jamaica; on the trial, after 81 days and 5 hours it was 5 seconds slow, an error of about one nautical mile. The Board was persuaded that the accuracy could have been luck and demanded another trial. The £20,000 reward was never paid in a lump sum, but Harrison received payments totalling £23,065.