Instead, it's in a 3:2 resonance - in other words, Mercury's day is two-thirds as long as its year.Ĭloser to the Roche limit the body (an exoplanet) is deformed by tidal forces. Mercury's eccentric orbit prevents it from being in a 1:1 spin-orbit resonance. Pluto and Charon are tidally locked to each other. Examples of this are common in our Solar System. Another way of saying this is that the Moon is in a 1:1 spin-orbit resonance - the ratio of its rotational (spin) period to its orbital period is 1 to 1. We always see the same face of the Moon from the Earth because the Moon's rotation period is the same as the time it takes to complete one orbit around the Earth. This is why most satellites, like the Moon, face toward their planet - they are "tidally locked" in that orientation. Just as the Earth's rotation is slowing due to the Moon's tidal force on it, the Moon's rotation has slowed until it is locked into this position. The same tidal force that stretches a satellite also tends to slow its rotation until the longest axis of the satellite lines up with the planet. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth.
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