$LastChangedDate: 27-Juil-2011 $

# CL_mod_precessionAngles

Precession angles (Lieske 1976)

## Calling Sequence

       [K[,KP[,KPP]]]=CL_mod_precessionAngles(jj_tai,sec_tai[,flag])

## Description

- Computes the precession angles according to Lieske precession theory.

  if P0 is the pole of J2000 equator, and P is the pole at time t; if G0 is the equinox of J2000 and G is the equinox at time t:

  zetaa: angle between the meridians (pole P0) P0-P and P0-G0

  thetaa: angle between the equateur J2000 and the equateur t

  za: angle between the meridians (pole P) P-P0 and P-G

## Parameters

jj_tai :

  (integer) Modified (1950.0) Julian day (1xN)

ss_tai :

  seconds (TAI time scale) (1xN)

flag :

  (optional) "n" -> K; "p" -> K and Kp; "s" -> K, Kp, Kpp (default depends on number of outputs)

K :

  precession angles K = [zetaa;tetaa;za] [rad] (3xN)

KP :

  (optional) first time derivative of K [rad/s] (3xN)

KPP :

  (optional) second time derivative of K [rad/s^2] (3xN)

## Authors

- CNES - DCT/SB

## Bibliography

- 1) IERS Conventions (1996), Dennis D. McCarthy

- 2) Explanatory Supplement to the Astronomical Almanac, Seidelman (1992)

## See also

- CL_mod_nutationAngles

## Examples

    jj_tai = [19500:1:20500];
    ss_tai = 43200*ones(jj_tai);
    [K,KP,KPP]=CL_mod_precessionAngles(jj_tai,ss_tai,"s");
    plot(jj_tai'+ss_tai'/86400,K');