### SLA_FK54Z

FK5 to FK4, no P.M. or Parallax

ACTION:
Convert a J2000.0 FK5 star position to B1950.0 FK4 assuming FK5 zero proper motion and parallax. This routine converts star positions from the new, IAU 1976, FK5, Fricke system to the old, Bessel-Newcomb, FK4 system.
CALL:
CALL sla_FK54Z (R2000, D2000, BEPOCH, R1950, D1950, DR1950, DD1950)
##### GIVEN:
 R2000 D J2000.0 FK5 $\alpha$ (radians) D2000 D J2000.0 FK5 $\delta$ (radians) BEPOCH D Besselian epoch (e.g. 1950D0)

##### RETURNED:
 R1950 D B1950.0 FK4 $\alpha$ at epoch BEPOCH (radians) D1950 D B1950.0 FK4 $\delta$ at epoch BEPOCH (radians) DR1950 D B1950.0 FK4 proper motion in $\alpha$ (radians per tropical year) DD1950 D B1950.0 FK4 proper motion in $\delta$ (radians per tropical year)

NOTES:
(1)
The $\alpha$ proper motions are $\stackrel{̇}{\alpha }$ rather than $\stackrel{̇}{\alpha }cos\delta$, and are per year rather than per century.
(2)
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 routines before and after this routine is called.
(3)
Unlike in the sla_FK524 routine, the FK5 proper motions, the parallax and the radial velocity are presumed zero.
(4)
It was the intention that FK5 should be a close approximation to an inertial frame, so that distant objects have zero proper motion; such objects have (in general) non-zero proper motion in FK4, and this routine returns those fictitious proper motions.
(5)
The position returned by this routine is in the B1950 reference frame but at Besselian epoch BEPOCH. For comparison with catalogues the BEPOCH argument will frequently be 1950D0.
(6)