How heavy is Jupiter?
A similar method can be applied to find the mass of Jupiter, Saturn, or any planet with an observable moon. First, though, we need to find out how to work out an object’s size from observation, if we know how far away it is. Things look bigger close up than they do further away. This obvious truth is because an further object has a smaller angular diameter than a close one (see diagram). We can measure directly an objects angular diameter from where we are, so if we know how far away it is we can work out how big it is.
