Why does the Moon orbit Earth? The Moon revolves around Earth because the former is well within the latter’s Hill Sphere. Any object within Earth’s Hill Sphere will be dominated gravitationally by our planet. In fact, every celestial body is surrounded by this gravitational sphere, the extent of which is determined principally by its mass and its distance from its parent body.*
The above image shows the Moon within the boundary defined as Earth’s Hill Sphere. However, the image is not to scale because this zone extends 1.47 million kilometers above our planet’s surface. (Refer to the planetary radius chart below.) Since the Moon’s average distance equals about 0.384 million kilometers, it is safely within this boundary. Even though the Moon is slowly moving away from Earth at the glacial rate of 3.8 cm/year, it will never venture outside of this region. If it did, however, it would ‘detach’ from Earth and establish its own independent orbit around the Sun.
The chart above shows the Hill Sphere radii for the solar system’s major bodies. Note: Neptune’s zone is the largest because of Neptune’s greater distance from the Sun. Pluto’s radius is smaller because it is far less massive than Neptune.
[Note: Yes, a moon can in theory have a moon of its own—a sub-moon—provided that the smaller moon is within the larger moon’s region, but outside the Roche Limit, the boundary at which the sub-moon would be torn apart by the larger moon’s tidal forces.]
*The Hill Sphere of a body of mass m when in orbit around a larger body of mass M can be approximated by the following formula:
Hill sphere = a(1-e) (m/3M)^1/3
a = the smaller body’s semi-major axis
e = the smaller body’s orbital eccentricity
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