Editing Ecliptic (section)

==Plane of the Solar System==
{{main|Solar System}}

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|width=200px|[[File:Ecliptic plane top view.gif|200px]]
|width=200px|[[File:Ecliptic plane side view.gif|200px]]
|width=200px|[[File:FourPlanetSunset hao annotated.JPG|200px]]
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|colspan=2|Top and side views of the plane of the ecliptic, showing planets [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. Most of the planets orbit the [[Sun]] very nearly in the same plane in which Earth orbits, the ecliptic.
|colspan=1|Five planets (Earth included) lined up along the ecliptic in July 2010, illustrating how the planets orbit the Sun in nearly the same plane. Photo taken at sunset, looking west over Surakarta, Java, Indonesia.
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Most of the major bodies of the Solar System orbit the Sun in nearly the same plane. This is likely due to the way in which the Solar System formed from a [[protoplanetary disk]]. Probably the closest current representation of the disk is known as the ''[[invariable plane]] of the Solar System''. Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the invariable plane, Jupiter's orbit is within a little more than ½° of it, and the other major planets are all within about 6°. Because of this, most Solar System bodies appear very close to the ecliptic in the sky.

The invariable plane is defined by the [[angular momentum]] of the entire Solar System, essentially the vector sum of all of the [[orbit]]al and [[Rotation#Astronomy|rotational]] angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter.<ref name=meanplane>{{cite web |date=3 April 2009 |title=The Mean Plane (Invariable Plane) of the Solar System passing through the barycenter |url=http://home.surewest.net/kpheider/astro/MeanPlane.gif |access-date=10 April 2009 |url-status=dead |archive-url=https://web.archive.org/web/20130603143144/http://home.surewest.net/kheider/astro/MeanPlane.gif |archive-date=3 June 2013 }} produced with {{cite web |url=http://chemistry.unina.it/~alvitagl/solex/ |title=Solex 10 |first=Aldo |last=Vitagliano |format=computer program |access-date=10 April 2009 |archive-url=https://www.webcitation.org/5gOzK38bc?url=http://chemistry.unina.it/~alvitagl/solex/ |archive-date=29 April 2009 |url-status=dead }}</ref> That sum requires precise knowledge of every object in the system, making it a somewhat uncertain value. Because of the uncertainty regarding the exact location of the invariable plane, and because the ecliptic is well defined by the apparent motion of the Sun, the ecliptic is used as the reference plane of the Solar System both for precision and convenience. The only drawback of using the ecliptic instead of the invariable plane is that over geologic time scales, it will move against fixed reference points in the sky's distant background.<ref>{{cite book |last=Danby |first=J.M.A. |title=Fundamentals of Celestial Mechanics |publisher=Willmann-Bell, Inc., Richmond, VA |year=1988 |isbn=0-943396-20-4 |at=section&nbsp;9.1}}</ref><ref>{{cite book |last=Roy |first=A.E. |title=Orbital Motion |publisher=Institute of Physics Publishing |year=1988 |isbn=0-85274-229-0 |edition=third |at=section&nbsp;5.3}}</ref>

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