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CT2LST

CT2LST

Name


    CT2LST

Purpose


    To convert from Local Civil Time to Local Mean Sidereal Time.

Calling Sequence


    CT2LST, Lst, Lng, Tz, Time, [Day, Mon, Year]
                      or
    CT2LST, Lst, Lng, dummy, JD

Inputs


    Lng - The longitude in degrees (east of Greenwich) of the place for
            which the local sidereal time is desired, scalar. The Greenwich
            mean sidereal time (GMST) can be found by setting Lng = 0.
    Tz - The time zone of the site in hours, positive East of the Greenwich
          meridian (ahead of GMT). Use this parameter to easily account
          for Daylight Savings time (e.g. -4=EDT, -5 = EST/CDT), scalar
          This parameter is not needed (and ignored) if Julian date is
          supplied. ***Note that the sign of TZ was changed in July 2008
          to match the standard definition.***
    Time or JD - If more than four parameters are specified, then this is
              the time of day of the specified date in decimal hours. If
              exactly four parameters are specified, then this is the
              Julian date of time in question, scalar or vector

Optional Inputs


      Day - The day of the month (1-31),integer scalar or vector
      Mon - The month, in numerical format (1-12), integer scalar or vector
      Year - The 4 digit year (e.g. 2008), integer scalar or vector

Outputs


      Lst The Local Sidereal Time for the date/time specified in hours.

Restrictions


      If specified, the date should be in numerical form. The year should
      appear as yyyy.

Procedure


      The Julian date of the day and time is question is used to determine
      the number of days to have passed since 0 Jan 2000. This is used
      in conjunction with the GST of that date to extrapolate to the current
      GST; this is then used to get the LST. See Astronomical Algorithms
      by Jean Meeus, p. 84 (Eq. 11-4) for the constants used.

Example


      Find the Greenwich mean sidereal time (GMST) on 2008 Jul 30 at 15:53 pm
      in Baltimore, Maryland (longitude=-76.72 degrees). The timezone is
      EDT or tz=-4
      IDL> CT2LST, lst, -76.72, -4,ten(15,53), 30, 07, 2008
              ==> lst = 11.356505 hours (= 11h 21m 23.418s)
      The Web site http://tycho.usno.navy.mil/sidereal.html contains more
      info on sidereal time, as well as an interactive calculator.

Procedures Used


      jdcnv - Convert from year, month, day, hour to julian date

Modification History


    Adapted from the FORTRAN program GETSD by Michael R. Greason, STX,
              27 October 1988.
    Use IAU 1984 constants Wayne Landsman, HSTX, April 1995, results
              differ by about 0.1 seconds
    Longitudes measured *east* of Greenwich W. Landsman December 1998
    Time zone now measure positive East of Greenwich W. Landsman July 2008
    Remove debugging print statement W. Landsman April 2009



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