Q (Tidal Quality Factor)

Definition

The tidal Q is a measure of a body’s response to tidal distortion and is inversely proportional to the lag angle between the tidal bulge and the position of the perturbing body. For example, on a satellite orbiting close to a planet, the satellite becomes elongated as the force of gravity is stronger at the sub-planet point than on the opposite side. As the satellite orbits, this tidal bulge does not necessarily point toward the planet, due to the satellite’s rotation. Rocky bodies tend to have Q-values near 100; giant planets and stars have Q-values near 10⁶. However, these numbers are very difficult to measure, and the above values are probably only accurate to about an order of magnitude. The orbital evolution of the planet-satellite (or star-planet, or star-star) system is therefore a function of Q, with faster evolution for smaller Q-values.