SLA_DT - Approximate ET minus UT

ACTION:

Estimate $\Delta$T, the offset between dynamical time and Universal Time, for a given historical epoch.

CALL:

D = sla_DT (EPOCH)

GIVEN:

EPOCH

D

(Julian) epoch (e.g. 1850D0)

RETURNED:

sla_DT

D

approximate ET$-$UT (after 1984, TT$-$UT1) in seconds

NOTES:

1. Depending on the epoch, one of three parabolic approximations is used:

   	before AD 979	Stephenson & Morrison's 390 BC to AD 948 model
   	AD 979 to AD 1708	Stephenson & Morrison's AD 948 to AD 1600 model
   	after AD 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.

2. The accuracy is modest, with errors of up to $20^{\rm s}$ during the interval since 1650, rising to perhaps $30^{\rm m}$ by 1000 BC. Comparatively accurate values from AD 1600 are tabulated in the Astronomical Almanac (see section K8 of the 1995 edition).
3. The use of DOUBLE PRECISION for both argument and result is simply for compatibility with other SLALIB time routines.
4. The models used are based on a lunar tidal acceleration value of $-26\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.00$ per century.

REFERENCE:

Seidelmann, P.K. (ed), 1992. Explanatory Supplement to the Astronomical Almanac, ISBN 0-935702-68-7. This contains references to the papers by Stephenson & Morrison and by McCarthy & Babcock which describe the models used here.