[SOFA] Returns the equation of the equinoxes compatible with IAU 2000
resolutions but using the truncated nutation model IAU 2000B, for the
specified TT Julian Date
jdtt. The returned value has
the same dimensions as the parameter and is measured in radians.
Greenwich apparent sidereal time (gast) is equal to Greenwish mean sidereal time (gmst) plus the equation of the equinoxes.
See also: Astronomical Coordinate Calculations