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
- 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.
- 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!).
- 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!
- Following the closing of the RGO in the fall of 1998, the Old Greenwich Observatory was renamed the Royal Observatory Greenwich.
Greenwich Mean Time
- 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 earth’s orbit is slightly eccentric and the plane of the earth’s orbit is inclined with respect to the equator (about 23-1/2 degrees) hence at different times of the year the sun appears to move faster or slower in the sky. That’s why an uncorrected sundial can be “wrong” (if it is supposed to be telling mean time) by up to 16 minutes. So if the mean (i.e. corrected) sun is directly over the meridian through Greenwich, it is exactly 12 noon GMT or 12:00 GMT (Prior to 1925, astronomers reckoned mean solar time from noon so that when the mean sun was on the meridian, it was actually 00:00 GMT. This practice arose so that astronomers wouldn’t have a change in date during a night’s observing. Some in the astronomical community still use the pre-1925 definition of GMT in the analysis of old data although it is recommended that the term Greenwich Mean Astronomical Time now be used to refer to time reckoned from noon.)
- Mean time on selected meridians 15 degrees apart is generally known as standard time. For example, Eastern Standard Time (EST) is the mean solar time of the meridian at 75 degrees W.
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