Royal Observatory, Greenwich, London
To most people the question "what time is it?" is mundane. To those who have worked at the Royal Observatory since 1675 it has been the whole point, and answering it precisely changed navigation, commerce and the map of the world.
Charles II founded the Observatory in 1675 for a strictly practical reason: to fix the positions of the stars accurately enough that sailors could work out where they were. Christopher Wren, himself an astronomer, designed the first building, and John Flamsteed became the first Astronomer Royal.
The scale of the improvement since is hard to hold in the head. The cathedral clock at Salisbury, built around 1386 and among the oldest working clocks anywhere, has no face at all and simply strikes the hours. A caesium atomic standard today would drift by a second over roughly a hundred million years, and the best optical clocks are far better again.
The longitude problem
Latitude was straightforward: the height of the sun at noon gives it. Longitude was not, because it requires knowing the time at a reference place and the time where the ship is, simultaneously. Pendulum clocks were useless on a moving deck.
After the loss of a British fleet off the Scillies, Parliament passed the Longitude Act in 1714 and set up a Board of Longitude, offering up to £20,000 for a workable solution. The answer came from John Harrison, a Lincolnshire-born carpenter turned self-taught clockmaker, who spent more than forty years building a series of marine timekeepers of extraordinary ingenuity.
The prize was never actually awarded. The Board, whose members largely favoured the rival astronomical method, kept moving the requirements. Harrison eventually petitioned George III, and in 1773 Parliament voted him £8,750 as a settlement — bringing his lifetime payments to something over £23,000, but leaving the official prize unclaimed by anyone.
His timekeepers are the centre of the collection: the great brass H1, H2 and H3, and H4, the pocket-watch-sized instrument that finally worked. Note that the H1 on show is currently a replica, the original having gone to the museum's conservation studios.
The meridian
By the late nineteenth century most nations were still using their own reference meridians, which made charts and railway timetables a nuisance. The International Meridian Conference met in Washington in 1884 to settle it. Greenwich won by twenty-two votes to one, the single vote against coming from San Domingo; France and Brazil abstained rather than back a British line, and France did not formally adopt Greenwich time until 1911.
The argument for Greenwich was commercial rather than scientific. The United States had already based its railway time zones on it, and by then the great majority of the world's shipping was already navigating on charts drawn to Greenwich.
Standing astride the brass strip in the courtyard puts a visitor in two hemispheres, though not — as is often said — in two times, since the clocks read the same on both sides. There is a further wrinkle worth knowing: satellite navigation places the meridian about a hundred metres to the east of the brass line, because modern geodetic reference differs slightly from the local vertical the Victorian astronomers worked to. Phones consequently insist the line is in the wrong place. After dark, a green laser projects the meridian northwards across London.
What else to see
The red time ball on top of Flamsteed House has risen and dropped since 1833, making it one of the world's first public time signals. It rises at 12.55 and falls at exactly one o'clock — one rather than noon, because at midday the astronomers were occupied observing the sun. Ships on the Thames set their chronometers by it, and it still draws a small crowd.
Under the onion dome sits the Great Equatorial Telescope, a 28-inch refractor installed in 1893, the largest of its kind in Britain and in working service until 1970. Beyond it the collections hold clocks, chronometers and optical instruments of every kind, from opera glasses to gun sights.
The Observatory has not been a working research institution here for a long time. Light pollution and the growth of London pushed it out: the astronomers moved to Herstmonceux in Sussex between 1948 and 1957, then to Cambridge in 1990, and the Royal Greenwich Observatory was wound up altogether in 1998. The Greenwich buildings became a museum, now part of Royal Museums Greenwich alongside the National Maritime Museum, the Queen's House and the Cutty Sark.
Visiting
Get the station right: the Observatory is nowhere near North Greenwich, which serves the O2 arena. The right stops are Cutty Sark on the DLR, or Greenwich on the DLR and National Rail, or Maze Hill. From the river it is a steep ten- to fifteen-minute walk uphill through Greenwich Park, and the view from the top is part of the reward.
Entry to the Observatory is ticketed, and booking ahead is advised. The Astronomy Centre alongside is free, as are the National Maritime Museum and the Queen's House at the bottom of the hill; a day pass covers the ticketed sites together.
Two closures need planning around. The Peter Harrison Planetarium — London's only planetarium — closed in September 2025 for a major renovation and is not expected to reopen until 2028. The Observatory itself has announced a capital project that will close the site from November 2026, with reopening indicated for spring 2027. Dates on projects of this kind move, so the Royal Museums Greenwich site is the place to check before travelling.