Definition of Universal Coordinates

[Joe]

A dummy question :-Anyone can help me by explaining the Universal Coordinate defined by Celestia. Where is the original point of the Universal Coordinate frame and the axes?

[selden]

Ecliptic J2000

The fundamental plane is the J2000 Earth ecliptic, and the preferred direction (x-axis) is the J2000 equinox.

J2000 is shorthand for Noon of Julian date January 1, 2000.
http://en.wikipedia.org/wiki/Epoch_%28a ... 29#J2000.0

Does this help?

[Joe]

Thanks, Selden.A great help, as you have always been.