Why some moons rotate differently than other moons.
Published on 18 Sep 2011 at 12:00 am.
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Many celestial bodies, including moons, are known to rotate, or spin on an axis. Different moons rotate in different directions and at different rates. The reason for these differences is that different forces are acting, and have acted in the past, on the different moons. Usually, properties of the planet that a moon orbits around have the greatest effect on the direction and rate of the rotation of that moon.
Nearly every moon in our solar system rotates roughly in the same direction of the planet that it orbits. Because not every planet rotates in the same direction, neither does every moon. There are also extreme cases, such as Hyperion, one of Saturn’s moons, which rotates chaotically and in no set direction due to its eccentric orbit and the gravitational influence of Titan, Saturn’s largest moon.
The rate of rotation of a moon is most notably affected by the gravitational pull of the planet it orbits. A typical moon eventually becomes tidally locked, which means that it rotates at the same rate that it orbits, keeping the same face toward its planet at all times. Thus, factors such as differing planet sizes cause moons to have differing rates of rotation.