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      SUBROUTINE slPREC (EP0, EP1, RMATP)

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      P R E C
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  Form the matrix of precession between two epochs (IAU 1976, FK5)
  (double precision)

  Given:
     EP0    dp         beginning epoch
     EP1    dp         ending epoch

  Returned:
     RMATP  dp(3,3)    precession matrix

  Notes:

     1)  The epochs are TDB (loosely ET) Julian epochs.

     2)  The matrix is in the sense   V(EP1)  =  RMATP * V(EP0)

     3)  Though the matrix method itself is rigorous, the precession
         angles are expressed through canonical polynomials which are
         valid only for a limited time span.  There are also known
         errors in the IAU precession rate.  The absolute accuracy
         of the present formulation is better than 0.1 arcsec from
         1960AD to 2040AD, better than 1 arcsec from 1640AD to 2360AD,
         and remains below 3 arcsec for the whole of the period
         500BC to 3000AD.  The errors exceed 10 arcsec outside the
         range 1200BC to 3900AD, exceed 100 arcsec outside 4200BC to
         5600AD and exceed 1000 arcsec outside 6800BC to 8200AD.
         The SLALIB routine slPREL implements a more elaborate
         model which is suitable for problems spanning several
         thousand years.

  References:
     Lieske,J.H., 1979. Astron.Astrophys.,73,282.
      equations (6) & (7), p283.
     Kaplan,G.H., 1981. USNO circular no. 163, pA2.

  Called:  slDEUL

  P.T.Wallace   Starlink   23 August 1996

  Copyright (C) 1996 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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