idlastro / Astronomical Utilities: ZENPOS

[Source code]

NAME
ZENPOS
PURPOSE
Return the zenith RA and Dec in radians for a given Julian date.
CALLING SEQUENCE
ZENPOS, Date, Ra, Dec
INPUT
Date  The Julian date, in double precision, of the date and time
        for which the zenith position is desired, scalar or vector.
OUTPUTS
Ra    The right ascension in RADIANS of the zenith.
Dec   The declination in RADIANS of the zenith.
PROCEDURE
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.
PROMPTS
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)
COMMON BLOCKS
SITE - This common block should contain the three scalars LAT, LNG
        and TZONE
PROCEDURE CALLS
CT2LST - Convert to Local Mean Sidereal Time
MODIFICATION HISTORY
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