Editing Tidal acceleration (section)

{{Short description|Natural phenomenon due to which tidal locking occurs}}
[[Image:The Earth and the Moon photographed from Mars orbit.jpg|thumb|right|200px|A picture of [[Earth]] and the [[Moon]] from [[Mars]]. The presence of the Moon (which has about 1/81 the mass of Earth), is slowing Earth's rotation and extending the day by a little under 2 milliseconds every 100 years.]]

'''Tidal acceleration''' is an effect of the [[tidal force]]s between an orbiting [[natural satellite]] (e.g. the [[Moon]]) and the primary [[planet]] that it orbits (e.g. [[Earth]]).  The [[acceleration]] causes a gradual recession of a satellite in a [[retrograde and prograde motion|prograde orbit]] (satellite moving to a higher orbit, away from the primary body, with a lower orbital velocity and hence a longer orbital period), and a corresponding slowdown of the primary's rotation. See [[supersynchronous orbit]]. The process eventually leads to ''[[tidal locking]]'', usually of the smaller body first, and later the larger body (e.g. theoretically with Earth-Moon system in 50 billion years).<ref name="NASA Tidal Locking">{{cite web |title=Tidal Locking |website=NASA Science |url=https://science.nasa.gov/moon/tidal-locking/ |publisher=NASA |access-date=2025-05-23}}</ref> The Earth–Moon system is the best-studied case.

The similar process of '''tidal deceleration''' occurs for satellites that have an orbital period that is shorter than the primary's rotational period, or that orbit in a [[Retrograde and prograde motion|retrograde]] direction. These satellites will have a higher and higher orbital velocity and a shorter and shorter orbital period, until a final collision with the primary. See [[subsynchronous orbit]].

The naming is somewhat confusing, because the average speed of the satellite relative to the body it orbits is ''decreased'' as a result of tidal acceleration, and ''increased'' as a result of tidal deceleration. This conundrum occurs because a positive acceleration at one instant causes the satellite to loop farther outward during the next half orbit, decreasing its average speed. A continuing positive acceleration causes the satellite to spiral outward with a decreasing speed and angular rate, resulting in a negative acceleration of angle. A continuing negative acceleration has the opposite effect.