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)

GIVEN:

SYSTEM precession to be applied: `FK4' or `FK5'

EP0,EP1 D starting and ending epoch

$$[\,\alpha,\delta\,]$$ RA,DC D

RETURNED:

$$[\,\alpha,\delta\,]$$ RA,DC

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.