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.