 As the earth formed, it was rotating on an axis perpendicular to the solar plane, just like all the other planets. Today, it's off by 23.5 degrees. The 1970s Apollo missions to the moon collected 400 kilograms of moon rocks, that's 900 pounds on earth. These rocks showed a remarkable similarity to rocks here on earth. This indicates that they were made at the same place. An analysis of the oldest moon rocks showed that they are the same age as the oldest rocks found on earth. This indicates that they formed at the same time. These along with many other earth-moon system characteristics, supports the idea that the earth-moon combination was the result of a massive collision called a giant impact hypothesis. There is an animation created by full dome that illustrates how this may have looked. It's a hypothesis instead of a theory because there are a number of variations and not enough data available to enable definitive selection of the right combination of possibilities. The hypothesis that currently comes closest to what happened has it that a planet the size of Mars, called Thea, moved in from the outer solar system with major quantities of water and collided with the earth shortly after it had formed. It hit at a 45 degree angle, traveling at around 4 kilometers per second, that's 2.5 miles per second. The collision would have tilted the earth, liquefied, vaporized, and homogenized the mantles of the two planets and ejected massive amounts of matter into space where it coalesced into the moon.