Editing Moon (section)

===Tidal effects===
{{Main |Tidal force |Tidal acceleration |Tide |Theory of tides}}
[[File:Tide animation.gif|thumb|Simplified diagram of Earth bulging, being pulled and stretched toward the Moon by its gravity, which is the main driver of the [[tide]]s. The Ocean and Earth are being pulled more where it is closer to the Moon, causing [[tidal force]]s to be weaker at the far-side of Earth creating a second bulge and high-tide. The animation shows the change of the Moon's position on its inclined orbit.]]
The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides closest to each other, resulting in [[tidal forces]]. [[Ocean tides]] are the most widely experienced result of this, but tidal forces also considerably affect other mechanics of Earth, as well as the Moon and their system.

The lunar solid crust experiences tides of around {{Convert |10 |cm |4=0 |abbr=on}} amplitude over 27&nbsp;days, with three components: a fixed one due to Earth, because they are in [[synchronous rotation]], a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun.<ref name="touma1994" /> The Earth-induced variable component arises from changing distance and [[libration]], a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides).<ref name="touma1994" /> According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining [[Earth's magnetic field]].<ref>{{cite web |author=Iain Todd |date=March 31, 2018 |title=Is the Moon maintaining Earth's magnetism? |url=https://www.skyatnightmagazine.com/news/is-the-moon-maintaining-earths-magnetism/ |url-status=live |archive-url=https://web.archive.org/web/20200922194637/https://www.skyatnightmagazine.com/news/is-the-moon-maintaining-earths-magnetism/ |archive-date=September 22, 2020 |access-date=November 16, 2020 |website=[[BBC Sky at Night]] Magazine}}</ref>

The cumulative effects of stress built up by these tidal forces produces [[moonquakes]]. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour&nbsp;– significantly longer than terrestrial quakes&nbsp;– because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from [[seismometer]]s placed on the Moon by [[Apollo program|Apollo]] [[astronaut]]s from 1969 through 1972.<ref>{{cite journal |last1=Latham |first1=Gary |date=1972 |last2=Ewing |first2=Maurice |last3=Dorman |first3=James |last4=Lammlein |first4=David |last5=Press |first5=Frank |last6=Toksőz |first6=Naft |last7=Sutton |first7=George |last8=Duennebier |first8=Fred |last9=Nakamura |first9=Yosio |title=Moonquakes and lunar tectonism |journal=[[Earth, Moon, and Planets]] |volume=4 |issue=3–4 |pages=373–382 |doi=10.1007/BF00562004 |bibcode=1972Moon....4..373L |s2cid=120692155}}</ref>

The most commonly known effect of tidal forces is elevated sea levels called ocean tides.<ref name="Lambeck1977" /> While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay the [[Spring tide|spring and neap tides]].<ref name="Lambeck1977" />

The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. The tide under the Moon is explained by the Moon's gravity being stronger on the water close to it. The tide on the opposite side can be explained either by the centrifugal force as the Earth orbits the [[barycenter]] or by the water's inertia as the Moon's gravity is stronger on the solid Earth close to it and it is pull away from the farther water.<ref>{{cite web |last1=Feynman |first1=Richard |title=Feynman's Lectures on Physics – The Law of Gravitation |url=https://www.youtube.com/watch?v=-UFr1X0prbo&t=1503s |website=YouTube |date=October 24, 2020 |access-date=5 December 2024}}</ref> 

Thus, there are two high tides, and two low tides in about 24 hours.<ref name="Lambeck1977" /> Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.

If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects:
* the frictional coupling of water to Earth's rotation through the ocean floors
* the [[inertia]] of water's movement
* ocean basins that grow shallower near land
* the sloshing of water between different ocean basins<ref>{{cite journal |last=Le Provost |first=C. |author2=Bennett, A.F. |author3=Cartwright, D.E. |date=1995 |title=Ocean Tides for and from TOPEX/POSEIDON |pages=639–642 |journal=[[Science (journal)|Science]] |pmid=17745840 |volume=267 |issue=5198 |bibcode=1995Sci...267..639L |doi=10.1126/science.267.5198.639 |s2cid=13584636}}</ref>
As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

====System evolution====
Delays in the tidal peaks of both ocean and solid-body tides cause [[torque]] in opposition to the Earth's rotation. This "drains" [[angular momentum]] and rotational [[kinetic energy]] from Earth's rotation, slowing the Earth's rotation.<ref name="Lambeck1977" /><ref name="touma1994" /> That angular momentum, lost from the Earth, is transferred to the Moon in a process known as [[tidal acceleration]], which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.

Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction.<ref name="touma1994" /> Measurements from laser reflectors left during the Apollo missions ([[lunar Laser Ranging experiments|lunar ranging experiments]]) have found that the Moon's distance increases by {{Convert |38 |mm |abbr=on}} per year (roughly the rate at which human fingernails grow).<ref>{{cite journal |last=Chapront |first=J. |author2=Chapront-Touzé, M. |author3=Francou, G. |date=2002 |title=A new determination of lunar orbital parameters, precession constant and tidal acceleration from LLR measurements |journal=[[Astronomy and Astrophysics]] |volume=387 |issue=2 |pages=700–709 |bibcode=2002A&A...387..700C |doi=10.1051/0004-6361:20020420 |doi-access=free |s2cid=55131241}}</ref><ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-12311119 |title=Why the Moon is getting further away from Earth |newspaper=BBC News |date=February 1, 2011 |access-date=September 18, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150925185706/http://www.bbc.co.uk/news/science-environment-12311119 |archive-date=September 25, 2015}}</ref><ref>{{Cite journal |last1=Williams |first1=James G. |last2=Boggs |first2=Dale H. |date=2016 |title=Secular tidal changes in lunar orbit and Earth rotation |url=https://doi.org/10.1007/s10569-016-9702-3 |journal=Celestial Mechanics and Dynamical Astronomy |language=en |volume=126 |issue=1 |pages=89–129 |doi=10.1007/s10569-016-9702-3 |bibcode=2016CeMDA.126...89W |s2cid=124256137 |issn=1572-9478 |access-date=July 30, 2022 |archive-date=July 30, 2022 |archive-url=https://web.archive.org/web/20220730084922/https://link.springer.com/article/10.1007/s10569-016-9702-3 |url-status=live}}</ref>
[[Atomic clock]]s show that Earth's Day lengthens by about 17&nbsp;[[microsecond]]s every year,<ref>{{cite web |last=Ray |first=R. |date=May 15, 2001 |url=http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html |title=Ocean Tides and the Earth's Rotation |publisher=IERS Special Bureau for Tides |access-date=March 17, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100327084125/http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html |archive-date=March 27, 2010}}</ref><ref>{{Cite journal |last1=Stephenson |first1=F. R. |last2=Morrison |first2=L. V. |last3=Hohenkerk |first3=C. Y. |date=2016 |title=Measurement of the Earth's rotation: 720 BC to AD 2015 |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=472 |issue=2196 |pages=20160404 |doi=10.1098/rspa.2016.0404 |pmc=5247521 |pmid=28119545 |bibcode=2016RSPSA.47260404S}}</ref><ref>{{Cite journal |last1=Morrison |first1=L. V. |last2=Stephenson |first2=F. R. |last3=Hohenkerk |first3=C. Y. |last4=Zawilski |first4=M. |date=2021 |title=Addendum 2020 to 'Measurement of the Earth's rotation: 720 BC to AD 2015' |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=477 |issue=2246 |pages=20200776 |doi=10.1098/rspa.2020.0776 |bibcode=2021RSPSA.47700776M |s2cid=231938488 |doi-access=free}}</ref> slowly increasing the rate at which [[UTC]] is adjusted by [[leap second]]s.

This tidal drag makes the rotation of the Earth, and the orbital period of the Moon very slowly match. This matching first results in [[tidal locking|tidally locking]] the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years,<ref name="Universe Today 2016">{{cite web |title=When Will Earth Lock to the Moon? |website=Universe Today |date=April 12, 2016 |url=https://www.universetoday.com/128350/will-earth-lock-moon/ |access-date=January 5, 2022 |archive-date=May 28, 2022 |archive-url=https://web.archive.org/web/20220528015905/https://www.universetoday.com/128350/will-earth-lock-moon/ |url-status=live}}</ref> the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become a [[red giant#The Sun as a red giant|red giant]], most likely engulfing the Earth–Moon system long before then.<ref>{{cite book |last1=Murray |first1=C.D. |last2=Dermott |first2=Stanley F. |title=Solar System Dynamics |date=1999 |publisher=[[Cambridge University Press]] |isbn=978-0-521-57295-8 |page=184}}</ref><ref>{{cite book |last=Dickinson |first=Terence |author-link=Terence Dickinson |title=From the Big Bang to Planet X |date=1993 |publisher=[[Camden House]] |location=Camden East, Ontario |isbn=978-0-921820-71-0 |pages=79–81}}</ref>

If the Earth–Moon system isn't engulfed by the enlarged Sun, the drag from the solar atmosphere can cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of {{convert|18470|km|mi|abbr=on}}, it will cross Earth's [[Roche limit]], meaning that tidal interaction with Earth would break apart the Moon, turning it into a [[ring system]]. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow up Earth, the planet may be left moonless.<ref name=space070122>{{citation |first=David |last=Powell |date=January 22, 2007 |title=Earth's Moon Destined to Disintegrate |work=Space.com |publisher=Tech Media Network |url=http://www.space.com/scienceastronomy/070122_temporary_moon.html |access-date=June 1, 2010 |archive-date=September 6, 2008 |archive-url=https://archive.today/20080906222127/http://www.space.com/scienceastronomy/070122_temporary_moon.html |url-status=live}}</ref>