Expert: Courtney Seligman - 2/27/2010

Question
Why doesn't our 'Moon' spin on its axis ( rotates ) like the moon's of Jupiter and Saturn?

Answer
Our Moon does rotate. It just rotates synchronously -- that is, it rotates once, each time it goes around us. Its rotation period is 27.3 days, and so is its orbital period. So when it goes 1/4 or 1/2 or 3/4 of the way around us, it rotates 1/4 or 1/2 or 3/4 of the way around its axis. The result is that it always keeps the same face to us; but the stars move westward across its sky, once every 27.3 days.

To visualize this, take two balls, one representing the Earth, and the other the Moon. Paint one side of the "Moon" differently from the other, so you can tell which side it is. Let's call that the "near" side (in fact, you might just paint "NEAR" on it). Now stand so that the NEAR side of the "Moon" is facing the "Earth", and also some distant point (a wall behind the Earth, in a room; a distant tree or some such, outdoors). Walk around the Earth, keeping the "Moon" always facing the same way (toward the wall or tree, as it originally faced). If you do that, the "NEAR" side of the "Moon" won't face the "Earth" anymore, which would mean that people on the "Earth" could see different parts of the "Moon" at different times.

Do the experiment again, but as you walk around the "Earth", keep turning the "Moon" so that the "NEAR" side is always toward the "Earth". By the time you've gone all the way around and back to the start, you'll have turned the "Moon" once around. That's what happens with the real Earth and Moon. As the Moon goes around the Earth, it turns on its axis; but since it turns at the same rate it goes around us, we always see the same side. But that is not the same as not rotating at all. (Incidentally, almost all the moons of the other planets rotate the same way; so although they do rotate, in most cases, they always keep the same face to their planet. In fact, Pluto and its moon, Charon, always keep the same face to each other, so someone on the "far" side of either would never see the other one.)