ZENPOS

       Return the zenith RA and Dec in radians for a given Julian date.

       ZENPOS, Date, Ra, Dec

       Date  The Julian date, in double precision, of the date and time
               for which the zenith position is desired, scalar or vector.

       Ra    The right ascension in RADIANS of the zenith.
       Dec   The declination in RADIANS of the zenith.

       The local sidereal time is computed; this is the RA of the zenith.
       It and the observatories latitude (corresponding to the Dec.) are
       converted to radians and returned as the zenith direction.

       ZENPOS will prompt for the following 3 parameters if they are not
       defined in the common block SITE (see below)
       LAT,LNG - north latitude and east longitude of the desired location
               in DEGREES
       TZONE - Time Zone (in hours) of the desired location (e.g. 4 = EDT,
               5 = EST)

       SITE - This common block should contain the three scalars LAT, LNG
               and TZONE

       CT2LST - Convert to Local Mean Sidereal Time

       Written by Michael R. Greason, STX, 14 October 1988.
       Converted to IDL V5.0   W. Landsman   September 1997
       Update documentation, longitude now *east* of Greenwich W.L. July 2000

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