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It all seems more arbitrary than many other systems of differentiation. Perhaps we should classify things in terms of what they are a "moon" of, meaning what they orbit. The Earth is a moon of the Sun, and the Sun is a moon of the galaxy, and the galaxy is a moon of the universe.
The most widely held theory is that the moon is comprised of partly some object colliding with earth, and taking a chunk of it away. Maybe we should call it earth theif?
Anyhow, what I don't get is if the moon is partly composed of earth, then why aren't the same diversity of elements and compounds found there?
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