Editing Orbital elements (section)

{{Short description|Parameters that define a specific orbit}}
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'''Orbital elements''' are the [[parameter]]s required to uniquely identify a specific [[orbit]]. In [[celestial mechanics]] these elements are considered in [[two-body system]]s using a [[Kepler orbit]]. There are many different ways to mathematically describe the same orbit, but certain schemes are commonly used in [[astronomy]] and [[orbital mechanics]].

A real orbit and its elements change over time due to gravitational [[Perturbation (astronomy)|perturbations]] by other objects and the effects of [[general relativity]]. A Kepler orbit is an idealized, mathematical approximation of the orbit at a particular time.

When viewed from an [[inertial frame]], two orbiting bodies trace out distinct trajectories. Each of these trajectories has its [[Focus (geometry)|focus]] at the common [[center of mass]]. When viewed from a non-inertial frame centered on one of the bodies, only the trajectory of the opposite body is apparent; Keplerian elements describe these non-inertial trajectories. An orbit has two sets of Keplerian elements depending on which body is used as the point of reference. The reference body (usually the most massive) is called the ''[[Primary (astronomy)|primary]]'', the other body is called the ''secondary''. The primary does not necessarily possess more mass than the secondary, and even when the bodies are of equal mass, the orbital elements depend on the choice of the primary.

Orbital elements can be obtained from [[orbital state vectors]] (position and velocity vectors along with time and magnitude of acceleration) by manual transformations or with computer software through a process known as [[orbit determination]].<ref>For example, with {{cite web |title=VEC2TLE |url=http://www.amsat.org/amsat-new/information/faqs/sv_keps.php |url-status=dead |archive-url=https://web.archive.org/web/20160520165738/http://www.amsat.org/amsat-new/information/faqs/sv_keps.php |archive-date=20 May 2016 |access-date=19 June 2013 |website=amsat.org}}</ref>

It non-closed orbits exist, although these are typically referred to as trajectories and not orbits, as they are not periodic. The same elements used to describe closed orbits can also typically be used to represent open trajectories.