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erfasrc/src/pfw06.c  view on Meta::CPAN

#include "erfa.h"

void eraPfw06(double date1, double date2,
              double *gamb, double *phib, double *psib, double *epsa)
/*
**  - - - - - - - - -
**   e r a P f w 0 6
**  - - - - - - - - -
**
**  Precession angles, IAU 2006 (Fukushima-Williams 4-angle formulation).
**
**  Given:
**     date1,date2  double   TT as a 2-part Julian Date (Note 1)
**
**  Returned:
**     gamb         double   F-W angle gamma_bar (radians)
**     phib         double   F-W angle phi_bar (radians)
**     psib         double   F-W angle psi_bar (radians)
**     epsa         double   F-W angle epsilon_A (radians)
**
**  Notes:
**
**  1) The TT date date1+date2 is a Julian Date, apportioned in any
**     convenient way between the two arguments.  For example,
**     JD(TT)=2450123.7 could be expressed in any of these ways,
**     among others:
**
**            date1          date2
**
**         2450123.7           0.0       (JD method)
**         2451545.0       -1421.3       (J2000 method)
**         2400000.5       50123.2       (MJD method)
**         2450123.5           0.2       (date & time method)
**
**     The JD method is the most natural and convenient to use in
**     cases where the loss of several decimal digits of resolution
**     is acceptable.  The J2000 method is best matched to the way
**     the argument is handled internally and will deliver the
**     optimum resolution.  The MJD method and the date & time methods
**     are both good compromises between resolution and convenience.
**
**  2) Naming the following points:
**
**           e = J2000.0 ecliptic pole,
**           p = GCRS pole,
**           E = mean ecliptic pole of date,
**     and   P = mean pole of date,
**
**     the four Fukushima-Williams angles are as follows:
**
**        gamb = gamma_bar = epE
**        phib = phi_bar = pE
**        psib = psi_bar = pEP
**        epsa = epsilon_A = EP
**
**  3) The matrix representing the combined effects of frame bias and
**     precession is:
**
**        PxB = R_1(-epsa).R_3(-psib).R_1(phib).R_3(gamb)
**
**  4) The matrix representing the combined effects of frame bias,
**     precession and nutation is simply:
**
**        NxPxB = R_1(-epsa-dE).R_3(-psib-dP).R_1(phib).R_3(gamb)
**
**     where dP and dE are the nutation components with respect to the
**     ecliptic of date.
**
**  Reference:
**