Editing Orbital inclination (section)

===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.