Celestia Forums

First, let me say great work by the developers of Celestia! I'm just an amateur astronomy enthusiast, but I'm also a computer science student and C++ programmer with Qt experience. I would like to learn more about the intricacies of the orbital charateristics of celestial bodies so I can contribute to this awesome software project.

1. Does Celestia use simple Keplerian laws to predict orbital paths of celestial bodies?
2. Does Celestia factor in precession and nutation of orbit, such as the precessional and nutational effects the moon and sun have on the earth?
3. What is the latest greatest mathmatical formulae for elliptical orbits?
4. When a celestial body orbits another body, does the point of focus, always occur along the major axis or can it be off to one side? Simply put, can a satellite orbit another primary body where the primary body is neither located on the major or axis or near the center of ellipse?

Keep up the great work!

Epimetheus wrote:1. Does Celestia use simple Keplerian laws to predict orbital paths of celestial bodies?

I believe so.

Epimetheus wrote:2. Does Celestia factor in precession and nutation of orbit, such as the precessional and nutational effects the moon and sun have on the earth?

No (not yet?)

Epimetheus wrote:3. What is the latest greatest mathmatical formulae for elliptical orbits?

I'm not sure exactly. Selden likely knows. This has been discussed before.

Epimetheus wrote:4. When a celestial body orbits another body, does the point of focus, always occur along the major axis or can it be off to one side? Simply put, can a satellite orbit another primary body where the primary body is neither located on the major or axis or near the center of ellipse?

The body being orbited will always be a focus.

Epimetheus wrote:1. Does Celestia use simple Keplerian laws to predict orbital paths of celestial bodies?

That's what's used when EllipticalOrbit is specified in an SSC file. Celestia can also use VSOP87 or JPL DE405/406 ephemides (CustomOrbit) or NAIF SPICE kernels (SpiceOrbit). See the Celestia Wikibook for details http://en.wikibooks.org/wiki/Celestia/Trajectories

2. Does Celestia factor in precession and nutation of orbit, such as the precessional and nutational effects the moon and sun have on the earth?

When a CustomOrbit or SpiceOrbit is used, the planet's orbital position is determined by an ephemeris. That ephemeris would have to include all the appropriate orbital effects (and they do). EllipticalOrbits are simple ellipses.

Planetary orientations (rotations) are handled separately from orbital positions. Celestia v1.5.0's SSC files can include precession. Celestia v1.6.0 (not yet released) will include a CustomRotation function for Solar System planets and many major moons which will include appropriate nutations, etc. Either version of Celestia allows the user to specify arbitrary orbital positions and orientations by defining ScriptedOrbit and ScriptedRotation functions. Again, see the Celestia Wikibook for details http://en.wikibooks.org/wiki/Celestia

3. What is the latest greatest mathmatical formulae for elliptical orbits?

Only the notation has changed since Newton worked them out. But orbits aren't actually ellipses. That approximation assumes that there are no disturbing influences. Orbiting objects actually are affected by the gravitation of nearby objects. Except for very special cases, there is no analytical formula for calculating orbits. That calculation has to be done numerically. ( Celestia doesn't calculate gravitational effects. They have to be precalculated and included in the ephemerides. There are various college level courses on the topic. )

4. When a celestial body orbits another body, does the point of focus, always occur along the major axis or can it be off to one side? Simply put, can a satellite orbit another primary body where the primary body is neither located on the major or axis or near the center of ellipse?

Objects orbit around their common center of mass, which is not necessarily at the center of any of the objects. That center of mass is commonly called the barycenter of the system.

Thank you both for your prompt replies. Great info, selden, concerning the different orbital parameters. This will keep me busy for a while!

BTW, would the barycenter, or center of mass, fall along the major axis of the elliptical orbit?

Thanks again!

Epimetheus wrote:Thank you both for your prompt replies. Great info, selden, concerning the different orbital parameters. This will keep me busy for a while!

BTW, would the barycenter, or center of mass, fall along the major axis of the elliptical orbit?

Thanks again!

When approximating an orbit with an ellipse, yes, the barycenter is at one of the foci.