slaPreces -

SYNOPSYS

PARAMETERS

char *sys

double ep0

double ep1

double *ra

double *dc

DESCRIPTION

  Precession - either FK4 (Bessel-Newcomb, pre-IAU1976) or
  FK5 (Fricke, post-IAU1976) as required.

Given

     sys        char[]     precession to be applied: "FK4" or "FK5"
     ep0,ep1    double     starting and ending epoch
     ra,dc      double     RA,Dec, mean equator & equinox of epoch ep0

Returned

     *ra,*dc    double     RA,Dec, mean equator & equinox of epoch ep1

Called

 slaDranrm, slaPrebn, slaPrec, slaDcs2c,
             slaDmxv, slaDcc2s

Notes

  1)  The epochs are Besselian if sys='FK4' and Julian if 'FK5'.
      For example, to precess coordinates in the old system from

          slaPreces ( "FK4", 1900.0, 1950.0, &ra, &dc )

  2)  This routine will not correctly convert between the old and
      the new systems - for example conversion from B1950 to J2000.
      For these purposes see slaFk425, slaFk524, slaFk45z and
      slaFk54z.

  3)  If an invalid sys is supplied, values of -99.0,-99.0 will
      be returned for both ra and dc.

  P.T.Wallace   Starlink   22 December 1993

SEE ALSO

• slalib.3