How might draw a trail of a point on a planet?

[jiveabillion]

If you go to http://phet.colorado.edu/sims/my-solar- ... em_en.html and choose Sun, planet, moon from the dropdown in the top right and click START. Se how the line is drawn to follow the path of the moon, but only from behind the moon? I would like to be able to draw a path like this, but by tracking any point on the surface of a planet or moon. This should allow us to see all of the parabolic arcs we travel in as we travel through the galaxy.

Any idea how I could do this?

[Marco Klunder]

Hello jiveabillion,

This is not really a scripting issue.
What you ask cannot be solved by scripting.

Unfortunately for you, the way Celestia handles orbit drawing, differs from your wish, and there is no option to set it.

There is however a way Celestia can draw these kind of parabolic arcs by predefined trajectories (like the trajectories of spacecrafts).
However, in that case the whole trajectory will be drawn, and not only the trailing part.
You also need to build and define those trajectories in your own *.xyz or *.xyzv files.

Marco

[jiveabillion]

Is there a way to record the coordinates of a point on a planet as it moves? Perhaps plot a point every second or millisecond while time is running at 1 million times faster. If I could record this I believe I could parse the results and create some logic to connect the points for only "forward" movement around the center of its orbit. It's kind of like a 3D spirograph. Can we write to a file with a script?

[Marco Klunder]

Yes, you can write a file with a script. See http://en.wikibooks.org/wiki/Celestia/Celx_Scripting/CELX_Lua_Methods/Celx_celestia#requestsystemaccess for an example.

Marco