[SOFA] Returns Greenwich apparent sidereal time in radians consistent
with IAU 2000 resolutions.
jdut is the Julian Date
jdtt is the Julian Date (TT) of the same
jdut is needed to predict the Earth rotation,
and TT to predict the effects of precession. If
set equal to
jdut then errors of order 100
microarcseconds result. Both parameters must have the same
dimensions, and the result has those same dimensions, too.
See also: Astronomical Coordinate Calculations