SLA_PRECES

Precession

ACTION:: Precession – either the old “FK4” (Bessel-Newcomb, pre IAU 1976) or new “FK5” (Fricke, post IAU 1976) as required.
CALL:: CALL sla_PRECES (SYSTEM, EP0, EP1, RA, DC)

SYSTEM	C	precession to be applied: ‘FK4’ or ‘FK5’

EP0,EP1	D	starting and ending epoch

RA,DC	D	$[α, δ]$ , mean equator & equinox of epoch EP0

RA,DC	D	$[α, δ]$ , mean equator & equinox of epoch EP1

NOTES:

(1): Lowercase characters in SYSTEM are acceptable.
(2): The epochs are Besselian if SYSTEM=‘FK4’ and Julian if ‘FK5’. For example, to precess coordinates in the old system from equinox 1900.0 to 1950.0 the call would be:

CALL sla_PRECES (’FK4’, 1900D0, 1950D0, RA, DC)
(3): This routine will NOT correctly convert between the old and the new systems – for example conversion from B1950 to J2000. For these purposes see sla_FK425, sla_FK524, sla_FK45Z and sla_FK54Z.
(4): If an invalid SYSTEM is supplied, values of $-$ 99D0, $-$ 99D0 are returned for both RA and DC.