To disprove the idea put forward by Gilbert that the entire Earth was a magnet, Kircher attempted to calculate the force it would exert and the weight it could move if it were so. He concluded that if the Earth were a magnet and the Moon ferrous, the Earth's magnetic force would cause the Moon to crash into it. He likewise argued that if the Earth were a magnet then all the iron on its surface, even in mountains, would be pulled into itself and could not remain where anyone could find it. Kepler had argued that the Sun was like a magnet, holding the planets in their orbits around it. Kircher found this unsatisfactory: how could a single, consistent magnetic force emanating from the Sun explain the different motions of the planets, with their varying speeds and trajectories? How could a force powerful enough to move the planets leave the fixed stars beyond them unmoved? In any case, he argued, it was contrary to the observable behaviour of magnets to suggest that a motionless magnet could draw magnetised bodies around it in orbit.