Greenwich Mean Time (GMT) is, properly speaking, the hour angle of the "mean sun" at the meridian of the old Greenwich Observatory.

This means that the astronomical day would start at noon and not at midnight as in civil usage. The "mean sun" is actually a fiducial point on the sky which moves uniformly on the equator and not with variable speed along the ecliptic as the "apparent sun" does. The mean sun is so defined that over a year it represents the average position of the sun. therefore, mean time which is referenced to this point and not to the apparent sun, is a good average approximation to apparent time, the time which a sundial shows. the difference between these two times is the equation of time, a correction which leads from one to the other. (There are two different sign conventions in use; caution is therefore required in the application of tables).

Over the last 50 years a number of changes have taken place which have caused changes of concepts and of nomenclature which, however, have not yet become widely known.

The first of these events has been the establishment of the International Time Bureau (BIH) at the Paris Observatory. The Paris Time Service simply acted as a coordinating bureau to assure that the time determination by the various time services were all coordinated and the results published. This coordination essentially means that the longitudes of all stations have to be established in a consistent system. For this it is not practical to refer the longitudes of the stations to a single arbitrary instrument. over the years this led to the practice of defining the standard meridian in respect to the average of all contributing observatories instead in reference to the transit instrument in Greenwich.

This is an obvious improvement because instrumental errors or motions of that instrument are then not reflected in the international reference. When The Royal Greenwich Observatory (RGO) moved away to the better sky of southern England after World War II, the reference meridian was not affected by this at all. in fact, had RGO stopped observations altogether, GMT would still have been available, as it is now, from the observations of all the other national observatories (in the USA from the USNO and its time station in Florida).

Indeed, more recently the RGO has lost interest in time having become more of a pure research operation with primary concern in advanced astrophysics. It will even move away from Herstmonceau to a university campus. the castle would then be sold as a source of support. the other buildings of the RGO are also to be converted into a hotel, etc. all of the remaining activities of the RGO, i.e. the support for the new observing site in the canary islands, will be moved to cambridge university into a new, quite modest building.

These developments are due to the change in mission and oversight which took place in 1965 when the RGO was put under the Science and Engineering Research Council (SERC). It's activities are now mainly the operation of the new major observatory on the Roque De Los Muchachos on the island of La Palma (RDLM Observatory).

Another change came about by the decision of various bodies, EG: the International Astronomical Union (IAU), to change the beginning of the day to midnight, in conformance with civil usage. mean time at the standard meridian (i.e., according to the Washington convention of 1884, the Greenwich meridian) was therefore to be the hour angle of the mean sun plus 12 hours. regarding the name of this "world time" confusion arose because the British did not accept a us proposal to simply name it GTC for Greenwich civil time. on the other hand, most people will agree that it is not wise to use the same name for two different things, in this case the old and the new style. for these reasons, the IAU recommended the new designation "Universal Time" (ut) for what otherwise would have to be called GMT new style. ut was quickly accepted by most countries. the only exception was britain (for obvious reasons) and the usa which followed britain because of the close coordination of the astronomical and nautical almanacs.

Yet another development has been the introduction of a distinction between various measures of time: first the time as observed, then the time corrected for polar variation (in the order of 100ms), furthermore a time corrected and averaged in rate over a year by the application of a seasonal correction, and finally the time of the clocks and time signals. the GMT designation which simply continued to exist because of the reluctance of the British and American authorities to confuse the navigators (that is the most frequently heard explanation) was not adapted to these new needs. instead, the IAU established the system of universal time which comprised the ut0, ut1, ut2 and UTC (universal time coordinated) in the meanings as listed above.

Finally when other professions entered the timing field and demanded even closer international coordination and consultation, the international telecommunications union (ITU with its consultative body the CCIR), and the international bureau for weights and measures as part of the metric convention, in addition to the scientific unions IAU, URSI, and IUGG, established a system of definitions and conventions which deal exclusively with the system of universal time, GMT being considered as obsolete.

For practical purposes GMT may be considered as the equivalent of the general expression UT. UT without further index is in use wherever a precision of 1 second is sufficient, or where no greater precision is to be implied. for the use in almanacs where the hour angle of celestial objects is needed, ut1 is to be used. for all general timing of clocks or electronic systems, UTC is the appropriate reference.

The official documents of these resolutions, recommendations, and decisions, have been published by The Bureau International De Poids et Mesures (BIPM), and by the Consultative Committee for Radio (CCIR as a body of the itu). The CCIR documents are contained in the CCIR "green book" VOLUME VII.

Any of these documents may be obtained from the UNSO Time Service upon request.

LITERATURE: Derek Howse (1980) Greenwich Time and the Discovery of the Longitude.

Oxford University Press, ISBN 0-19-215948-8 qb213 79-40052

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