- Describe the likely origin of the Moon (the giant impact hypothesis)
- Describe the evidence that allowed astronomers to develop the giant impact hypothesis.

There are four theories about how the Moon originated:

  1. The Moon evolved in a different part of the Solar System and was captured by Earth's gravity
  2. It formed in the same part of the Solar System near the Earth
  3. The Earth spun rapidly during its early evolution and Earth material flung out to form the Moon.
  4. A Mars size object crashed into the early Earth, changing its axis and jettisoning debris into its orbit and the Moon formed from this.

Since astronauts visited the Moon in 1969 our understanding about its origins has increased.

Analysis of rock from the Apollo landings confirm the Moon is made of similar material and rock as the Earth from about the same time. Theory 4 (the 'Giant Impact Hypothesis' ) is now the more accepted theory of how the Moon formed, although there is some disagreement in the scientific community and many questions remain. This has been nicknamed both 'The Big Splash' and 'The Big Whack'.

Another reason for the theory is because scientists believe the Moon's crust was molten and an impact would have provided this.

The Moon was closer to the Earth (it is now moving away from us at 4cm a year) and spun on its axis more quickly. Tidal forces slowed down the Moon's rotation. Tidal friction performs as a brake on the Earth's rotation and eventually in billions of years time the Earth day will be 55 of our present days long and so will the Moon's. The Moon will appear to hang in the same position in the sky. When this happens the Moon will be said to have a synchronous orbit.


Image New Window

Links New Window

ESA Kids Birth of the Moon

NOVA The Big Whack animation


  • What evidence is there to suggest the Giant Impact Hypothesis is a likely explaination for the origin of the Moon?
  • Compare different theories about the origin of the Moon.