Editing Sun-synchronous orbit (section)

== Orbital precession ==
A Sun-synchronous orbit is achieved by having the [[Osculating orbit|osculating]] orbital plane [[Nodal precession|precess]] (rotate) approximately one degree eastward each day with respect to the [[celestial sphere]] to keep pace with the Earth's movement around the [[Sun]].<ref name="me">{{cite journal|first=M.|last=Rosengren|title=ERS-1 - An Earth Observer that exactly follows its Chosen Path|journal=ESA Bulletin |issue=72|date=November 1992|volume=72|page=76|bibcode=1992ESABu..72...76R|publisher=European Space Agency}}</ref> This precession is achieved by tuning the inclination to the altitude of the orbit (see [[#Technical details |Technical details]]) such that Earth's [[equatorial bulge]], which perturbs inclined orbits, causes the orbital plane of the spacecraft to precess with the desired rate. The plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis.

Typical Sun-synchronous orbits around Earth are about {{convert|600|–|800|km|abbr=on}} in altitude, with periods in the 96–100-[[minute]] range, and inclinations of around 98°. This is slightly [[retrograde motion|retrograde]] compared to the direction of Earth's rotation: 0° represents an equatorial orbit, and 90° represents a polar orbit.<ref name="me"/>

Sun-synchronous orbits are possible around other [[oblate spheroid|oblate]] planets, such as [[Mars]]. A satellite orbiting a planet such as [[Venus]] that is almost spherical will need an outside push to maintain a Sun-synchronous orbit.<!-- If a satellite is given sideways momentum from an outside source, the orbital plane will then also turn; conservation of momentum -->