Py ERFA:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- PyERFA v2.0.0.1 »
- erfa Package »
- pmat76

# pmat76

erfa.pmat76(date1, date2)

  Precession matrix from J2000.0 to a specified date, IAU 1976 model.

  Parameters
    date1 double array
    date2 double array
  Returns
    rmatp double array

  Notes

  Wraps ERFA function `eraPmat76`. The ERFA documentation is:

      - - - - - - - - - -
       e r a P m a t 7 6
      - - - - - - - - - -

      Precession matrix from J2000.0 to a specified date, IAU 1976 model.

      Given:
         date1,date2 double       ending date, TT (Note 1)

      Returned:
         rmatp       double[3][3] precession matrix, J2000.0 -> date1+date2

      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) The matrix operates in the sense V(date) = RMATP * V(J2000),
         where the p-vector V(J2000) is with respect to the mean
         equatorial triad of epoch J2000.0 and the p-vector V(date)
         is with respect to the mean equatorial triad of the given
         date.

      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.  In addition, the IAU 1976
         precession rate is known to be imperfect.  The absolute accuracy
         of the present formulation is better than 0.1 arcsec from
         1960AD to 2040AD, better than 1 arcsec from 1640AD to 2360AD,
         and remains below 3 arcsec for the whole of the period
         500BC to 3000AD.  The errors exceed 10 arcsec outside the
         range 1200BC to 3900AD, exceed 100 arcsec outside 4200BC to
         5600AD and exceed 1000 arcsec outside 6800BC to 8200AD.

      Called:
         eraPrec76    accumulated precession angles, IAU 1976
         eraIr        initialize r-matrix to identity
         eraRz        rotate around Z-axis
         eraRy        rotate around Y-axis
         eraCr        copy r-matrix

      References:

         Lieske, J.H., 1979, Astron.Astrophys. 73, 282.
          equations (6) & (7), p283.

         Kaplan,G.H., 1981. USNO circular no. 163, pA2.

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.