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    Re: Time
    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{at}hoki.ibp.fhg.de
    

       
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