Maritime Greenwich World Heritage Site

The red time ball on the roof of the Royal Observatory in Greenwich drops each day at 1:00pm to set world time. It has done this every day since 1833. It drops at 1:00pm because historically astronomers were busy with their telescopes with the midday sun at noon.

Pictured above is our GPS unit on "the prime meridian line" at Greenwich. Note the reading: 51.4779 N 00.00146 W.

The prime meridian line is the line of longitude running from the north pole to the south pole defined as 0 degrees. The Airy Transit Circle at Greenwich was originally the zero of longitude for the World. However, a GPS receiver operated on the zero meridian at Greenwich indicates a longitude differing by about 100 metres from zero. This is because the basic longitude from a GPS receiver is referred to WGS 84 (World Geodetic System 84), which is a geocentric frame, with longitudes, latitudes and heights referred to a spheroid that best fits mean sea level over the whole globe. The longitude of Greenwich is not zero in this system. See the Royal Observatory Page for more information.

The "flaming bird" from New Year's Eve, 2001.


Greenwich postage stamps from my album.

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Lynn Salmon <>{

Last updated: September 10, 2015