- 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).

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 ).

For convenience, a direct to 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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk