NASA's orbital elements given for Charon's orbit are given at https://ssd.jpl.nasa.gov/?sat_elem. The ascending node is defined as being "measured from the node of the reference plane on the ICRF equator". For Charon, the plane being used is given as the "Mean equatorial (IAU equator)". I'm assuming this is Pluto's equatorial plane. What then is Charon's ascending node in reference to? Normally this would be ecliptic vernal equinox, however this obviously cannot be it.

Some sources have the ascending node as undefined, but that then begs the question, what is the argument of perihelion in reference to if the ascending node is undefined?


1 Answer 1


I'm assuming this is Pluto's equatorial plane.

That is incorrect. Charon's right ascension of ascending node with respect to Pluto's equator is undefined. Seemingly paradoxically, it is well defined with respect to Earth's equatorial plane, and since that has become the universal plane of reference, that is what is used.

Charon and Pluto are tidally locked to one another. Their mean axes of rotation and orbital axes are identical. This means that Charon's orbit has zero mean inclination with respect to Pluto's mean equator, which in turn means that Charon's mean right ascension of ascending node with respect to Pluto's mean equator is undefined.

Charon's orbit about Pluto is however inclined with respect to the Earth's mean equator. In particular, it's inclined with respect to the Earth's mean equator at the J2000 epoch (noon Terrestrial Time on 1 Jan 2000). The Earth's mean equator at the J2000 epoch is the ICRF equator, to within a fraction of a milliarcsecond. The linked data clearly states that Charon's right ascension of ascending node is measured with respect to the ICRF equator.


Brozović, Marina, et al. "The orbits and masses of satellites of Pluto." Icarus 246 (2015): 317-329.

  • 2
    $\begingroup$ I wonder why Earth's equatorial plane is the universal reference plane. $\endgroup$
    – BMF
    Jan 27, 2020 at 17:49
  • 3
    $\begingroup$ @BMFForMonica - Any standardization by us humans is by definition universal as there's only one intelligent, star-observing species in the universe -- that we know of. Regarding other frames, mean of 1950 (also based on Earth's equator) turns out to be a rotating frame. True of date and mean of date (both based on the Earth's equator) are rapidly falling out of favor, as are ecliptic-based frames (based on the Sun's orbit about the Earth). It all sound's a bit geocentric, but that's how it goes. That includes the very modern, Earth equator-based International Celestial Reference Frame. $\endgroup$ Jan 27, 2020 at 18:41
  • $\begingroup$ @BMFForMonica Agreed. That is totally earthism. Or planetism. Or. hmm. $\endgroup$
    – Stian
    Jan 28, 2020 at 9:04
  • $\begingroup$ @DavidHammen If it's the ICRF being used, then the inclination should be approximately 280 degrees. Instead, the inclination is zero as per Pluto's equator. If these values were used as is to orientate Charon's orbit based on the ICRF, you'd have an orbital plane parallel with earth's equator which is clearly not the case. Are you able to clarify this? $\endgroup$ Jan 28, 2020 at 11:52
  • $\begingroup$ I would like to know (but don't think it's worthy of an actual question) when, or how often, is the or will the J2000 epoch be updated/changed? Assuming nothing else happens, will the J2000 epoch still be used in 2100? 3000? IOW, will all references need to be calculated back to J2000 forever, or is there a schedule for updating the reference? (apologies for what may very well be a "stupid" question, as I am not knowledgeable to space or orbital references) $\endgroup$
    – CGCampbell
    Jan 28, 2020 at 15:46

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