"Brian P. McCarty (N9IWP)" <UCSBPM@UWPLATT.EDU> (02/02/90)
not exact quotes > The BBC will be using it's own time signals rather than those > provided by Greenwich Just a note, the Royal Greenwich Observatory has not been in Greenwich since 1948, when in moved Hurstmonceux. So the actual time signals for GMT are NOT from Greenwich (though the meridian still goes through the old observatory). Also, I've heard that GB is moving to UTC (due to budget cuts). Does anyone know when this will happen? Does anyone know the EXACT difference btween UTC and GMT? The information came from the July-August "Sea Frontiers", p 195. Brian McCarty (N9IWP) in chronophile mode interenet:UCSBPM@UWPLATT.EDU bitnet:UCSBPM@UWPLATT
LANG@UNB.CA (02/03/90)
On Fri, 2 Feb 90 13:02:00 CST "Brian P. McCarty (N9IWP)" <UCSBPM@UWPLATT.EDU> writes: > Just a note, the Royal Greenwich Observatory has not been in Greenwich > since 1948, when in moved Hurstmonceux. So the actual time signals > for GMT are NOT from Greenwich (though the meridian still goes > through the old observatory). Also, I've heard that GB is > moving to UTC (due to budget cuts). Does anyone know when this > will happen? Does anyone know the EXACT difference btween UTC and GMT? A few facts concerning RGO, GMT, and UT: 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 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. 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. o In 1928, the International Astronomical Union recommended that the time used in the compilation of astronomical almanacs, essentially GMT, be referred to as Universal Time. The terms "Universal Time" and "Universal Day" were introduced at the various conferences in the 1800's held to set up the standard time system. o There are actually a couple of variants of UT. UT as determined by actual astronomical observations at a particular observatory is known as UT0. It is affected by the motion of the earth's rotation pole with respect to the crust of the earth. If UT0 is corrected for this effect, we get UT1 which is a measure of the true angular orientation of the earth in space. However, because the earth does not spin at exactly a constant rate, UT1 is not a uniform time scale. So rather than base our civil time keeping on the rotation of the earth we now use Atomic Time, time based on the extremely constant frequency of a radio emission from cesium atoms when they change between two particular energy states. The unit of Atomic Time is the atomic d. second. 86,400 atomic seconds define the length of the nominal day. But because of the variations in the earth's spin the length of the actual day can be shorter or longer than the nominal day of 86,400 seconds. The time scale based on the atomic second but corrected every now and again to bring it into sync with the earth's rotation is known as UTC or Coordinated Universal Time. The corrections show up as the leap seconds put into UTC from time to time - the last one being inserted last New Year's Eve. So, GMT (if corrected for variations in the earth's spin) and UTC are for all intents and purposes one and the same. o The BBC began transmitting time signals in 1924. The chimes of Big Ben were first broadcast at midnight beginning 1 January and on 5 February, at the recommendation of the then Astonomer Royal, Frank Dyson, the six pips time signal was inaugurated. o Control of the BBC's six pips was taken over by the Royal Observatory in 1949 from Abinger to where the time service had moved during the war. The time service moved to Herstmonceux in 1957. As you can tell from the lengthy discussion above, I have an interest in time, both as a shortwave listener and as a professor who teaches astronomy and time to budding geodesists and surveying engineers. If you'd like to learn more about time you might look for the book "Greenwich Time and the Discovery of Longitude" by Derek Howse published in 1980 by the Oxford University Press. ======================================================================== Richard B. Langley BITnet: LANG@UNB.CA or SE@UNB.CA Geodetic Research Laboratory Phone: (506) 453-5142 Dept. of Surveying Engineering Telex: 014-46202 University of New Brunswick FAX: (506) 453-4943 Fredericton, N.B., Canada E3B 5A3 ========================================================================