Celestia Forums

What are the coordinate axes in "chase" mode? Obviously, the first axis is the direction of the velocity vector of the celestial object chosen. But what is the second axis?

thanks for answers.
lidocorc

The second axis is (somewhat arbitrarily) the primary axis of rotation of the chased object. The actual axis used for the orthogonal basis is the direction 90 degrees from the velocity vector in the plane defined by the velocity vector and rotation axis. Obviously there are troubles if you're traveling exactly the direction of the rotation axis.

--Chris

Thank you for reply.
May I repeat your explanation in terms of vector analysis: The second axis is the cross product of the spin vector of the body, I went to, and the first axis. Thus it is perpendicular to the first axis by definition. The problem arising, if spinvector and first axis are (nearly) collinear is evident.

I have some more questions about the exact definitions of the coordinate systems used by Celestia. Is there a reference about this topic available anywhere? I don't want to bother you with all the questions coming up the more I come in touch with celestia.

lidocorc wrote:Thank you for reply.
May I repeat your explanation in terms of vector analysis: The second axis is the cross product of the spin vector of the body, I went to, and the first axis. Thus it is perpendicular to the first axis by definition. The problem arising, if spinvector and first axis are (nearly) collinear is evident.

Well, not quite what I said, but aside from the labeling of the axes--first, second, third--it will give you the same coordinate system.

I have some more questions about the exact definitions of the coordinate systems used by Celestia. Is there a reference about this topic available anywhere? I don't want to bother you with all the questions coming up the more I come in touch with celestia.

I don't think that there is any detailed documentation of the coordinate systems accessible in the user interface. The reference frames that may be specified in ssc files however are precisely documented here:

http://en.wikibooks.org/wiki/Celestia/Reference_Frames

--Chris

Chris et al:

I know it may have once been explained in the forum, but damn if I can find the reference to it.

What specifically is the difference between follow and chase? I've tried both commands and cannot detect any difference?

Frank

Frank,

the difference is the used coordinate system.
Most significant: In chase mode one axis is given by the velocity vector of the selected object.
You should see the stars in the background move in chase mode, while in follow mode they stand still.
In chase mode you can stay on the sunlit side of a planet for example.

Andy