Planets travel in an ellipse around the sun. Muggle sources can be used to find the distance from the sun. Albedo is constant, while angular size varies. Angular size is expressed by arcunits.

Von Rheticus telescopes have a 'S' button for angular size. If an object is not round, the angular size is its greatest dimension. The other button is 'A' for albedo. If the whole object is seen, you will get the average. To calculate magical albedo, you need to know what the surface of the planet is made of. Rocky surfaces cut it in half, and water cuts that in half again.