SLALIB support for precession and nutation

SLALIB offers a choice of three precession models:

• The old Bessel-Newcomb, pre IAU 1976, ``FK4'' model, used for B1950 star positions and other pre-1984.0 purposes (sla_PREBN).
• The new Fricke, IAU 1976, ``FK5'' model, used for J2000 star positions and other post-1984.0 purposes (sla_PREC).
• A model published by Simon et al. which is more accurate than the IAU 1976 model and which is suitable for long periods of time (sla_PRECL).

In each case, the named SLALIB routine generates the $(3\times3)$precession matrix for a given start and finish time. For example, here is the Fortran code for generating the rotation matrix which describes the precession between the epochs J2000 and J1985.372 (IAU 1976 model):

            DOUBLE PRECISION PMAT(3,3)
             :
            CALL sla_PREC(2000D0,1985.372D0,PMAT)

It is instructive to examine the resulting matrix:

            +0.9999936402  +0.0032709208  +0.0014214694
            -0.0032709208  +0.9999946505  -0.0000023247
            -0.0014214694  -0.0000023248  +0.9999989897

Note that the diagonal elements are close to unity, and the other elements are small. This shows that over an interval as short as 15 years the precession isn't going to move a position vector very far (in this case about $0^{\circ} \hspace{-0.37em}.\hspace{0.02em}2$).

For convenience, a direct $[\,\alpha,\delta\,]$ to $[\,\alpha,\delta\,]$ precession routine is also provided (sla_PRECES), suitable for either the old or the new system (but not a mixture of the two).

SLALIB provides only one nutation model, the new, IAU 1980 model, implemented in the routine sla_NUTC. This returns the components of nutation in longitude and latitude (and also provides the obliquity) from which a nutation matrix can be generated by calling sla_DEULER (and from which the equation of the equinoxes, described later, can be found). Alternatively, the nutation matrix can be generated in a single call by using sla_NUT.

A rotation matrix for applying the entire precession/nutation transformation in one go can be generated by calling sla_PRENUT.