Where did the Moon come from

The Moon is obviously a very familiar object to us Earthlings. It has been orbiting the Earth for longer than humans have inhabited the planet (much, much longer, if scientists are correct). Not only is the Moon familiar, but it is also responsible for another familiar feature of Earth, the tides of the oceans. However, despite all of this familiarity, there is much about the Moon that we don’t know, or have just recently learned.  And perhaps because of the Moon’s proximity to the Earth, we have a lot of questions about it. One of these questions is, “Where did the Moon come from?”.

Scientists have been using what we already know about the Moon to try and get a grasp of where it came from. We do know a bit about the composition of the surface of the moon through samples taken by astronauts. We know that the Moon is a little more than four billion years old and that it is covered in craters that were probably acquired during the late stages of the galaxy’s development. The Moon is roughly 2,159.9 miles in diameter and lacks the molten core that the Earth possesses. It has no atmosphere. Based on this information and much, much more, scientists have come up with four theories regarding the origins of the Moon, one of which is currently the most popular.

Co-accretion or Condensation Theory

The Co-accretion or Condensation Theory is the idea that the Moon and the Earth were formed at the same time; when the solar system was formed. The Moon may have slipped into orbit around the Earth immediately or soon after both bodies were formed.

Fission Theory

The Fission Theory suggests that a large chunk of the Earth broke free and flew off into space, back when the Earth was rotating much faster than it is now. The piece then fell into orbit around the Earth and began to take on its present shape. Where this chunk would have broken off is anyone’s guess, but a popular theory is that the Moon used to occupy the Pacific Basin.

Capture Theory

The Capture theory suggests that an object (which would become Earth’s Moon) was innocently floating through space when it was pulled into orbit around the Earth and became trapped there. Of course, the conditions would have had to have been highly favorable for such an event to have occurred and many scientists dismiss the Capture theory because of this. However, you just never know. It’s not as if the universe is reliably predictable.

Impact or Collision Theory

The Impact or Collision Theory is based on the idea that a celestial object, roughly the size of Mars, collided with the Earth, billions of years ago. The idea is that when the two objects collided, some of the Earth was blasted into its own orbit and pieces of the other object did the same. These pieces would have eventually formed into one, the Earth’s Moon. This is the most widely favored of all the Moon origin theories.

Of course, none of these theories has been proven conclusively and, given that these events most likely occurred millions or billions of years ago, all of the possibilities we can imagine will likely remain theories and nothing more. Nonetheless, it doesn’t hurt to try and figure out how that big hunk of rock came to be circling this planet and pulling at its oceans.


The Moon, retrieved 2/12/10, nineplanets.org/luna.html

Where did the moon come from?, retrieved 2/12/10, starchild.gsfc.nasa.gov/docs/starchild/questions/question38.html