There are actually many tidally locked bodies in the solar system. The technical term is "synchronous rotation", which means that there is a 1:1 ratio to the length of its orbital period and the length of its rotational period. Pluto and Charon are said to be "mutally synchronous" -- this means that they are both locked to one another, such as that one Pluto day is one Charon day which is also one Charon "month". That's what willpittenger is describing above. That's the only known example of mutally synchronous bodies, although there could well be others out in the Kuiper Belt and among the other minor planets.<br /><br />Most of Saturn's moons also rotate synchronously. This is especially interesting for bodies like Iapetus. Iapetus is astonishingly dark on its leading hemisphere, but brilliantly white on its trailing hemisphere. Dione and Rhea are the other way around; they are bright on their leading hemispheres, and somewhat darker on their trailing hemispheres (although the difference is not anywhere near as pronounced as it is with Iapetus). The synchronous rotation and bicolor pattern of Iapetus was first theorized centuries ago by its discoverer, Giovanni Cassini. He noticed that he could only see Iapetus on one side of Saturn. Therefore, it had to rotate synchronously, just like our own Moon, and its leading hemisphere had to be almost pitch black. When the Voyagers arrived at Saturn in the late 20th Century, they proved him right. <img src="/images/icons/wink.gif" /> Just an interesting random factoid. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>