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# num00b

erfa.num00b(date1, date2)

  Form the matrix of nutation for a given date, IAU 2000B model.

  Parameters:
    date1 double array
    date2 double array
  Returns:
    rmatn double array

  Notes

  Wraps ERFA function `eraNum00b`. The ERFA documentation is:

      - - - - - - - - - -
       e r a N u m 0 0 b
      - - - - - - - - - -

      Form the matrix of nutation for a given date, IAU 2000B model.

      Given:
         date1,date2  double         TT as a 2-part Julian Date (Note 1)

      Returned:
         rmatn        double[3][3]   nutation matrix

      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(true) = rmatn * V(mean), where
         the p-vector V(true) is with respect to the true equatorial triad
         of date and the p-vector V(mean) is with respect to the mean
         equatorial triad of date.

      3) The present function is faster, but slightly less accurate (about
         1 mas), than the eraNum00a function.

      Called:
         eraPn00b     bias/precession/nutation, IAU 2000B

      Reference:

         Explanatory Supplement to the Astronomical Almanac,
         P. Kenneth Seidelmann (ed), University Science Books (1992),
         Section 3.222-3 (p114).

      This revision:  2021 May 11

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.