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Royal Observatory, Greenwich

The Royal Observatory, Greenwich

The observatory is built on top of a hill overlooking Greenwich Park.

Greenwich park

This makes it easily visible from the river Themes, where the ships could see the ball drop and synchronise their clocks to the observatory.

The Observatory

The observatory defined where the location of prime meridian, or longitude zero. Most tourists come here, stand astride the prime meridian line, and get the photo taken.

Prime Meridian

Since the position of the prime meridian is an arbitrary standard, there has actually been several different meridians. Over time, the prime meridian has moved towards the east as extensions were made to the observatory building. This photo is looking south, so the meridians move towards the left of the photo.

The four meridians

The first meridian is

John Flamsteed's meridian
Edmund Halley's meridian
James Bradley's meridian

The most obvious: George Airy’s meridian.

George Airy's meridian

Modern GPS systems use a coordinate system called WGS-84 (World Geodetic System) which has established a new prime meridian a bit further east of the observatory – roughly from where this photo was taken.

WGS-84 meridian
View from The Royal Observator
National Maritime Museum, Greenwich