How come the same side of the moon always faces the Earth? asks Gordon Thomas, a reader in San DiegoThe moon never turns its back on us, but it's not alone: Most of the other moons in our solar system also keep one face pointed toward their home planets.
Contrary to popular belief, that doesn't mean that the moon isn't rotating. Moons, and the planets they orbit, are always spinning. Instead, our moon's steady rotation is neatly synchronized to its orbit. As the moon rotates, one complete turn takes about 655 hours in relation to the distant stars. Which exactly matches how long the rocky satellite takes to journey once around the Earth: 27.3 days.
(However, there's a twist: If you found yourself on the moon, an actual day, noon to noon, would last about 708 hours. As the Earth-moon system travels together around the sun, the moon has to rotate a bit more than 360 degrees for noon to come around again.)
Synchronous rotation may be hard to picture without trying it for yourself. So get a small ball, mark it on one side, and make it travel around a lit lightbulb, keeping the marked side pointed at the bulb. Notice that by the time it goes around once, the marked side has faced all directions of the room. Just like the moon, the ball has rotated once on its axis on its trip around the lightbulb.
But why is the moon's rotation perfectly synchronized to its orbit? Thank gravity for our never-ending view of the Man on the Moon. Just as the moon's gravity creates tides in Earth's oceans, so has the Earth's even greater pull raised tides in the (once-molten) stony body of the moon.
Billions of years ago, the then-closer moon was spinning faster. The Earth's tugging, countered by the tendency of a moving body like the moon to fly off on a tangent, deformed our satellite. As the moon lost energy to its seismic struggle, its rotation gradually slowed.
Result: The moon was left bulging more on one side than the other -- about two miles more. Like a barbell with more weight on one end, the moon's distribution of mass means that one half feels more of Earth's gravitational pull. Which is how the Man on the Moon side ended up facing us when the moon settled into equilibrium, tidally locked by planet Earth.
A sampling of the dozens of other moons in synchronous rotation in our solar system include Mars' tiny Phobos and Deimos, Jupiter's Ganymede and Callisto; Saturn's Titan and Iapetus; Uranus' Miranda and Ariel, and Neptune's Triton.