**ACTION:**- Estimate 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:

The breakpoints are chosen to ensure continuity: they occur at places where the adjacent models give the same answer as each other.`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` - 2.
- The accuracy is modest, with errors of up to during
the interval since 1650, rising to perhaps 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 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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk