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      DOUBLE PRECISION FUNCTION slDT (EPOCH)

     - - -
      D T
     - - -

  Estimate the offset between dynamical time and Universal Time
  for a given historical epoch.

  Given:
     EPOCH       d        (Julian) epoch (e.g. 1850D0)

  The result is a rough estimate of ET-UT (after 1984, TT-UT) at
  the given epoch, in seconds.

  Notes:

  1  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.

  2  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).

  3  The use of double-precision for both argument and result is
     purely for compatibility with other SLALIB time routines.

  4  The models used are based on a lunar tidal acceleration value
     of -26.00 arcsec per century.

  Reference:  Explanatory Supplement to the Astronomical Almanac,
              ed P.K.Seidelmann, University Science Books (1992),
              section 2.553, p83.  This contains references to
              the Stephenson & Morrison and McCarthy & Babcock
              papers.

  P.T.Wallace   Starlink   1 March 1995

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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