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The Winter 2010 issue of The Earth Scientist includes a variety of educational resources, ranging from astronomy to glaciers. Check out the other publications and classroom materials in our online store.
This drawing illustrates how forming moons drew gas to themselves.
Image from: The New Solar System

The Co-Formation Theory

The co-formation theory explains the origin of the moon as an object which formed out of the primitive solar nebula at the same time and roughly the same place as the Earth. As shown in this picture, while they were forming in the solar nebula, the nucleii of the moons-to-be (called protomoons) drew material to themselves from the cloud of gas and dust around them.

Because the proto-moon was so close to the proto-earth the nebular material out of which they both formed ought to be very similar, composed mostly of rocky material rather than volatile gases.

The co-formation theory explains why the moon appears in the location it does but it does not explain the evidence that the Earth and Moon do not appear to be made of the same material.

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