Astronomical Applications Department, U.S. Naval Observatory - NOVASC (Page 8)

TOPO_STAR

short int topo_star (double tjd, short int earth, double deltat,
fk5_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)
TDT Julian date for topocentric place.
earth (short int)
Body identification number for the Earth.
deltat (double)
Difference TDT-UT1 at 'tjd', in seconds.
*star (struct fk5_entry)
Pointer to catalog entry structure (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 TDT 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 TDT­UT1
(often called

T) is passed to the function via argument

deltat

. Values of

T are published in the annual

Astronomical Almanac or can be obtained from the National Earth Orientation Service (NEOS) home page
on the World Wide Web (http://maia.usno.navy.mil/). The user's choice of the version of function
solarsystem determines the value of the argument

earth

that the calling program must supply to

app_star.

The input mean place at standard epoch J2000.0 is assumed to be the true mean place, similar to the

mean places in the FK5, and not contain the so-called "E-terms".