I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is always on the equator. And I suppose it usually is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?

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    $\begingroup$ The J2000 vernal equinox is on the J2000 celestial equator. Similarly, the vernal equinox today is on the equator today if you use the equator and equinox of today. The only discrepancy would occur if you use J2000 for one set of coordinates and the ecliptic or equator of the date for another. $\endgroup$
    – user21
    Apr 18, 2020 at 4:35

1 Answer 1


The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.

Given the various ways that objects in orbit over Earth are perturbed, any set of keplerian orbital parameters that describe the orbit of any object over the Earth (including the Moon!) will need updating long before the drift of the Vernal Equinox along the celestial equator becomes significant.


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