It is commonly held that Galileo gave little or no attention to the problem of planetary motion. Some have taken the position that his principle of so-called "circular inertia" eliminated the need for a planetary mover. Galileo was certainly aware of Kepler’s theory that a rotating Sun was the cause of the motions of the planets, but never commented on the astronomer’s ideas on planetary motion. After the publication of Galileo’s Sidereus nuncius, Kepler several times urged upon Galileo the notion of a rotating Sun as planetary mover. With his discovery and close observation of sunspots, however, Galileo concluded that the Sun indeed rotates, and went on to propose that Sun’s rotation moved the planets. In this belief he was echoing positions taken earlier, not only by Kepler, but also by Giordano Bruno. All three were modifying and elaborating a tradition concerning the role of the Sun that went back to Platos’s Timaeus.