Gilbert, a physician to Queen Elisabeth I, wrote the first experimental treatise devoted to magnetism. Gilbert discerned analogies between the Earth and magnets, and reasoned that the Earth itself is a magnet. The Earth’s magnetic field behaves in the same manner as a spherical magnet one may directly observe. Because Gilbert believed that a spherical magnet floating on water rotates on its axis every 24 hours, he supposed that the Earth would do the same. Gilbert’s ideas on magnetism stimulated Galileo, Kepler and many others in their thinking about the motion of the Earth, Sun and planets.