Transcript: Is there something that sets the maximum size of the moon of a planet? The answer to this question complex, but one part of the answer is Roche's limit. The tidal force on a moon increases both with the size of the moon and it's proximity to the planet. Thus, a moon that is large and close to a planet will have a large tidal force, and at some point that tidal force will disrupt the solid material and break apart the moon leading perhaps to a ring system. Thus, moons that are too close and too large to their planets will be disrupted, destroyed, and eventually end up as ring systems. Thus, we should not be surprised that most moons are substantially smaller then their parent planets and relatively far away. Earth's moon in this regard is relatively large being only a quarter the size of the Earth and about one-eightieth of the mass. We should also remember that not all moons and rings are related in an evolutionary sense; some moons have been captured gravitationally from other parts of the solar system.