In my blog post on Meteorites I mentioned the Moon formation theory regarding what is known as the Giant Impact Hypothesis. This theory is widely becoming regarded as the best model science has for the Moon’s formation.

Regardless of the evidence for and against this model, here is how it works…

We set the scene around 4 and a half billion years ago, just a few tens of millions of years after the Earth’s formation. At the same time as the Earth has formed, another body, dubbed Theia, has also been created at one of the Langrangian points (proably L5) along the Earth’s orbit. Lagrangian points are those at which the gravity forces of the Sun and Earth are minimal or non-existent - The Hubble Sapce telescope sits at L2 for example.

As Theia grew, it eventually got too big for its position (around the size of Mars) and began to sway in its orbit, eventually being flung at an angle toward the Earth, attracted by its gravitational pull.

It hit the Earth, throwing of huge amounts of debris and itself being destroyed in the impact, merging eventually with the Earth and also the material flung outward.

Caught in the Earth’s gravity, the debris coalesces itself into a body - the Moon - and settles into a tidally locked orbit, meaning that the same face always presents itself to the Earth.

I went looking for an animation of these events and found one on Wikipedia:

The object was named Theia after the greek Titan who gave birth to Selene, goddess of the Moon.