**ACTION:**- Form the matrix of precession between two epochs (IAU 1976, FK5).
**CALL:**`CALL sla_PREC (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.
- Though the matrix method itself is rigorous, the precession angles are expressed through canonical polynomials which are valid only for a limited time span. There are also known errors in the IAU precession rate. The absolute accuracy of the present formulation is better than from 1960AD to 2040AD, better than from 1640AD to 2360AD, and remains below for the whole of the period 500BC to 3000AD. The errors exceed outside the range 1200BC to 3900AD, exceed outside 4200BC to 5600AD and exceed outside 6800BC to 8200AD. The SLALIB routine sla_PRECL implements a more elaborate model which is suitable for problems spanning several thousand years.

**REFERENCES:**- 1.
- Lieske, J.H., 1979.
*Astr.Astrophys.***73**, 282; equations 6 & 7, p283. - 2.
- Kaplan, G.H., 1981.
*USNO circular no. 163*, pA2.

Starlink User Note 67

P. T. Wallace

12 October 1999

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