Many satellites in the solar system always show the same face to their primary. For example, we always see the same side of the moon. Other examples include the Galilean satellites of Jupiter (Io, Europa, Ganymede and Callisto), as well as most of the regular satellites of the other giant planets. Tides raised in the satellites by the gravitational field of the planet is responsible for this, hence the name tidal locking. A simplified account of the process follows. Imaging that we start with a rapidly spinning satellite with its spin axis not aligned with its orbital axis
A body that is tidally locked to its primary is said to be in a one to one spin orbit resonance, since the spin period and the orbital period are in a 1:1 ratio. There are other possible end states of tidal evolution. For example, Mercury is in a 3:2 spin orbit resonance.
Tidal interactions between a satellite and its primary also affect the spin of the primary. For example, the length of the day is increasing. In outline form, this works as follows:
Note that the tidal interaction that affects the spin period of the primary also affects the orbital period of the moon. Since the moon is moving away from the Earth, the month is getting longer. In other cases, the orbital period of the moon may decrease. If the orbital period of the moon is shorter than the length of day, or of the moon is retrograde, like Neptune's moon Triton, the month will shorten over time.
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