Royal Observatory ★★

The popular tradition of standing in two hemispheres along the official Prime Meridian (Longitude 0º) at the Royal Observatory in Greenwich, Royal Observatory, London (Photo © Reid Bramblett)
The popular tradition of standing in two hemispheres along the official Prime Meridian (Longitude 0º) at the Royal Observatory in Greenwich
The popular tradition of standing in two hemispheres along the official Prime Meridian (Longitude 0º) at the Royal Observatory in Greenwich, Royal Observatory, London (Photo © Reid Bramblett)
The Royal Observatory in Greenwich, Royal Observatory, London (Photo by Katie Chan)
The back garden at the Royal Observatory in Greenwich, Royal Observatory, London (Photo by Mike Peel)
A room inside the Royal Observatory in Greenwich, Royal Observatory, London (Photo © Reid Bramblett)
The crowds along the official Prime Meridian (Longitude 0º) at the Royal Observatory in Greenwich, Royal Observatory, London (Photo © Reid Bramblett)
John Harrison's successful H4 clock that finally determined longitude, Royal Observatory, London (Photo © Reid Bramblett)
John Harrison's H1 marittime clock in the attempt to determine longitude, Royal Observatory, London (Photo © Reid Bramblett)
The 28-inch and 40-inch telescopes at the Royal Observatory in Greenwich, Royal Observatory, London (Photo by Benjamín Núñez González)

Set your watch by the actual Greenwich Mean Time clock, straddle the Prime Meridian line that divides the eastern and western hemispheres, and tour the fascinating little museum about it all

This is where they keep the Prime Meridian—well, the marker for it—the line that marks 0° longitude and divides the Earth into East and West hemispheres.

Inside are loads of historic scientific devices, including the four clocks made famous by the book Longitude.

A digital clock outside, set right at the Prime Meridian, lets you know down the millisecond exactly what time it is — the original Greenwich Mean Time, by which most of the world sets its clocks.

Also on the grounds are a Planetarium and Astronomy Centre (with a hunk of 4.5 billion-year-old meteorite you can touch)—the only bit of the Royal Observatory that is free anymore.

 
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