In 2025, to commemorate the 350th anniversary of the home of modern British astronomy – London’s Royal Observatory Greenwich – Great Britain’s Royal Mail has produced some very fine items.

The Miniature Sheet seen here has four First Class stamps featuring English carpenter and clockmaker John Harrison (1693-1776). He created marine timekeeper prototypes for measuring longitude at sea, including his H4, with its convention-defying watch design. The stamps celebrate a pioneer whose ideas would be adopted by future clockmakers and would inspire the marine chronometer that would revolutionise 18th-century celestial navigation.

John Harrison spent 5 years building his first marine timekeeper, now referred to as H1. Instead of a pendulum it had two linked dumbbell balances to compensate for movement. It was large and in its case took up much of the space in the captain’s tiny cabin. On its first sea-trial from England to Lisbon and back, it proved its value on the return leg by correcting the longitude of the ship and preventing almost certain disaster. Harrison however, always looking for improvements, was not satisfied and so set about making a second marine timekeeper.

Harrison felt that H1 could be further improved and started work on a new version, H2, seen on above stamp. This clock contained many improvements, including a more reliable and constant source of power to the escapement – the remontoire – and better temperature compensation. But he soon realised there was a fundamental design flaw as the clock was susceptible to errors occasioned by sharp forward-and backward rotational movement. Although the H2 passed rigorous testing, Harrison was dissatisfied with it and started again on a new clock. H2 was never trialled at sea.

H3 took some 19 years to complete (1740-1759), by which time Harrison was in his late 60s. The clock incorporated many of the features of H2, but significantly used a pair of linked circular balances rather than the dumbbell balances in H1 and H2. Two further innovations were the caged roller bearing to minimise friction and the bimetallic strip to combat changes in temperature, two devices that remain in wide use today. Like H2, H3 never underwent sea trials as Harrison had devised a new and better solution – his famous timekeeper H4.

And finally we come to H4. Pocket watches driven by a mainspring had been around for many years but were very inaccurate compared to pendulum clocks, being incapable of accuracy of less than a minute a day. In the early 1750s while working on H3, Harrison designed for himself a relatively conventional-looking spring-driven pocket watch which he had made for him by John Jefferys (1701–1754) an English clockmaker and watchmaker. This included a number of novel refinements including temperature compensation and a larger balance than normal, oscillating faster and at greater amplitude. The accuracy of his new watch pleased him so much that he recognised that a spring-driven watch could potentially solve many of the problems H2 and H3 had revealed. After six years of work and innovation he developed the watch that came to be known as H4, accurate enough to solve the Longitude problem. It was the first true marine chronometer and is now recognised as one of the most important watches ever made. It proved that spring-driven watches could perform well enough to determine longitude at sea. It was tested on a voyage to Jamaica, where after a voyage of 81 days it was found to be just 5 seconds slow, equivalent to one nautical mile of longitude. King George III tested the second version of this watch (H5) himself at the Kew observatory in London and after ten weeks of daily observations between May and July 1772, found it to be accurate to within one third of one second per day.

And the above six 1st Class stamps depict important developments connected to the Royal Observatory. In chronological order they show: Flamsteed House, the original Royal Observatory building at Greenwich. King Charles II instructed Christopher Wren, who was also an astronomer, to design the building in 1675 and it was completed 1676. The role of Astronomer Royal was created by Charles II in 1675, when he appointed John Flamsteed to draw up a map of the heavens with enough accuracy to be reliable for navigation;
Airy Transit Circle Telescope, observations made to define Meridian in 1851;
Shepherd Gate Clock, 1852, the Observatory’s first public clock;
Great Equatorial Telescope, 1893, large refracting telescope installed;
Prime Meridian, 1884, Greenwich chosen as the centre of time and longitude; and
Annie Maunder Astrographic Telescope, 2018, new telescope installed for astrophotography.