**ACTION:**- Form the matrix of precession between two epochs, using the
model of Simon
*et al*. (1994), which is suitable for long periods of time. **CALL:**`CALL sla_PRECL (EP0, EP1, RMATP)`

**GIVEN:**-

*EP0***D**beginning epoch *EP1***D**ending epoch

**RETURNED:**-

*RMATP***D(3,3)**precession matrix

**NOTES:**- 1.
- The epochs are TDB Julian epochs.
- 2.
- The matrix is in the sense:

where**v**_{1}=**M****v**_{0}**v**_{1}is the star vector relative to the mean equator and equinox of epoch EP1,**M**is the matrix RMATP and**v**_{0}is the star vector relative to the mean equator and equinox of epoch EP0. - 3.
- The absolute accuracy of the model is limited by the uncertainty in the general precession, about per 1000 years. The remainder of the formulation provides a precision of 1 milliarcsecond over the interval from 1000AD to 3000AD, from 1000BC to 5000AD and from 4000BC to 8000AD.

**REFERENCE:**- Simon, J.L.
*et al*., 1994.*Astr.Astrophys.***282**, 663.

Starlink User Note 67

P. T. Wallace

12 October 1999

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