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Moon's Origin
The hypothesis has been raised that the moon resulted from a massive collision between the Earth and a Mars-sized planetary body more than 4.6 millions years ago. The debris from the collision coalesced and formed the solid mass of the moon we see today. The fact that Earth has a satellite as a result of the collision means the trajectory of the Mars-sized impacting body approached Earth, not the sun. Had the Mars-sized object approached the sun, it might have been captured as a satellite, provided the right circumstances for orbit existed.
Moon's Formation
The collision between the Earth and the impacting Mars-sized object had to contain the right speed and angle of impact in order for the moon to form. If the angle of the impact was any less severe than it was, the impacting body would have struck a glancing blow, letting it continue with its momentum out into space, where it might have been captured by the sun's gravitational attraction. Due to the glancing blow, the impacting body disintegrated, sending up vast rocks the size of small moonlets to an altitude of 13,700 miles. The moonlets stayed within the gravitational field of the Earth, where they aggregated and congealed into the much larger body of the moon.
Orbital Conditions
The new moon that formed as a result of the collision had to follow certain laws of physics to remain in orbit around the Earth. The moon's mass amounted to 1/8 that of the Earth. The moon's speed, as of today, equals about 1 kilometer per second on its journey around the Earth, while the Earth's speed around the sun equals 30 kilometers per second. Yet the moon travels at the same speed around the sun as does the Earth, remaining in equilibrium. The moon's mean distance from the Earth averages around 238,000 miles, and its rotation profile resembles that of a curved 12-sided circle.
Hill's Sphere
American astronomer George William Hill calculated that in order for a smaller body to orbit a larger body it must reside in a volume of space where the primary body dominates gravitational influence over the smaller body, rather than any other distant object. The gravity of the primary body and that of the secondary body must be calculated, along with the centrifugal force that moves around the sun in same orbit as Earth. The three factors, the two gravitational calculations and the centrifugal force, when added together determines whether the moon can be held in orbit or not. Earth's Hill sphere has a radius of 1.5 million kilometers, and the moon's Hill sphere radius measures 400,000 kilometers, which keeps the moon "locked" to the Earth, preventing it from escaping to the sun.