Royal Observatory Greenwich

Charles II founded the observatory in 1675 to solve the problem of finding longitude at sea, and Christopher Wren, an astronomer before he was an architect, built Flamsteed House on the hill. The line running through the courtyard is the prime meridian because an international conference in 1884 chose the instrument standing here as zero. The red time ball on the roof has dropped at one o'clock since 1833, originally so that ships on the river could set their chronometers by it. Inside are John Harrison's sea clocks, the four timekeepers that eventually made the longitude problem solvable, and their ticking is the quietest thing in Greenwich.

History

The hilltop once held Greenwich Castle, a Tudor hunting lodge above Greenwich Palace, and in 1674 Sir Jonas Moore, Surveyor-General of the Ordnance, persuaded Charles II to found an observatory; it was founded on 22 June 1675, Christopher Wren chose the site, building began on 10 August, and Flamsteed House was finished in 1676. The Longitude Act of 1714 made the Astronomer Royal a Commissioner of Longitude, Nevil Maskelyne began publishing the Nautical Almanac in 1767, daily time signals by a dropping time ball started in 1833, and in 1884 the International Meridian Conference in Washington chose the Greenwich meridian as the prime meridian for the world. The observatory gave its name to Greenwich Mean Time, the forerunner of today's Coordinated Universal Time (Wikipedia, Royal Observatory, Greenwich).

What to look for

• The prime meridian line - zero longitude set into the courtyard paving.
• Harrison's sea clocks - the timekeepers H1 to H4, three of them still running.
• The time ball - dropped from the roof at one o'clock daily since 1833.
• The Great Equatorial Telescope - the 28-inch refractor under its onion dome.

When to go

Open year-round, busiest between eleven and three when the meridian queue forms. Early entry or the last hour gets you the courtyard almost alone. Planetarium shows book separately and sell out at weekends. The park view over the Queen's House and the river is best in low winter sun, and the hill is a stiff climb in any weather.

Good to know

Photographing yourself astride the line means queueing, and satellite positioning puts true zero about a hundred metres east, which visitors enjoy pointing out. The observatory is ticketed while much of Greenwich is free, and the split confuses people. The hill is steep. Astronomy content is slighter than the horology, which is the real strength here.

Nearby

The Queen's House, the National Maritime Museum and the Cutty Sark down at the river.

Key facts