void edlimb_c ( SpiceDouble a,
SpiceEllipse * limb )
Find the limb of a triaxial ellipsoid, viewed from a specified
Variable I/O Description
-------- --- --------------------------------------------------
a I Length of ellipsoid semi-axis lying on the x-axis.
b I Length of ellipsoid semi-axis lying on the y-axis.
c I Length of ellipsoid semi-axis lying on the z-axis.
viewpt I Location of viewing point.
limb O Limb of ellipsoid as seen from viewing point.
c are the lengths of the semi-axes of a triaxial
ellipsoid. The ellipsoid is centered at the
origin and oriented so that its axes lie on the
x, y and z axes. a, b, and c are the lengths of
the semi-axes that point in the x, y, and z
viewpt is a point from which the ellipsoid is viewed.
viewpt must be outside of the ellipsoid.
limb is a CSPICE ellipse that represents the limb of
1) If the length of any semi-axis of the ellipsoid is
non-positive, the error DEGENERATECASE is signaled.
limb is not modified.
2) If the length of any semi-axis of the ellipsoid is zero after
the semi-axis lengths are scaled by the reciprocal of the
magnitude of the longest semi-axis and then squared, the error
SPICE(DEGENERATECASE) is signaled. limb is not modified.
3) If the viewing point viewpt is inside the ellipse, the error
SPICE(INVALIDPOINT) is signaled. limb is not modified.
4) If the geometry defined by the input ellipsoid and viewing
point is so extreme that the limb cannot be found, the error
SPICE(DEGENERATECASE) is signaled.
5) If the shape of the ellipsoid and the viewing geometry are
such that the limb is an excessively flat ellipsoid, the
limb may be a degenerate ellipse. You must determine whether
this possibility poses a problem for your application.
The limb of a body, as seen from a viewing point, is the boundary
of the portion of the body's surface that is visible from that
viewing point. In this definition, we consider a surface point
to be `visible' if it can be connected to the viewing point by a
line segment that doen't pass through the body. This is a purely
geometrical definition that ignores the matter of which portions
of the surface are illuminated, or whether the view is obscured by
any additional objects.
If a body is modelled as a triaxial ellipsoid, the limb is always
an ellipse. The limb is determined by its center, a semi-major
axis vector, and a semi-minor axis vector.
We note that the problem of finding the limb of a triaxial
ellipsoid is mathematically identical to that of finding its
terminator, if one makes the simplifying assumption that the
terminator is the limb of the body as seen from the vertex of the
umbra. So, this routine can be used to solve this simplified
version of the problem of finding the terminator.
1) We'd like to find the apparent limb of Jupiter, corrected for
light time and stellar aberration, as seen from a spacecraft's
position at time ET.
Find the viewing point in Jupiter-fixed coordinates. To do
this, find the apparent position of Jupiter as seen from the
spacecraft in Jupiter-fixed coordinates and negate this
vector. In this case we'll use light time and stellar
aberration corrections to arrive at the apparent limb. jstat
is the Jupiter's state (position and velocity) as seen
from the spacecraft. scpos is the spacecraft's
position relative to Jupiter.
spkez_c( jupid, et, "IAU_JUPITER", "LT+S", scid, scstat, <);
vminus_c ( scstat, scpos );
Get Jupiter's semi-axis lengths.
bodvcd_c ( jupid, "RADII", 3, &n, rad );
Find the apparent limb. limb is a CSPICE ellipse
representing the limb.
edlimb_c ( rad, rad, rad, scpos, &limb );
lcentr, smajor, and sminor are the limb's center,
semi-major axis, and semi-minor axis.
el2cgv_c ( &limb, center, smajor, sminor );
N.J. Bachman (JPL)
-CSPICE Version 1.1.0, 24-JUN-2014 (NJB)
Edit to correct chkout_c call passing the wrong routine name.
-CSPICE Version 1.0.1, 24-OCT-2005 (NJB)
Header update: reference to bodvar_c was replaced with
reference to bodvcd_c.
-CSPICE Version 1.0.0, 13-JUN-1999 (NJB)