Astronomical Applications Department, U.S. Naval Observatory - NOVASC (Page 10)

TOPO_PLANET

short int topo_planet (double tjd, short int planet, short int earth,
double deltat, site_info *location,

double *ra, double *dec, double *dis)

PURPOSE:
Computes the topocentric place of a planet, given the location of
the observer.

INPUT
ARGUMENTS:
tjd (double)
TDT Julian date for topocentric place.
planet (short int)
Body identification number for desired planet.
earth (short int)
Body identification number for the Earth.
deltat (double)
Difference TDT-UT1 at 'tjd', in seconds.
*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'.
*dis (double)
True distance from observer to planet at 'tjd' in AU.

RETURNED
VALUE:
(short int)
0...Everything OK.
>0...Error code from function 'solarsystem'.

Discussion:

This function computes the topocentric place of a planet or other solar system body (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

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 solarsystem determines the values of the arguments

planet

and

earth

, which identify the

planet and the Earth, respectively. The source of the rectangular coordinates is, of course, also determined
by the version of solarsystem in use.