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.

What you actually see

• 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.

Honest expectations

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.