Galileo's interest in magnetism is contemporaneous with the publication, in 1600, of William Gilbert's De Magnete, which immediately had a major impact. Kepler, in his Astronomia Nova, based the physical explanation of planetary motions on magnetism. According to Kepler, the Sun was a magnetic body, and the motion of the planets was due to the action of the magnetic vortex produced by the Sun's rotation. Hostile to occult properties, Galileo rejected the explanation of tides based on lunar attraction. Nonetheless, he performed experiments on magnetic needles, magnetic declination, and the arming of magnets—both during his stay in Padua (in collaboration with Paolo Sarpi and Giovanfrancesco Sagredo) and after his return to Tuscany.