Editing Orbital inclination (section)

==Orbits==
The inclination is one of the six [[orbital elements]] describing the shape and orientation of a celestial [[orbit]]. It is the [[angle]] between the orbital plane and the [[plane of reference]], normally stated in [[degree (angle)|degrees]]. For a satellite orbiting a [[planet]], the plane of reference is usually the plane containing the planet's [[equator]]. For planets in the Solar System, the plane of reference is usually the [[ecliptic]], the plane in which the Earth orbits the Sun.<ref>{{cite book
 | first=Vladimir A. | last=Chobotov | date=2002
 | title=Orbital Mechanics | pages=28–30
 | edition=3rd | publisher=[[AIAA]] | isbn=1-56347-537-5 }}</ref><ref>{{cite book
 | author=McBride, Neil
 | author2=Bland, Philip A.
 | author3=Gilmour, Iain
 | title=An Introduction to the Solar System | date=2004
 | page=248 | publisher=[[Cambridge University Press]]
 | isbn=0-521-54620-6 }}</ref> This reference plane is most practical for Earth-based observers. Therefore, Earth's inclination is, by definition, zero.

Inclination can instead be measured with respect to another plane, such as the [[Sun]]'s equator or the [[invariable plane]] (the plane that represents the angular momentum of the Solar System, approximately the orbital plane of [[Jupiter]]).

===Natural and artificial satellites===
The inclination of orbits of [[natural satellite|natural]] or [[artificial satellite]]s is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the plane perpendicular to the axis of rotation of the central body.

An inclination of 30° could also be described using an angle of 150°. The convention is that the normal orbit is [[Direct motion|prograde]], an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe [[Retrograde motion|retrograde orbits]] (backward). Thus:
* An inclination of 0° means the orbiting body has a prograde orbit in the planet's equatorial plane.
* An inclination greater than 0° and less than 90° also describes a prograde orbit.
* An inclination of 63.4° is often called a '''critical inclination''', when describing artificial satellites orbiting the Earth, because they have [[Orbital perturbation analysis#Perturbation of the orbital plane 2|zero apogee drift]].<ref>[http://www.diva-portal.org/smash/get/diva2:611689/FULLTEXT02.pdf Arctic Communications System Utilizing Satellites in Highly Elliptical Orbits], Lars Løge – Section 3.1, Page 17</ref>
* An inclination of exactly 90° is a [[polar orbit]], in which the spacecraft passes over the poles of the planet.
* An inclination greater than 90° and less than 180° is a retrograde orbit.
* An inclination of exactly 180° is a retrograde equatorial orbit.

For impact-generated moons of [[terrestrial planet]]s not too far from their star, with a large planet–moon distance, the orbital planes of moons tend to be aligned with the planet's orbit around the star due to tides from the star, but if the planet–moon distance is small, it may be inclined. For [[gas giant]]s, the orbits of moons tend to be aligned with the giant planet's equator, because these formed in circumplanetary disks.<ref>[http://epjwoc.epj.org/articles/epjconf/pdf/2011/01/epjconf_ohp2010_04003.pdf Moon formation and orbital evolution in extrasolar planetary systems-A literature review], K Lewis – EPJ Web of Conferences, 2011 – epj-conferences.org</ref> Strictly speaking, this applies only to regular satellites. Captured bodies on distant orbits vary widely in their inclinations, while captured bodies in relatively close orbits tend to have low inclinations owing to tidal effects and perturbations by large regular satellites.

===Exoplanets and multiple star systems===
The inclination of [[exoplanet]]s or members of [[Star system|multi-star star systems]] is the angle of the plane of the orbit relative to the [[plane of the sky]]: a plane perpendicular to the line of sight from Earth to the object: .<ref name="Campante">{{cite journal
|author=Tiago L. Campante
|date=27 October 2016
|title=Spin-orbit alignment of exoplanet systems: Analysis of an ensemble of asteroseismic observations
|url=https://www.cambridge.org/core/services/aop-cambridge-core/content/view/10DAA84B787CEF3DE91E5398CFB20123/S1743921316006232a.pdf
|journal=Proceedings of the International Astronomical Union
|publisher=Cambridge University Press
|volume=11
|issue=General Assembly A29B
|pages=636–641
|doi=10.1017/S1743921316006232
|bibcode=2016IAUFM..29B.636C
|s2cid=126328423
|access-date=27 February 2022
}}</ref>
* An inclination of 0° is a face-on orbit, meaning the plane of the exoplanet's orbit is perpendicular to the line of sight with Earth.
* An inclination of 90° is an edge-on orbit, meaning the plane of the exoplanet's orbit is parallel to the line of sight with Earth.

Since the word "inclination" is used in exoplanet studies for this line-of-sight inclination, the angle between the planet's orbit and its star's rotational axis is expressed using the term the "spin-orbit angle" or "spin-orbit alignment".<ref name="Campante"/> In most cases the orientation of the star's rotational axis is unknown.

Because the [[doppler spectroscopy|radial-velocity method]] more easily finds planets with orbits closer to edge-on, most exoplanets found by this method have inclinations between 45° and 135°, although in most cases the inclination is not known. Consequently, most exoplanets found by radial velocity have [[Binary mass function|true masses]] no more than 40% greater than their [[minimum mass]]es.{{citation needed|date=May 2014}} If the orbit is almost face-on, especially for superjovians detected by radial velocity, then those objects may actually be [[brown dwarf]]s or even [[red dwarf]]s. One particular example is [[HD 33636]] B, which has true mass 142 M<sub>J</sub>, corresponding to an M6V star, while its minimum mass was 9.28 M<sub>J</sub>.

If the orbit is almost edge-on, then the planet can be seen [[transit method|transiting]] its star.