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An observatory that sits on the Prime Meridian, marking Greenwich Mean Time.

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The Royal Observatory, or the Old Royal Observatory, is located in Greenwich along the River Thames. The Observatory is famous for sitting along the Prime Meridian, giving its name to Greenwich Mean Time (also known as Universal Time). For years, the Observatory has played a major role in the history of astronomy and navigation, and is currently number 000 (first on the list) among the International Astronomical Union.

Commissioned in 1675 by King Charles III, the first stone was laid on August 10th and was completed the follwoing year. John Flamsteed was the first Astronomer Royal, and thus the main house is often known as Flamsteed House in reference to him. By 1767, the Astronomer Royal Nevil Maskelyne had begun the publication of the Nautical Almanac based on the observations he made at the Observatory. This was the first almanac that had data dedicated to the determination of longitude at sea.

The Royal Observatory has a brass strip that runs through the courtyard. This brass strip was put in place in the 1960’s to mark the Prime Meridian. The Prime Meridian is used around the world for mapping and timekeeping. It was important for helping ships to navigate their positions at sea.

If you look at the top of the observatory you will notice a large red ball. This ball helped mariners to synchronize their clocks before heading out to sea. Each day, at exactly 1pm, the ball would be dropped. Ships in port could see the ball dropping and they would align their clocks to match Greenwich Mean Time.

Today the Royal Greenwich Observatory is located in Cambridge and the Greenwich site has been converted into part of the National Maritime Museum and has been incorporated into the Royal Museums Greenwich. The Museum features a new planetarium as well as the Annie Maunder Astrographic Telescope, which is a cluster of four separate instruments.