idlastro / Astronomical Utilities: ZENPOS

[Source code]

NAME
ZENPOS
PURPOSE
Return the zenith RA and Dec for a given Julian date and observer longitude
        and latitude
CALLING SEQUENCE
ZENPOS, Date, Ra, Dec, [ Latitude= , Longitude = , /Degrees ]
INPUT
Date  The Julian date, in double precision, of the date and time
        for which the zenith position is desired, scalar or vector.
OPTIONAL INPUT KEYWORDS
LATITUDE  -  Latitude of the desired location (degrees)
LONGITUDE -  Longitude of the desired location (degrees)
OUTPUTS
RA - The right ascension of the zenith (in radians unless /Degrees is set)
Dec - The declination of the zenith (in radians unless /Degrees is set)
RA and Dec will have same number of elements as the input Julian date.
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 2 parameters if they are not
defined in the common block SITE (see below) or provided as
keywords.  Keywords override information in the common block.
LAT,LNG - north latitude and east longitude of the desired location 
        in DEGREES
COMMON BLOCKS
SITE - This common block should contain the scalars LAT, LNG
PROCEDURE CALLS
CT2LST - Convert to Local Mean Sidereal Time
MODIFICATION HISTORY
Written by Michael R. Greason, STX, 14 October 1988.
Update documentation, longitude now *east* of Greenwich W.L. July 2000
Add keywords for non-interactive use  J. Sapp   July 2016
Remove unneeded time zone info, add /Degrees  W. Landsman Mar 2018