You'd be hard-pressed to find someone who's never seen the moon in the sky. It comes up, it goes down, it changes phases. But when you stop and really think about it, the moon is pretty strange and pretty fascinating. Here are some facts to refresh your memory about our closest neighbor in space:

The average distance from the Earth to the moon is 238,855 miles (384,400 kilometers). It took three days for Apollo astronauts to make the trip. The moon was formed about 4.5 billion years ago, shortly after Earth, and according to the prevailing wisdom, a Mars-sized object named Theia collided with our infant planet. The impact produced the debris that eventually became the moon. This isn't some Biblical creation story; that's what the chemical signatures of moon rocks tell us.

One thing you might not appreciate about the moon is its size. Compared to Earth, it's pretty big. Many moons are tiny compared to the planet they orbit. The moon is about one-fourth the diameter of Earth. Some scientists even consider Earth and the moon a double planet. That way of looking at it can help you remember the relationship between our world and its moon.

The moon's size is important. Its gravity stabilizes Earth's axis, which is tilted at about 23.5 degrees and has been relatively stable for millions of years. Without the moon, our axis might have wobbled drastically, leading to dramatic climate shifts. So, the moon is not just something pretty to look at. In a way, it has been helping to keep Earth habitable.

The moon also causes the tides. Its gravity pulls the oceans back and forth on Earth, and that has affected coastlines, marine life and navigation for thousands of years. The sun also has a role, but the moon has the greater effect. Spring tides occur when the Earth, moon and sun are aligned. Neap tides occur when they form a right angle.

Take a look at the surface of the moon. The dark splotches you see are huge basins of solidified lava called maria. These formed when volcanoes filled the impact craters with lava billions of years ago. The craters you see are, well, craters. The moon has no atmosphere and almost no geology, so they remain. The moon preserves much of its history for us to see.

There is water on the moon, too. Ice is trapped in some craters near the lunar poles, where the sun does not shine. Scientists confirmed this within the last two decades, and it has affected how they and some space agencies have been planning to return to the moon. That ice can provide drinking water, for example, or even rocket fuel.

Twelve men have walked on the moon. That was all a long time ago (between 1969 and 1972). No human has visited since. This is one of the oddest facts in the history of exploration, and it may change soon as agencies prepare to return to the moon.