Editing Moon (section)

=== Eclipses ===
{{Main |Solar eclipse |Lunar eclipse |Solar eclipses on the Moon|Eclipse cycle}}
{{multiple image
| total_width       = 330
| image1            = Solar_eclipse_1999_4_NR.jpg
| image2            = Full Eclipse of the Moon as seen in from Irvine, CA, USA (52075715442) (cropped).jpg
| caption1          = A [[solar eclipse]] causes the Sun to be covered, revealing the white [[Stellar corona|corona]].
| caption2          = The Moon, tinted reddish, during a [[lunar eclipse]]
}}

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "[[Syzygy (astronomy)|syzygy]]"). [[Solar eclipse]]s occur at [[new moon]], when the Moon is between the Sun and Earth. In contrast, [[lunar eclipse]]s occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon, but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both [[total eclipse|total]] (with the Moon appearing larger than the Sun) and [[annular eclipse|annular]] (with the Moon appearing smaller than the Sun) solar eclipses.<ref>{{cite web |first=F. |last=Espenak |date=2000 |url=http://www.mreclipse.com/Special/SEprimer.html |title=Solar Eclipses for Beginners |publisher=MrEclip |access-date=March 17, 2010 |archive-date=May 24, 2015 |archive-url=https://web.archive.org/web/20150524172606/http://www.mreclipse.com/Special/SEprimer.html |url-status=dead}}</ref> In a total eclipse, the Moon completely covers the disc of the Sun and the [[solar corona]] becomes visible to the [[naked eye]].

Because the distance between the Moon and Earth is very slowly increasing over time,<ref name="Lambeck1977" /> the angular diameter of the Moon is decreasing. As it evolves toward becoming a [[red giant]], the size of the Sun, and its apparent diameter in the sky, are slowly increasing.{{efn|name=size changes}} The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.<ref name=fourmilab>{{cite web |last=Walker |first=John |url=http://www.fourmilab.ch/images/peri_apo/ |title=Moon near Perigee, Earth near Aphelion |publisher=[[Fourmilab]] |date=July 10, 2004 |access-date=December 25, 2013 |url-status=live |archive-url=https://web.archive.org/web/20131208153430/http://www.fourmilab.ch/images/peri_apo/ |archive-date=December 8, 2013}}</ref>

As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the [[ecliptic|orbit of Earth around the Sun]], eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.<ref name="eclipse" /> The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the [[Saros (astronomy)|saros]], which has a period of approximately 18&nbsp;years.<ref>{{cite web |url=http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html |last=Espenak |first=F. |title=Saros Cycle |publisher=NASA |access-date=March 17, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20071030225501/http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html |archive-date=October 30, 2007}}</ref>

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,{{Efn |name=area}}<ref>{{cite magazine |title=The Square Degree as a Unit of Celestial Area |author=Guthrie, D.V. |date=1947 |magazine=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=55 |pages=200–203 |bibcode=1947PA.....55..200G}}</ref> the related phenomenon of [[occultation]] occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the [[precession]] of the lunar orbit, each year different stars are occulted.<ref>{{cite web |url=http://occsec.wellington.net.nz/total/totoccs.htm |title=Total Lunar Occultations |publisher=[[Royal Astronomical Society of New Zealand]] |access-date=March 17, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100223022627/http://occsec.wellington.net.nz/total/totoccs.htm |archive-date=February 23, 2010}}</ref>