fk54z¶

erfa.fk54z(r2000, d2000, bepoch)[source]¶

Convert a J2000.0 FK5 star position to B1950.0 FK4, assuming zero proper motion in FK5 and parallax.

Parameters:

r2000double array
d2000double array
bepochdouble array

Returns:

r1950double array
d1950double array
dr1950double array
dd1950double array

Notes

Wraps ERFA function eraFk54z. The ERFA documentation is:

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 e r a F k 5 4 z
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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.

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fk54z¶