[SOFA] Returns the equation of the origins measured in radians, given
the classical nutation-precession-bias matrix
rnpb must have its first two
dimensions equal to 3.
s has the same dimensions as
rnpb except that it lacks those first two dimensions
equal to 3.
The equation of the origins is the distance between the true equinox and the celestial intermediate origin, or, equivalently, between the Earth rotation angle and Greenwich apparent sidereal time. It is equal to the precession in right ascension since J2000.0 plus the equation of the equinoxes including the small correction terms.
See also: Astronomical Coordinate Calculations