# What caused the Moon's initial inertia?

I.e. if we removed the Earth the Moon would keep moving in a straight line if there was no Sun (question edited based on correction below).

What was the source of the original force or push which first caused the moon's movement along its initial path?

• It would keep moving in orbit around the Sun, which doesn't seem much like a straight line to me. And that orbit rests in the angular momentum of the primordial nebula that resulted in our solar system (ultimately). – Jon Custer Oct 13 '16 at 21:52
• Sorry could you please explain, "and that orbit rests in the angular momentum of the primordial nebula that resulted in our solar system (ultimately)." I just read up on the primordial nebula so I may be missing something. "Solar system formed about 4.6 billion year ago, when gravity pulled together low-density cloud of interstellar gas and dust (called a nebula)" Does that mean that gravity was the cause of the initial push that resulted in the moons inertia? – K-Feldspar Oct 13 '16 at 21:57

The modern theory is that a small planet named "Theia" collided with the young earth, spewing a lot of debris during the clash. The average velocity of the swirled debris was smaller than the escape velocity $\sqrt{\text{2 G M}/{\text{r}}}$ where M is the earth's mass, but high enough to make it into orbit. So the momentum was transfered like it is in billard, but with a lot more debris particles involved.