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Re: Time
From: Thomas Schmidt
Date: 2000 Nov 09, 5:55 PM
From: Thomas Schmidt
Date: 2000 Nov 09, 5:55 PM
Dave Weilacher wrote: > > Anyone got concise definitions of various "time"s > and formulas for converting from/to. > > For example: > Zulu time > UCT > UTC > Zone Time > Watch time > Ship's time > etc. > > How about local apparent noon > Meridian passage of sun > .... TAI = International Atomic Time, a strictly uniform time scale, based on atomic clocks. Unit is the SI second. UT = Universal time, based on the rotation of the Earth and therefore irregular. Important for us, however, since day and night comply with UT. Unit is the mean solar day. UTC = A practical compromise between TAI and UT. Unit is the SI second, so the rate of UTC is strictly predictable (in contrast to UT). Occasional leap seconds assure that it doesn't get out of step with day and night (which TAI does slowly but surely) Meridian Passage of the Sun = the moment when the sun is exactly due south. True Solar Day = the period of time between two meridian passages of the sun. Not a uniform measure of time because in the course of a year some true solar days are longer, some are shorter ("equation of time"). Can be observed directly. Mean Solar Day = the period of time between two meridian passages of the sun, with the effect of the equation of time averaged out over one year. Can not be observed directly, must be taken from appropriately regulated clocks. Local True Solar Time = is noon whenever the sun crosses the meridian as seen from your current location. At the same instant, locations at different geographical longitudes have differnt LTSTs. Local Mean Solar Time = a succession of Mean Solar Days so that noon LMST is _on average_ when the sun crosses the meridian. (that is, LTST is always a bit faster or slower than LMST, but they agree on average). At the same instant, locations at different geographical longitudes also have different LMSTs. Local times are different at different longitudes. Zone times are the local time at a specific reference longitude, adopted by all people who live in the vicinity of the reference meridian. Therefore, different locations at different longitudes but within the same zone have the same time. If you live to the east of the reference meridian, zone time noon will be later than your local meridian passage of the sun; if you live to the west of the reference meridian, zone time noon will be earlier than your local meridian passage. ZULU is the zone for the reference meridian "zero longitude" and has by definition UTC as time scale. All other zones differ by an integer number of hours (some exotic zones by half-integer hours). Therefore ZULU and UTC are the same, and they are practically the same as the antiquated and now obsolete "Greenwich time". The difference between UTC and Greenwich time is that UTC is based on atomic clocks while Greenwich time was based on the rotation of the Earth, with some corrections applied. -- ------------------------------------------------------------------- Thomas Schmidt e-mail: schmidt@hoki.ibp.fhg.de