Waltzing with Luna

2/25/07 – 3/3/07
C. Zaitz

Did you ever notice that when you gaze at the full moon, it always looks the same? It’s not just a round bland face; it has features. Some say it has a smile, or that the dark areas look like a bunny or frog or even an astronaut tickling the chin of a poodle. Next full moon, take a look and notice what you see. It’s what you’ve seen your entire life.

Maybe you don't miss the other side of the moon, but isn’t it odd that we never get to see it? It’s a sphere, so it must have other faces, but we only see the same one, over and over. In fact, no one had ever seen the back side of the moon until 1959, when we sent rockets around our cosmic companion. It turns out that the other side of the moon is very different from the side we see. There are no dark areas to make smiles or bunnies. It’s all “highlands,” with very few “maria,” the dark lava “seas” that make the shapes so familiar to us on our side of the moon.

Notice I didn’t say “dark side of the moon.” No offense to Pink Floyd, but technically it’s not correct to refer to the other side of the moon as the “dark side,” since over the course of a month it gets just as much sunlight as the side facing us.

So the question is: why do we see only one face of the moon? Is it because the moon isn’t rotating? Actually, with respect to the stars, it is rotating. If it didn’t rotate, over the course of the month we’d see all of it. It would slowly show each nook and crater to us. Instead, it rotates at the same speed that it circles us. This is called synchronous rotation, and it turns out that it is no accident. The majority of the moons in our solar system are synchronized with their parent planet. It’s all about tides.

We may remember that the moon tugs on the earth and creates tidal bulges in the oceans, but what we may not know is that even rocks and dirt feel the effects of that tug and experience tidal bulging. Tidal bulges occur in any body that is tugged on by other bodies. The result of these bulges is that they act like little friction brakes to the spin of the object. The moon is causing the earth to slow down. As a thank you, the earth is sending the moon farther away from us to conserve angular momentum. Meanwhile, the two are facing off like a bullfighter and his bull. The moon has already succumbed, but eventually, earth will slow enough so that one face will point eternally toward the moon as well. Then they will dance, staring each other down, until the sun itself burns out and goes dark.

This effect is called tidal locking, and is the opposite of rare in the solar system. Given enough time, most objects will have orbital and rotational resonance with their nearest gravitational partners. If that sounds like heavy duty physics, it is. But it’s also beautiful, because the whole system of planets, moons and the sun will be in sync, in a cosmic waltz, pirouetting at the same time, locked in eternal embraces. I wonder what the band will be playing for the dance. Perhaps it will be the music of the spheres.

Until next week, my friends, enjoy the view.