Convert J2000.0 FK5 star data to B1950.0 FK4. (double precision) This routine converts stars from the new, IAU 1976, FK5, Fricke system, to the old, Bessel-Newcomb, FK4 system. The precepts of Smith et al (Ref 1) are followed, using the implementation by Yallop et al (Ref 2) of a matrix method due to Standish. Kinoshita's development of Andoyer's post-Newcomb precession is used. The numerical constants from Seidelmann et al (Ref 3) are used canonically. Given: (all J2000.0,FK5) r2000,d2000 double J2000.0 RA,Dec (rad) dr2000,dd2000 double J2000.0 proper motions (rad/Jul.yr) p2000 double parallax (arcsec) v2000 double radial velocity (km/s, +ve = moving away) Returned: (all B1950.0,FK4) *r1950,*d1950 double B1950.0 RA,Dec (rad) *dr1950,*dd1950 double B1950.0 proper motions (rad/trop.yr) *p1950 double parallax (arcsec) *v1950 double radial velocity (km/s, +ve = moving away)
1) The proper motions in RA are dRA/dt rather than cos(Dec)*dRA/dt, and are per year rather than per century. 2) Note that conversion from Julian epoch 2000.0 to Besselian epoch 1950.0 only is provided for. Conversions involving other epochs will require use of the appropriate precession, proper motion, and E-terms routines before and/or after FK524 is called. 3) In the FK4 catalogue the proper motions of stars within 10 degrees of the poles do not embody the differential E-term effect and should, strictly speaking, be handled in a different manner from stars outside these regions. However, given the general lack of homogeneity of the star data available for routine astrometry, the difficulties of handling positions that may have been determined from astrometric fields spanning the polar and non-polar regions, the likelihood that the differential E-terms effect was not taken into account when allowing for proper motion in past astrometry, and the undesirability of a discontinuity in the algorithm, the decision has been made in this routine to include the effect of differential E-terms on the proper motions for all stars, whether polar or not. At epoch 2000, and measuring on the sky rather than in terms of dRA, the errors resulting from this simplification are less than 1 milliarcsecond in position and 1 milliarcsecond per century in proper motion.
1 Smith, C.A. et al, 1989. "The transformation of astrometric catalog systems to the equinox J2000.0". Astron.J. 97, 265. 2 Yallop, B.D. et al, 1989. "Transformation of mean star places from FK4 B1950.0 to FK5 J2000.0 using matrices in 6-space". Astron.J. 97, 274. 3 Seidelmann, P.K. (ed), 1992. "Explanatory Supplement to the Astronomical Almanac", ISBN 0-935702-68-7. Defined in slamac.h: D2PI P.T.Wallace Starlink 20 December 1993