Celestia Forums

Okay, this is probably going to be a massively dumb question, but....

When I define the distance of a moon or other body from a planet, is the orbital radius used coming from the surface of the planet, or the planet's center?

...John...

You don't define the distance, you define the Semimajor Axis of an elliptical orbit. If the orbit is a circle, the semimajor axis is the same as the circle's radius.

If the satellite is much smaller than the planet, then the center of the planet is at one focus of the ellipse.

If the orbit is a circle (i.e. if the eccentricity is 0), both focii are at the same location, the center of the circle, and the center of the orbit is at the center of the planet. In other words, the radius of the orbit is the distance from the center of the planet.

(However, the distance that Celetia displays for your viewpoint is from the surface of the planet.)

Does this help?

It definitely helps, thanks! And it explains why, on occasion, when I place a moon in a low orbit it is drawn *within* the planet itself.

Thanks, Selden!

...John...