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The tidal Q (for tidal Q factor or tidal quality factor) is a number associated with an orbiting body that characterizes the rate at which tidal force converts kinetic energy to heat. It is the maximum potential energy reached in the body's tidal distortion, divided by the energy lost over the course of a cycle (the tidal period), typically this ratio multiplied by 2π. For the tidal Q associated with an orbit, the cycle lasts for an orbital period, adjusted if necessary to accommodate the central body's rotation. The "lost" energy is typically converted to heat, a source of heat within such a body, e.g., a moon. A tidal Q much greater than one indicates the conversion is occurring very slowly. A tidally locked moon in a perfectly circular orbit around a uniform spherical host theoretically has an infinite tidal Q (no energy converted) since there is no variation in the stress and strain over the course of the orbit. An eccentric orbit has a varying tidal force, more when the moon is nearer, and internal friction from the strain generates heat, which can be significant, i.e., keep a moon measurably warmer than would otherwise be expected. Also, in such an eccentric orbit, the locking is imperfect since the moon rotates basically uniformly once per period, but the direction facing the host planet, is shifting, thus the tidal forces shift accordingly. Viscosity contributes to the tidal Q of gas planets, but in such a case, inertial waves can play a larger role.
A body may be affected by more than one tide mechanism (e.g., the Earth's tides due both to the Sun and the Moon), each with its Q. Also, tides affect particular portions of a body, such as the Earth's sea tides, atmospheric tides, and tides within Earth's solids. Q may be subscripted to indicate it is the quality factor specific to some mechanism and/or portion of the body.
The tidal Q is a type of dissipation function (i.e., a tidal dissipation function), meaning the tide's dissipation of energy, and those terms are sometimes used for the tidal Q.
The terms Q factor, quality factor, or just Q, are used more generally for any oscillators that have a damping mechanism (e.g., a pendulum that experiences friction) and tidal Q can be thought of as an adaption of this concept to the tidal cycle.