eraLtp

rp = ERFA.ltp(epj)

Long-term precession matrix.

Given:

   epj     double         Julian epoch (TT)

   rp      double[3][3]   precession matrix, J2000.0 to date

The matrix is in the sense

P_date = rp x P_J2000,

where P_J2000 is a vector with respect to the J2000.0 mean equator and equinox and P_date is the same vector with respect to the equator and equinox of epoch epj.
The Vondrak et al. (2011, 2012) 400 millennia precession model agrees with the IAU 2006 precession at J2000.0 and stays within 100 microarcseconds during the 20th and 21st centuries. It is accurate to a few arcseconds throughout the historical period, worsening to a few tenths of a degree at the end of the +/- 200,000 year time span.

   eraLtpequ    equator pole, long term
   eraLtpecl    ecliptic pole, long term
   eraPxp       vector product
   eraPn        normalize vector

Vondrak, J., Capitaine, N. and Wallace, P., 2011, New precession expressions, valid for long time intervals, Astron.Astrophys. 534, A22

Vondrak, J., Capitaine, N. and Wallace, P., 2012, New precession expressions, valid for long time intervals (Corrigendum), Astron.Astrophys. 541, C1

This revision: 2021 May 11