Hill sphere
In the example to the right, Earth's Hill sphere extends between the Lagrangian points and , which lie along the line of centers of the two bodies (the Earth and the Sun). The region of influence of the second body is shortest in that direction, and so it acts as the limiting factor for the size of the Hill sphere. Beyond that distance, a third object in orbit around the second (e.g. the Moon) would spend at least part of its orbit outside the Hill sphere, and would be progressively perturbed by the tidal forces of the central body (e.g. the Sun), eventually ending up orbiting the latter.