And if the band you’re in starts playing different tunes
I’ll see you on the dark side of the Moon.
One of my favorite readers, Zrinka, asks us why we’re only able to see one side of the Moon from Earth. Seriously, look at the different phases; we always see the same side of the Moon:
How does this happen? Well, the Moon makes one revolution around the Earth about every 29 days, and that’s what causes the Moon phases. But the Moon also rotates once every 29 days also. Because of this, the same side of the Moon always faces the Earth:
We call this “being locked.” We’re not the only system like this, by the way. Both of Mars’ moons, Phobos and Deimos, always have the same side facing Mars. All of Jupiter’s, Saturn’s, Uranus’, and Neptune’s moons are locked to those planets as well. And in a really weird case, Pluto and its largest moon, Charon, are locked to each other, so that both always show one another the same face:
But what made this happen? Why do Moons wind up locked to the planets they orbit? Like everything else in the solar system, the culprit is gravity. When you stand up, the Earth pulls down on you. But your feet are just a little bit closer to the center of the Earth than your head. This slight difference means that the Earth pulls with a slightly larger gravitational force on the lower half of you than the upper half; this slight difference is called a tidal force. When something like a Moon is close enough to a planet, these gravitational tidal forces cause the Moon to spin at the same rate that it revolves around the planet. When this happens, we call it tidal locking, and that’s why we always see the same side of the Moon!
This is true of all the planets’ moons that we know of, and is even true for some asteroids that are bound to each other. In fact, based on what we know, we think that an extra-solar planet, Gliese 581 c, is tidally locked to its Sun, just like our Moon is locked to us. Pretty neat!
Update: Here are a couple of images for you to compare. First up is the “near side” of the Moon, or the side that always faces us:
and next is the side you’ll never see from Earth, the “far side,” or the side always facing directly away from Earth:
These pictures were taken by the Clementine spacecraft, which was the first astronomical data I ever worked with as an undergraduate (back in 1998). Isn’t it neat how different these sides are? And now you know a little more about the side of the Moon you’ll never see!