Estimate the offset between dynamical time and UT palDt
Depending on the epoch, one of three parabolic approximations is used:
before 979 Stephenson & Morrison’
s 390 BC to AD 948 model 979 to 1708 Stephenson & Morrison’
s
948 to 1600 model after 1708 McCarthy & Babcock’
s post-1650 model
The breakpoints are chosen to ensure continuity: they occur at places where the adjacent models give the same answer as each other.
The accuracy is modest, with errors of up to 20 sec during the interval since 1650, rising to perhaps 30 min by 1000 BC. Comparatively accurate values from AD 1600 are tabulated in the Astronomical Almanac (see section K8 of the 1995 AA).
The use of double-precision for both argument and result is purely for compatibility with other SLALIB time routines.
The models used are based on a lunar tidal acceleration value of -26.00 arcsec per century.