SOLARSYSTEM
short int solarsystem (double tjd, short int body, short int origin,
double *pos, double *vel)
PURPOSE:
Provides the position and velocity vectors of a planet or other
solar system body at a specific time. The origin of coordinates
may be either the barycenter of the solar system or the center
of mass of the Sun.
INPUT
ARGUMENTS:
tjd (double)
TDB Julian date.
body (short int)
Body identification number for the solar system object of
interest; Mercury = 1,...,Pluto = 9, Sun = 10, Moon = 11.
origin (short int)
Origin code; solar system barycenter = 0,
center of mass of the Sun = 1.
OUTPUT
ARGUMENTS:
pos[3] (double)
Position vector of 'body' at tjd; equatorial rectangular
coordinates in AU referred to the mean equator and equinox
of J2000.0.
vel[3] (double)
Velocity vector of 'body' at tjd; equatorial rectangular
system referred to the mean equator and equinox of J2000.0,
in AU/Day.
RETURNED
VALUE:
(short int)
0...Everything OK.
Other values depend upon version in use
Discussion:
This subroutine supplies values for the components of the position vector
pos
and velocity vector
vel
for body
body
at time
tjd
. The vectors computed by solarsystem are in the equatorial rectangular
coordinate system which is oriented to the mean equator and equinox of standard epoch J2000.0. The
vectors are barycentric if
origin
=0 and heliocentric if
origin
=1.
There are two different versions of solarsystem supplied in NOVAS-C, each with its own internal
logic. One uses internally-stored data or series expansions, the other refers to external data files.
Additional documentation is provided on the following pages for the proper use of each version. The user
is free to supply alternative versions, providing that the arguments conform to the above specifications.
The values of the body identification number,
body
, will in general differ from one solarsystem
version to another; consult the documentation for the specific version in use. Usually,
body
=1 refers to
Mercury,
body
=2 refers to Venus,
body
=3 refers to the Earth, etc., but the identification numbers for
bodies such as the Sun or Moon vary. Furthermore, some versions of solarsystem support only a subset
of the major solar system bodies. The minimum requirement is support for the Earth. It is also
sometimes necessary to distinguish between the Earth and the Earth/Moon barycenter; for computing
quantities related to observables (e.g., apparent, topocentric, or astrometric places) it is the position and
velocity of the Earth that is required.
