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+.help dt Jun99 "Slalib Package"
+.nf
+
+      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.
+
+.fi
+.endhelp