The moon formed as a consequence of cosmic debris colliding with Earth.
Billions of years ago, while Earth was still a spry young thing, it came into touch with Theia, a "Mars-sized planet." According to the Natural History Museum, the impact caused "almost all of Earth and Theia to melt and rebuild as one body, with a little fraction of the new mass spinning out to become the Moon as we know it." The mechanism by which the moon formed from the debris of the enormous collision is known as "rotational fission," and it was confirmed by Apollo astronauts when samples of moon rock were found to have isotopes identical to those found on Earth.
Did you Know? The footsteps of the Apollo astronauts on the moon will most likely remain there for at least 100 million years.
Did you Know? Jupiter's Great Red Spot is actually a planetary-sized storm that has been raging on for centuries