Consider the following diagram: All 4 orbits are inclined 30 degrees to the xy plane (I tilted the diagram a bit for clarity, so we're not looking from directly in the xy plane. The green orbit and the purple orbit are inclined 0 degrees from each other. The orange orbit is rotated 180 degrees from the green and purple orbits. It appears to be inclined 60 degrees to the green and purple orbits. The blue orbit is rotated 90 degrees with respect to the green and purple orbits. I'm guessing that it is inclined 30 degrees from the green and purple orbits and also 30 degrees from the orange orbit. So in trying to come up with a formula to describe how one orbital plane is inclined relative to another my first guess is : i1+cos(rotation)*i2 Is this correct?