• According to the most widely accepted theory of the formation of the Moon, the Moon formed when a Mars-sized object collided with the Earth about 4.5 billion years ago ('Giant Impact Theory').
• Consistent with the collision theory:
  • The composition of the Moon is very similar to the Earth's mantle. The Moon has very little iron in its core.
  • Computer simulations confirm the impact hypothesis.
• This video goes over some of the history of this theory and has some nice animations.

• Fission theory--A molten Earth budded off the Moon. This theory failed because it would imply:
  • a ring of smaller moons rather than one big one
  • the moons would fly off and then fall back to Earth (i.e., stable orbit is unlikely)
• Co-accretion theory--The Moon formed at the same time as the Earth from the same material. This theory failed because it would imply:
  • Moon should have an iron-rich core like the Earth, bit it does not.
  • The Moon is moving away from the Earth and, on "playback" appears to have been born from the Earth.
• Capture theory--The Moon was captured by the Earth like many of the moons of Jupiter. This theory failed because:
  • A captured moon would follow an elliptical orbit. However, the Moon follows an almost circular orbit.
  • The Moon is too big to be captured. A collision with Earth would be more likely.
     

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