Occasionally in Celestia it is useful to have a completely invisible object that other objects can orbit around. The obvious example is the barycentre of a pair of bodies in mutual orbit - Celestia needs an object positioned at the centre of gravity to "carry" the orbits of the two real bodies, but we don't want to be able to see anything in that position when we view the simulated orbits.
With the advent of Celestia 1.3.0, the "empty object" I created for version
1.2.5 is no longer required - its function has been taken over by Celestia's
"invisible" class of objects. As an example of the use of Celestia's
"invisible" object, I've adapted my original binary_asteroid.ssc file,
replacing the rather clumsy definition necessitated by the "empty" object
with the more streamlined definition possible in version 1.3.0.
I've also created an alternative, barycentric definition of the Pluto-Charon
system, which can be used to replace the Pluto-centred system defined in
solarsys.ssc. This new version shows Pluto and Charon orbiting realistically
around their mutual centre of gravity.
"Binary Asteroids" "Sol"
{
Class "invisible"
EllipticalOrbit
{
Period 1 #
SemiMajorAxis 1 # An Earth Trojan, for fun
MeanLongitude 160 #
}
Obliquity 30 # A random tilt to the binary orbits
}
"A" "Sol/Binary Asteroids"
{
Mesh "roughsphere.cms"
Texture "asteroid.jpg"
EllipticalOrbit
{
Period 20.5
SemiMajorAxis 45
Eccentricity 0.5
ArgOfPericenter 180
MeanLongitude 90
}
Radius 20
Albedo 0.07
}
"B" "Sol/Binary Asteroids"
{
Mesh "roughsphere.cms"
Texture "asteroid.jpg"
EllipticalOrbit
{
Period 20.5 # Periods must be the same
SemiMajorAxis 360
Eccentricity 0.5 # Eccentricities must be the same
ArgOfPericenter 0 # Pericenters opposite each other
MeanLongitude 270 # Longitudes opposite each other
}
Radius 10
Albedo 0.07
}
"Pluto-Charon" "Sol"
{
Class "invisible"
CustomOrbit "pluto"
}
"Pluto" "Sol/Pluto-Charon"
{
Texture "pluto.jpg"
BumpMap "plutobump1k.jpg"
BumpHeight 2.5
Radius 1137
EllipticalOrbit
{
Period 6.387246
SemiMajorAxis 1499
Eccentricity 0.0002
Inclination 115.602
AscendingNode 228.328
ArgOfPericenter 47.314
MeanAnomaly 93.440
}
Obliquity 115.6
EquatorAscendingNode 228.3
RotationOffset 320.8
Albedo 0.55
}
"Charon" "Sol/Pluto-Charon"
{
Texture "charon.jpg"
Radius 586
EllipticalOrbit
{
Period 6.387246
SemiMajorAxis 17906
Eccentricity 0.0002
Inclination 115.602
AscendingNode 228.328
ArgOfPericenter 227.314
MeanAnomaly 93.440
}
Obliquity 115.9
EquatorAscendingNode 227.3
RotationOffset 140.4
Albedo 0.35
}
This Web page is maintained by Selden Ball
at Wilson Lab.
Please send any comments or corrections to
seb@lepp.cornell.edu