longTermGmstMinusEra function

double longTermGmstMinusEra(
  1. double t
)

Greenwich mean sidereal time minus the Earth rotation angle at t (radians): the angle from the CIO to the mean equinox of date along the equator, from the long-term precession model.

The CIO is carried along the equator without rotation (its kinematic definition) from its J2000 position, the GCRS x axis, as the equator pole moves; tabulated on first use every 0.01 century and interpolated.

Implementation

double longTermGmstMinusEra(double t) {
  final (n, k) = _poles(t);
  final equinox = n.cross(k).unit;
  final cio = _cio.at(t);
  return -math.atan2(cio.cross(equinox).dot(n), cio.dot(equinox));
}