John Harrison

John Harrison (1693–1776) was an English clockmaker who developed marine timekeepers precise enough to help ships determine longitude at sea.

Marine chronometer: A portable timekeeper designed to maintain accurate time aboard a ship. Harrison’s sea clocks became foundational examples of this instrument.
Longitude problem: The historical challenge of determining longitude accurately, especially at sea. It was the navigational problem Harrison set out to solve.
William Harrison: John Harrison’s son, who assisted in testing and advocating for his father’s marine timekeepers. He carried H4 on its successful 1761–1762 trial voyage to Jamaica.
Harrison's H2: John Harrison’s second large sea clock, completed in 1739. Its design revised H1 but retained a large, beam-based form.
Longitude by chronometer: A method of finding longitude by comparing local time with the time at a known reference meridian. Harrison’s timekeepers made this method workable for ships at sea.
Harrison's H1: The first of John Harrison’s large sea clocks, completed in 1735. Its linked, counter-oscillating beams helped resist a ship’s motion.
Longitude Act: A 1714 British law offering rewards for methods of finding longitude at sea. Its prize gave Harrison a route to recognition and financial support.
Nevil Maskelyne: An English astronomer who served as Astronomer Royal and promoted lunar-distance methods for finding longitude. His advocacy of an alternative method shaped the Board’s assessment of Harrison.
Harrison's H3: John Harrison’s third sea clock, developed over nearly two decades. Its experimental features included a bimetallic strip and rolling-ball bearings.
Lunar-distance method: A method of finding longitude by measuring the Moon’s angular distance from stars or the Sun. It was the principal practical alternative to carrying an accurate clock.

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