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There will be three points-of-interest in the geometry:
- The position in space (and orientation) of the camera, and the dimensions of its viewport.
- The position-in-space of the sun, the center of orbit.
- The distance and angle (polar coordinates) of each planet, which may be converted to absolute (x,y,z).
All of this math lets you determine, at any point in time, whether a given planet is visible to the camera, where it appears on the camera's film, and how far away from the camera it is. You then sort this display-list in descending order by z-order, and draw it.