prec76, jdtdb1, jdtdb2, zeta, z,
[SOFA] Returns the three Euler angles
zeta (1st rotation,
radians clockwise around z),
z (3rd rotation, radians
clockwise around z),
theta (2nd rotation, radians
counterclockwise around y) which implement general precession between
epochs (TDB Julian Dates)
using the IAU 1976 model (as for the FK5 catalog).
The precession matrix is
r_3(-z) r_2(+theta) r_3(-zeta).
The absolute accuracy is better than 0.1 arcsec from year 1960 through 2040, better than 1 arcsec from 1640 through 2360, and remains below 3 arcsec during -500 through 3000. The errors exceed 10 arcsec outside -1200 through 3900, exceed 100 arcsec outside -4200 through 5600, and exceed 1000 arcsec outside -6800 through 8200.
See also: Astronomical Coordinate Calculations