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fk54z.c
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fk54z.c
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#include "erfa.h"
void eraFk54z(double r2000, double d2000, double bepoch,
double *r1950, double *d1950,
double *dr1950, double *dd1950)
/*
** - - - - - - - - -
** e r a F k 5 4 z
** - - - - - - - - -
**
** Convert a J2000.0 FK5 star position to B1950.0 FK4, assuming zero
** proper motion in FK5 and parallax.
**
** Given:
** r2000,d2000 double J2000.0 FK5 RA,Dec (rad)
** bepoch double Besselian epoch (e.g. 1950.0)
**
** Returned:
** r1950,d1950 double B1950.0 FK4 RA,Dec (rad) at epoch BEPOCH
** dr1950,dd1950 double B1950.0 FK4 proper motions (rad/trop.yr)
**
** Notes:
**
** 1) In contrast to the eraFk524 function, here the FK5 proper
** motions, the parallax and the radial velocity are presumed zero.
**
** 2) This function converts a star position from the IAU 1976 FK5
** (Fricke) system to the former FK4 (Bessel-Newcomb) system, for
** cases such as distant radio sources where it is presumed there is
** zero parallax and no proper motion. Because of the E-terms of
** aberration, such objects have (in general) non-zero proper motion
** in FK4, and the present function returns those fictitious proper
** motions.
**
** 3) Conversion from J2000.0 FK5 to B1950.0 FK4 only is provided for.
** Conversions involving other equinoxes would require additional
** treatment for precession.
**
** 4) The position returned by this function is in the B1950.0 FK4
** reference system but at Besselian epoch bepoch. For comparison
** with catalogs the bepoch argument will frequently be 1950.0. (In
** this context the distinction between Besselian and Julian epoch
** is insignificant.)
**
** 5) The RA component of the returned (fictitious) proper motion is
** dRA/dt rather than cos(Dec)*dRA/dt.
**
** Called:
** eraAnp normalize angle into range 0 to 2pi
** eraC2s p-vector to spherical
** eraFk524 FK4 to FK5
** eraS2c spherical to p-vector
**
** This revision: 2023 March 5
**
** Copyright (C) 2013-2023, NumFOCUS Foundation.
** Derived, with permission, from the SOFA library. See notes at end of file.
*/
{
double r, d, pr, pd, px, rv, p[3], w, v[3];
int i;
/* FK5 equinox J2000.0 to FK4 equinox B1950.0. */
eraFk524(r2000, d2000, 0.0, 0.0, 0.0, 0.0,
&r, &d, &pr, &pd, &px, &rv);
/* Spherical to Cartesian. */
eraS2c(r, d, p);
/* Fictitious proper motion (radians per year). */
v[0] = - pr*p[1] - pd*cos(r)*sin(d);
v[1] = pr*p[0] - pd*sin(r)*sin(d);
v[2] = pd*cos(d);
/* Apply the motion. */
w = bepoch - 1950.0;
for ( i = 0; i < 3; i++ ) {
p[i] += w*v[i];
}
/* Cartesian to spherical. */
eraC2s(p, &w, d1950);
*r1950 = eraAnp(w);
/* Fictitious proper motion. */
*dr1950 = pr;
*dd1950 = pd;
/* Finished. */
}
/*----------------------------------------------------------------------
**
**
** Copyright (C) 2013-2023, NumFOCUS Foundation.
** All rights reserved.
**
** This library is derived, with permission, from the International
** Astronomical Union's "Standards of Fundamental Astronomy" library,
** available from http://www.iausofa.org.
**
** The ERFA version is intended to retain identical functionality to
** the SOFA library, but made distinct through different function and
** file names, as set out in the SOFA license conditions. The SOFA
** original has a role as a reference standard for the IAU and IERS,
** and consequently redistribution is permitted only in its unaltered
** state. The ERFA version is not subject to this restriction and
** therefore can be included in distributions which do not support the
** concept of "read only" software.
**
** Although the intent is to replicate the SOFA API (other than
** replacement of prefix names) and results (with the exception of
** bugs; any that are discovered will be fixed), SOFA is not
** responsible for any errors found in this version of the library.
**
** If you wish to acknowledge the SOFA heritage, please acknowledge
** that you are using a library derived from SOFA, rather than SOFA
** itself.
**
**
** TERMS AND CONDITIONS
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions
** are met:
**
** 1 Redistributions of source code must retain the above copyright
** notice, this list of conditions and the following disclaimer.
**
** 2 Redistributions in binary form must reproduce the above copyright
** notice, this list of conditions and the following disclaimer in
** the documentation and/or other materials provided with the
** distribution.
**
** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
** the International Astronomical Union nor the names of its
** contributors may be used to endorse or promote products derived
** from this software without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
** POSSIBILITY OF SUCH DAMAGE.
**
*/