Tidal Effects

Abstract

The relative acceleration between two nearby bodies produced by another gravitating, far away mass is determined by the gradient of the acceleration caused by the latter; this produces a tidal acceleration, proportional to their distance. Tidal processes are ubiquitous in the universe. They also appear when two bodies interact gravitationally and the size of at least one of them is not negligible with respect to their mutual distance. In this case gravity gradients give again rise to tidal forces and torques, that in turn cause deformations and changes in the rotational state of the affected bodies. In many cases tidal deformations are time-dependent, and thus dissipation of energy occurs, owing to violations of Hooke’s law of elastic behaviour, viscosity in fluid regions or friction at interfaces. This dissipation is frequently the main source of long-term, accumulating changes in the rotational and orbital parameters of the celestial bodies. Although tidal physics has many astrophysical applications concerning stellar evolution and high energy processes occurring in close binary systems, as well as in galactic dynamics and morphology (close encounters between galaxies are a relatively frequent phenomenon), geophysics and planetary science provide the main arenas for tidal effects. We shall discuss some examples in this Chapter, leaving to Ch. 15 the analysis of tidal evolution processes in the solar system.