Earth

=== Moon ===
{{Main|Moon|Lunar theory|Orbit of the Moon}}
[[File:MarsReconnaissanceOrbiter-Views-EarthMoon-20220422.jpg|thumb|Earth and the Moon as seen from [[Mars]] by the [[Mars Reconnaissance Orbiter]]]]
[[File:Earthrise over Compton crater -LRO full res - edit1.jpg|thumb|View of Earth from the Moon by the [[Lunar Reconnaissance Orbiter]]]]

The Moon is a relatively large, [[Terrestrial planet|terrestrial]], [[Planetary-mass moon|planet-like natural satellite]], with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although [[Charon (moon)|Charon]] is larger relative to the [[dwarf planet]] [[Pluto]].<ref>{{cite web|url=https://astronomy.com/news/2019/06/whats-so-special-about-our-moon-anyway|title=What's so special about our Moon, anyway?|work=[[Astronomy (magazine)|Astronomy]]|last1=Klemetti|first1=Erik|date=17 June 2019|access-date=13 October 2020}}</ref><ref>{{cite web|url=https://solarsystem.nasa.gov/moons/pluto-moons/charon/in-depth/#:~:text=At%20half%20the%20size%20of,phenomenon%20called%20mutual%20tidal%20locking.|title=Charon|website=NASA|date=19 December 2019|access-date=13 October 2020}}</ref> The natural satellites of other planets are also referred to as "moons", after Earth's.<ref>{{cite web|url=https://theconversation.com/curious-kids-why-is-the-moon-called-the-moon-127899|title=Curious Kids: Why is the moon called the moon?|website=The Conversation|last1=Brown|first1=Toby|date=2 December 2019|access-date=13 October 2020}}</ref> The most widely accepted theory of the Moon's origin, the [[giant-impact hypothesis]], states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.<ref name="canup_asphaug2001b" />

The gravitational attraction between Earth and the Moon causes [[tide]]s on Earth.<ref>{{Cite journal|last1=Coughenour|first1=Christopher L.|last2=Archer|first2=Allen W.|last3=Lacovara|first3=Kenneth J.|author-link3=Kenneth Lacovara|date=2009|title=Tides, tidalites, and secular changes in the Earth–Moon system|url=http://www.sciencedirect.com/science/article/pii/S0012825209001445|journal=Earth-Science Reviews|language=en|volume=97|issue=1|pages=59–79|doi=10.1016/j.earscirev.2009.09.002|bibcode=2009ESRv...97...59C|issn=0012-8252}}</ref> The same effect on the Moon has led to its [[tidal locking]]: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet.<ref>{{Cite news|last=Kelley|first=Peter|date=17 August 2017|title=Tidally locked exoplanets may be more common than previously thought|url=https://www.washington.edu/news/2017/08/14/tidally-locked-exoplanets-may-be-more-common-than-previously-thought/|access-date=8 October 2020|newspaper=Uw News|language=en}}</ref> As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the [[lunar phase]]s.<ref>{{Cite web|title=Lunar Phases and Eclipses {{!}} Earth's Moon|url=https://solarsystem.nasa.gov/moons/earths-moon/lunar-phases-and-eclipses|access-date=8 October 2020|website=NASA Solar System Exploration}}</ref> Due to their [[Tidal interactions|tidal interaction]], the Moon recedes from Earth at the rate of approximately {{convert|38|mm/yr|in/yr|abbr=on}}. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23&nbsp;[[Microsecond|µs]]/yr—add up to significant changes.<ref name="espenak_meeus20070207" /> During the [[Ediacaran]] period, for example, (approximately {{val|620|u=Ma}}) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.<ref>{{Cite journal |last=Williams |first=G.E. |date=2000 |title=Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit |journal=Reviews of Geophysics |volume=38 |issue=1 |pages=37–59 |doi=10.1029/1999RG900016 |bibcode=2000RvGeo..38...37W |s2cid=51948507|doi-access=free }}</ref>

The Moon may have dramatically affected the development of life by moderating the planet's climate. [[Paleontology|Paleontological]] evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.<ref name="aaa428_261" /> Some theorists think that without this stabilization against the [[torque]]s applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting large changes over millions of years, as is the case for Mars, though this is disputed.<ref>{{cite web|url=https://phys.org/news/2015-01-earth-moon-critical-life.html#:~:text=Lissauer's%20team%20found%20that%20without,day%20angle%20of%2023.5%20degrees.|title=Earth's moon may not be critical to life|work=[[Phys.org]]|last1=Cooper|first1=Keith|date=27 January 2015|access-date=26 October 2020}}</ref><ref>{{cite journal|url=http://web.mit.edu/perron/www/files/Daradich08.pdf|title=Equilibrium rotational stability and figure of Mars|journal=Icarus|last1=Dadarich|first1=Amy|first2=Jerry X.|last2=Mitrovica|author-link2=Jerry X. Mitrovica|first3=Isamu|last3=Matsuyama|first4=J. Taylor|last4=Perron|first5=Michael|last5=Manga|author-link5=Michael Manga|first6=Mark A.|last6=Richards|date=22 November 2007|volume=194|issue=2|pages=463–475|access-date=26 October 2020|doi=10.1016/j.icarus.2007.10.017|archive-date=1 December 2020|archive-url=https://web.archive.org/web/20201201094104/http://web.mit.edu/perron/www/files/Daradich08.pdf|url-status=dead}}</ref>

Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The [[angular size]] (or [[solid angle]]) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.<ref name="angular" /> This allows total and annular solar eclipses to occur on Earth.<ref>{{cite web|url=https://blogs.scientificamerican.com/life-unbounded/the-solar-eclipse-coincidence/|title=The Solar Eclipse Coincidence|work=[[Scientific American]]|last1=Sharf|first1=Caleb A.|date=18 May 2012|access-date=13 October 2020|author1-link=Caleb Scharf}}</ref>

On 1 November 2023, scientists reported that, according to computer simulations, remnants of a [[protoplanet]], named [[Theia (planet)|Theia]], could be inside the Earth, left over from a collision with the Earth in ancient times, and afterwards becoming the [[Moon]].<ref name="NYT-20231101">{{cite news |last=Chang |first=Kenneth |title=A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon. |url=https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |date=1 November 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20231101232849/https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |archivedate=1 November 2023 |accessdate=2 November 2023 }}</ref><ref name="NAT-20231101">{{cite journal |author=Yuan, Qian |display-authors=et al.|title=Moon-forming impactor as a source of Earth's basal mantle anomalies |url=https://www.nature.com/articles/s41586-023-06589-1 |date=1 November 2023 |journal=[[Nature (journal)|Nature]] |volume=623 |issue=7985 |pages=95–99 |doi=10.1038/s41586-023-06589-1 |pmid=37914947 |bibcode=2023Natur.623...95Y |s2cid=264869152 |url-status=live |archiveurl=https://archive.today/20231102061800/https://www.nature.com/articles/s41586-023-06589-1 |archivedate=2 November 2023 |accessdate=2 November 2023 }}</ref>