Maritime Clocks – CHRONOGRAPHIA

This subject deserves a special place when we are discussing the history of timekeeping. Until almost three hundred years ago, ships had problems navigating when sailing accross the seas and oceans. Navigators could determine their latitude by measuring the sun’s angle at noon or, in the Northern Hemisphere, by measuring the angle of the Polaris star from the horizon at twilight, but finding the longitude was a problem. Gemma Frisius (9 December, 1508 – 25 May, 1555) a Dutch physician, mathematician, cartographer, philosopher and instrument maker was the first to describe c.1553, how an accurate clock could be used to determine longitude. However, it was two centuries later before this was achieved. A clock was needed that would keep accurate time aboard a ship.

John Harrison, English inventor and horologist, 1767.
Science Museum, London. (CC BY-NC-SA)

In 1714, the British government offered a prize for a method of determining longitude at sea, with the awards ranging from £10,000 to £20,000 depending on accuracy. John Harrison (3 April, 1693 – 24 March, 1776) was an English clockmaker, who presented his first design in 1730, and worked over many years on improved designs, made several advances in time-keeping technology, finally turning to what were called sea watches. Harrison gained support from the Longitude Board in building and testing his designs. His marine chronometer invention, a long-sought-after device had solved the problem of calculating longitude while at sea. Harrison’s solution revolutionized navigation and greatly increased the safety of long-distance sea travel. Toward the end of his life, he received recognition and a reward from Parliament. Harrison is remembered in history as solving the problem of Longitude and has been recognized as one of the greatest Britons in history.

Several models of Harrison’s maritime clocks and later chronometers looking more like large watches were developed, which have been designated H1, H2, H3, H4, and H5.

Harrison H1 Maritime Clock – 1735

Harrison H1=1 Maritime Clock — Attribution: Royal Maritime Museum

Harrison H5 Maritime Chronometer – 1770

Restored H1, H2, H3 and H4 timepieces can be seen on display in the Royal Observatory at Greenwich. H1, H2 and H3 are still working, but H4 is kept in a stopped state, because it would degrade if running. The H5 is owned by the Worshipful Company of Clockmakers of London and is displayed at the Science Museum, London. For a more details of these important events in the history of clocks please visit the website of the Royal Museums Greenwich

Mathematicians call it “the arithmetic of congruences.” You can think of it as clock arithmetic. Temporarily replace the 12 on a clock face with 0. The 12 hours of the clock now read 0, 1, 2, 3, … up to 11. If the time is eight o’clock, and you add 9 hours, what do you get? Well, you get five o’clock. So in this arithmetic, 8 + 9 = 5; or, as mathematicians say, 8 + 9 ≡ 5 (mod 12), pronounced “eight plus nine is congruent to five, modulo twelve".
- John Derbyshire, Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics.

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