I recently came across a Wikipedia article about somebody's (?) law regarding limits on a moon's orbital radius because the sun's gravitational influence is greater than the planet's at some distance from the planet. As I recall, the law had two different names associated with it. In addition to the obvious relationship (shown below) for the values when the gravitational forces on the moon are equal for the sun and the planet, there was a useful diagram showing this effect together with consideration of the sun's influence on the Lagrangian points, L1 and L2. (rp/rm)2 = Ms/Mp, where rp is the distance between sun and planet rm is the distance planet and moon Ms is the mass of the sun Mp is the mass of the planet Unfortunately I did not take notes, and now I can not find that article. Can anyone help me? Does anyone know a name of this law?