Discover John Harrison's iconic marine timekeepers

Essential information

Opening times: 
10.00–17.00 daily
Royal Observatory, Meridian Line and Historic Royal Observatory, Time & Longitude gallery, Flamsteed House

Ever seen a clock that changed the world? We can show you timepieces that are revolutionary - and exquisitely made.

From 14th May until mid-September 2018, the outer case of John Harrison’s fourth timekeeper, H4, will be on loan to the Great Exhibition of the North. Visitors to the Royal Observatory during this period will still have the opportunity to view the original inner movement of H4.

John Harrison's marine timekeepers H1, H2, H3 and H4 are arguably the most important ever made, and you can see them on display at the Royal Observatory Greenwich, in our Time galleries. The clocks are extraordinarily beautiful objects of themselves, as well as being revolutionary in their ability to allow ships to determine their longitude at sea. This development drastically reduced the risk of ships and their crews, along with their precious cargoes, being lost at sea. Harrison was a carpenter by trade and was self-taught in the art of clock making.

Precision timekeeping

During the mid-1720s Harrison designed a series of remarkable precision longcase clocks. His regulators from this period achieved an accuracy of one second in a month, a performance far exceeding the best London clocks of the day. To solve the longitude problem, he needed to devise a portable clock which kept time to within three seconds a day – far more accurate than even the best watches of the time.
His first attempt H1 constructed between about 1728 and 1735 is essentially a portable version of Harrison's precision wooden clocks. H2 and H3 (1737-59) are larger and heavier than H1. Harrison worked on his third timekeeper from 1740 to 1759. After 19 years of labour, it failed to reach the accuracy required by the Longitude Act.

From clocks to watches

In about 1751-52, Harrison commissioned London watchmaker John Jefferys to make him a watch to a radical new design. Its success encouraged him to make a sea-watch, H4 (1755-59), using the same ideas. No one in the 1750s thought of the pocket watch as a serious precision timekeeper. However, Harrison discovered with his new watch that if certain improvements were made, it would be an excellent timekeeper.

Visit the Royal Observatory to discover the work of this unprepossessing man, and how he cracked a problem that had baffled astronomers and mathematicians for centuries. Along with Harrison's celebrated marine timekeepers, including the groundbreaking H4, you can explore displays of scientific objects, paintings and animations exploring the quest for longitude.

Find out more about John Harrison's groundbreaking timekeepers