NUMBER 9
Locked in orbit

Perhaps the coolest thing about the Moon is that it always shows us the same face. Since both the Earth and Moon are rotating and orbiting, how can this be?

Long ago, the Earth's gravitational effects slowed the Moon's rotation about its axis. Once the Moon's rotation slowed enough to match its orbital period (the time it takes the Moon to go around Earth) the effect stabilized. Many of the moons around other planets behave similarly.

What about phases? Here's how they work: As the Moon orbits Earth, it spends part of its time between us and the Sun, and the lighted half faces away from us. This is called a new Moon. (So there's no such thing as a "dark side of the Moon," just a side that we never see.)

As the Moon swings around on its orbit, a thin sliver of reflected sunlight is seen on Earth as a crescent Moon. Once the Moon is opposite the Sun, it becomes fully lit from our view -- a full Moon.