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PREMAT

PREMAT

Name


      PREMAT

Purpose


      Return the precession matrix needed to go from EQUINOX1 to EQUINOX2.

Explantion


      This matrix is used by the procedures PRECESS and BARYVEL to precess
      astronomical coordinates

Calling Sequence


      matrix = PREMAT( equinox1, equinox2, [ /FK4 ] )

Inputs


      EQUINOX1 - Original equinox of coordinates, numeric scalar.
      EQUINOX2 - Equinox of precessed coordinates.

Output


      matrix - double precision 3 x 3 precession matrix, used to precess
              equatorial rectangular coordinates

Optional Input Keywords


      /FK4 - If this keyword is set, the FK4 (B1950.0) system precession
              angles are used to compute the precession matrix. The
              default is to use FK5 (J2000.0) precession angles

Examples


      Return the precession matrix from 1950.0 to 1975.0 in the FK4 system
      IDL> matrix = PREMAT( 1950.0, 1975.0, /FK4)

Procedure


      FK4 constants from "Computational Spherical Astronomy" by Taff (1983),
      p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
      Supplement 1992, page 104 Table 3.211.1.

Revision History


      Written, Wayne Landsman, HSTX Corporation, June 1994
      Converted to IDL V5.0 W. Landsman September 1997



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