[SOFA] Returns the equation of the equinoxes complementary terms,
consistent with IAU 2000 resolutions, for the specified TT Julian Date
jdtt. The result has the same dimensions as the
parameter and is measured in radians.
The Greenwich apparent sidereal time (gast) is equal to the Greenwich mean standard time (gmst) plus the equation of the equinoxes, and that equation of the equinoxes is equal to dpsi*cos(eps) plus the complementary terms, where dpsi is the nutation in longitude and eps is the obliquity of date. The complementary terms are needed to make UT1 more regular.
See also: Astronomical Coordinate Calculations