Editing Orbital elements (section)

=== Two-line elements ===<!--This section is linked from [[Epoch (astronomy)]]-->
{{Main|Two-line element set}}

Orbital elements can be encoded as text in a number of formats. The most common of them is the [[NASA]] / [[NORAD]] '''"two-line elements"''' (TLE) format,<ref name="Kelso_FAQ2">{{cite web |last=Kelso |first=T.S. |title=FAQs: Two-line element set format |url=http://celestrak.com/columns/v04n03/ |url-status=live |archive-url=https://web.archive.org/web/20160326061740/http://celestrak.com/columns/v04n03/ |archive-date=26 March 2016 |access-date=15 June 2016 |website=celestrak.com |series=CelesTrak}}</ref> originally designed for use with 80 column punched cards, but still in use because it is the most common format, and 80-character ASCII records can be handled efficiently by modern databases.

The two-line element format lists the eccentricity ({{Mvar|e}}), inclination ({{Mvar|i}}), longitude of the ascending node ({{Math|Ω}}), argument of periapsis (''{{mvar|ω}}''), mean motion (''{{Mvar|n}}''), epoch ({{math|{{var|t}}{{sub|0}}}}), and mean anomaly at epoch ({{math|{{var|M}}{{sub|0}}}}).<ref name="Kelso_FAQ2" /><ref name=":02" /> Since the format is primarily meant for orbits of the Earth, the standard gravitational parameter (''{{Mvar|μ}}''), can be assumed and used to calculate the semi-major axis with the mean motion via the relations above.

Depending on the application and object orbit, the data derived from TLEs older than 30 days can become unreliable. Orbital positions can be calculated from TLEs through [[Simplified perturbations models|simplified perturbation models]]  ([[SGP4]] / [[SDP4]] / SGP8 / SDP8).<ref>{{cite book |title=Explanatory Supplement to the Astronomical Almanac |publisher=University Science Books |year=1992 |editor-last=Seidelmann |editor-first=K.P. |edition=1st |place=Mill Valley, CA}}</ref>

Example of a two-line element:<ref>{{cite web |title=SORCE |url=http://www.heavens-above.com/orbitdisplay.asp?lat=0&lng=0&alt=0&loc=Unspecified&TZ=CET&satid=27651 |archive-url=https://web.archive.org/web/20070927203806/http://www.heavens-above.com/orbitdisplay.asp?lat=0&lng=0&alt=0&loc=Unspecified&TZ=CET&satid=27651 |archive-date=2007-09-27 |website=Heavens-Above.com |series=orbit data}}</ref><pre>
1 27651U 03004A   07083.49636287  .00000119  00000-0  30706-4 0  2692
2 27651 039.9951 132.2059 0025931 073.4582 286.9047 14.81909376225249
</pre>