14
4.2
TOPO_STAR
short int topo_star (double tjd, body *earth, double deltat,
cat_entry *star, site_info *location,
double *ra, double *dec)
PURPOSE:
Computes the topocentric place of a star at date 'tjd', given its
mean place, proper motion, parallax, and radial velocity for
J2000.0 and the location of the observer.
INPUT
ARGUMENTS:
tjd (double)
TT (or TDT) Julian date for topocentric place.
*earth (struct body)
Pointer to structure containing the body designation for the
Earth (defined in novas.h).
deltat (double)
Difference TT (or TDT)-UT1 at 'tjd', in seconds.
*star (struct cat_entry)
Pointer to catalog entry structure containing J2000.0 catalog
data with FK5-style units (defined in novas.h).
*location (struct site_info)
Pointer to structure containing observer's location (defined
in novas.h).
OUTPUT
ARGUMENTS:
*ra (double)
Topocentric right ascension in hours, referred to true equator
and equinox of date 'tjd'.
*dec (double)
Topocentric declination in degrees, referred to true equator
and equinox of date 'tjd'.
RETURNED
VALUE:
(short int)
0...Everything OK.
>0...Error code from function 'solarsystem'.
Discussion:
This function computes the topocentric place of a star (neglecting atmospheric
refraction) for the location specified by the argument location, for the time specified by
the argument
tjd
. Note that
tjd
is the TT time at which the topocentric place is to be
computed. The word "star" as used here refers to any object outside the solar system. If
the values of
promora, promodec, parlax, or radvel
within structure
star
are unknown (or zero within the errors of measurement), the calling program
should set them to zero. For extragalactic objects, these input values should be set to
zero. The difference TTUT1 (often called
T) is passed to the function via argument
deltat
. Values of
T are published in the annual Astronomical Almanac [11] or can be
obtained from the National Earth Orientation Service (NEOS) home page on the World
