...or, "What is the inclination of a satellite that travels over Earth's equator?"

Considering Earth's axial tilt is 23.4°, it's a significant difference - is the inclination calculated relative to the plane of ecliptic, or relative to Earth's global coordinates?

Considering the sun-synchronous satellites travel at inclination of 98 degrees I'm inclined (pun unintended) to believe it's the former but I prefer to ask in case I'm guessing wrong.


1 Answer 1


This is another one of those data that you'd normally expect to be accompanied by with respect to what (plane of reference) metadata and explaining what they're quoted for. In Earth orbit, if this "w.r.t." isn't given, it's safe to assume that the reference zero angle is the equatorial plane in the direction of Earth's rotation on its axis (e.g. TLE, Two-Line Element sets also give values with the plane of reference being the equatorial plane and prograde as 0 angle of the primary body, i.e. ascending node), but you'd often also find sources quoting values relative to something else, say the Laplace's invariable plane, ecliptic, or alike. Sadly, this crucial metadata is often also missing and you're then required to find more reliable quotes somewhere else or dig for metadata to properly interpret quoted values, since they can be ambiguous and a subject to misinterpretation without it.

  • $\begingroup$ ...so what's the w.r.t. of the 98 degrees of sun-synchronous orbit? I assumed the ~8 degrees compensate for Earth rotation, while the 90 is perpendicular to the Earth-Sun line... $\endgroup$
    – SF.
    Oct 20, 2014 at 16:59
  • $\begingroup$ The equatorial plane. But you also need the semi-major axis (or orbital period) and J2 value of the primary body (see here) to calculate its precession. $\endgroup$
    – TildalWave
    Oct 20, 2014 at 17:10

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