Most people know the basics about the moon. It orbits our planet in a slight ellipse, just as our planet orbits the sun. The relationship between the Earth, the sun and the moon creates the moon's phases as well as solar and lunar eclipses. On clear nights, a full moon outshines many stars by reflecting light from the sun.

But the moon is more than just a pretty face. It contributes to the ocean's tides on Earth. Its presence also helps regulate the Earth's orbit. Space.com's article on moon mechanics explains many of the ways that the moon affects the Earth.


But how did it get here? There are four main theories:

  • It's a broken piece of Earth. During the early, rapidly spinning days of the solar nebula, the moon might have broken away from the Earth. Scientists question this theory because the Earth probably wasn't rotating fast enough to spin off a whole moon.

  • It's the Earth's baby brother. As the Earth was forming through accretion, the moon might have formed the same way, right next door. Scientists doubt this one because the moon doesn't have the same physical composition as the Earth.

  • It followed the Earth home, and the Earth kept it. The moon may have come from somewhere else in the universe and gotten caught in the Earth's gravity. It's relatively unlikely, though, that something the size of the moon could have passed by the Earth in just the right way to end up orbiting the planet rather than crashing into it.

  • It came from a fiery crash. The most popular scientific theory for the origin of the moon is that two large planetesimals, or planets-in-progress, collided in the solar nebula. The smaller object was about the size of Mars. The smaller planetesimal ended up in orbit around the larger. Neither object was really solid yet, so the iron core of the smaller got pulled back to Earth because of gravity. This explains the differences in composition between it and the Earth.
In addition to its physical composition, the moon differs from Earth in other ways. It has substantially less gravity because it's so much less massive. For the same reason, it also has very little atmosphere -- it's gravity isn't strong enough to hold an atmosphere to it surface.

Moon Myths
Many people have lots of misconceptions about the moon, and they're not just about insanity and green cheese. Here are a few:
  • The moon doesn't spin. It takes 29.5 days for the moon to spin one time. It also takes 29.5 days for the moon to orbit the Earth. This is why we see the same face of the moon all the time. This hasn't always been true - the moon's spin has gradually slowed down to match its orbit around the Earth.
  • One side of the moon is always dark. We see the same side of the moon every time we look at it. But the "back" of the moon gets light during the new moon, when the moon is between the Earth and the sun.
  • The moon will crash into the earth. The Earth's gravity does pull on the moon. But because of complex interactions between the Earth's gravity and the moon's, the moon is moving away by a few centimeters per year.
  • The moon orbits the Earth. It's common knowledge that the moon orbits the Earth, but it isn't exactly true. The moon does move around the Earth, but the Earth is moving, too. Both objects orbit a common point, called the barrycenter.

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