Astronomical Applications Department, U.S. Naval Observatory - novas c 2 doc (Page 17)

17

4.4

TOPO_PLANET

short int topo_planet (double tjd, body *ss_object, body *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)
TT (or TDT) Julian date for topocentric place.
*ss_object (struct body)
Pointer to structure containing the body designation for the
solar system body (defined in novas.h).
*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.
*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 `ephemeris'.

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 TT time at which the

topocentric place is to be computed. The difference TT­UT1 (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. The user's choice of ephemerides
determines the values to be used in structures

ss_object

and

earth

, which identify

the solar system object and the Earth, respectively.