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A Few Facts Concerning GMT, UT, and the RGO

A Few Facts Concerning GMT, UT, and the RGO A Few Facts Concerning GMT, UT, and the RGO ------------------------------------------- Richard B. Langley Geodetic Research Laboratory Dept. of Geodesy and Geomatics Engineering University of New Brunswick Fredericton, N.B., Canada E3B 5A3 E-mail: [email protected] (original version: 3 February 1990; this version: 20 January 1999) In answer to the question "Does anyone know the exact difference between GMT and UTC?" here are a few facts concerning Greenwich Mean Time, Universal Time, and the Royal Greenwich Observatory. Various versions of this document have been posted to Usenet newsgroups and made available on Web sites over the years under the original title "A Few Facts Concerning RGO, GMT, and UT". The current title has been in use since the 23 December 1995 version. The Royal Greenwich Observatory ------------------------------- o Prior to 1948, the observatory at Greenwich (located on a hill back from the Thames River with a view of the London Docks) was known as the Royal Observatory. o In 1948, the observatory moved to Herstmonceux Castle in Sussex, becoming the Royal Greenwich Observatory (yes, even though it wasn't at Greenwich any more!). o The site at Greenwich became known as the Old Greenwich Observatory and the historic buildings and instruments were progressively incorporated into the National Maritime Museum, the main buildings of which are located at the foot of Observatory Hill, close to the river. Highly recommended for a visit if you're in London! o Following the closing of the RGO in the fall of 1998, the Old Greenwich Observatory was renamed the Royal Observatory Greenwich (see "Where's the RGO Now?" below). Greenwich Mean Time ------------------- o Greenwich Mean Time is a time scale based on the apparent motion of the "mean" sun with respect to the meridian through the Old Greenwich Observatory (zero degrees longitude). The "mean" sun is used because time based on the actual or true apparent motion of the sun doesn't "tick" at a constant rate. The e

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