era.lteqec.md

eraLteqec

[dl, db] = ERFA.lteqec(epj, dr, dd)

Transformation from ICRS equatorial coordinates to ecliptic coordinates (mean equinox and ecliptic of date) using a long-term precession model.

Given:

   epj     double     Julian epoch (TT)
   dr,dd   double     ICRS right ascension and declination (radians)

Returned:

   dl,db   double     ecliptic longitude and latitude (radians)

1. No assumptions are made about whether the coordinates represent starlight and embody astrometric effects such as parallax or aberration.

2. The transformation is approximately that from mean J2000.0 right ascension and declination to ecliptic longitude and latitude (mean equinox and ecliptic of date), with only frame bias (always less than 25 mas) to disturb this classical picture.

3. 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.

Called:

   eraS2c       spherical coordinates to unit vector
   eraLtecm     J2000.0 to ecliptic rotation matrix, long term
   eraRxp       product of r-matrix and p-vector
   eraC2s       unit vector to spherical coordinates
   eraAnp       normalize angle into range 0 to 2pi
   eraAnpm      normalize angle into range +/- pi

References:

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

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.