Tidal acceleration

Tidal acceleration is the gradual change in an orbiting body's motion caused by gravitational tides raised on the body and its companion. In many systems, it transfers angular momentum between rotation and orbit, changing orbital distance and period.

Tidal force: The difference in gravitational force across an extended body, caused by a nearby mass. The uneven gravity that deforms bodies and creates the torques behind orbital change.
Lunar recession: The gradual increase in the Moon's distance from Earth, measured at about 3.8 centimeters per year. It is the present-day outward orbital change driven mainly by tides raised on Earth.
Earth's rotation: The turning of Earth about its axis, which sets the length of the day. Tidal transfer to the Moon's orbit gradually slows Earth's spin.
Tidal deceleration: A decrease in orbital angular motion caused by tidal torques, often accompanying inward migration. It distinguishes inward orbital slowing from outward tidal acceleration in common usage.
Tidal locking: A state in which a body's rotation period matches its orbital period around another body. Dissipation of rotational energy can synchronize a body's spin while its orbit evolves.
Earth–Moon system: The coupled Earth and Moon system, whose rotation and orbit evolve through mutual gravity and tides. Its changing day length and lunar distance provide a well-measured example of tidal acceleration.
Sidereal month: The time the Moon takes to complete one orbit relative to distant stars, about 27.3 days. Lunar orbital expansion lengthens this period over time.
Tidal torque: A torque produced when a body's tidal bulge is misaligned with the line connecting it to its companion. Its direction depends on rotation relative to orbital motion, determining whether an orbit expands or shrinks.
Angular momentum: A conserved physical quantity describing rotational motion, defined by position and momentum relative to an origin. Tidal torques redistribute it between rotation and orbital motion.
Io (moon): Jupiter's innermost large moon, whose orbit and interior are strongly affected by tidal heating. Resonant orbital forcing continually deforms Io and links its orbital evolution to Jupiter's rotation.

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