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# moon98

erfa.moon98(date1, date2)

  Approximate geocentric position and velocity of the Moon.

  Parameters
    date1 double array
    date2 double array
  Returns
    pv double array

  Notes

  Wraps ERFA function `eraMoon98`. The ERFA documentation is:

      - - - - - - - - - -
       e r a M o o n 9 8
      - - - - - - - - - -

      Approximate geocentric position and velocity of the Moon.

      n.b. Not IAU-endorsed and without canonical status.

      Given:
         date1  double         TT date part A (Notes 1,4)
         date2  double         TT date part B (Notes 1,4)

      Returned:
         pv     double[2][3]   Moon p,v, GCRS (AU, AU/d, Note 5)

      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.  The limited
         accuracy of the present algorithm is such that any of the methods
         is satisfactory.

      2) This function is a full implementation of the algorithm
         published by Meeus (see reference) except that the light-time
         correction to the Moon's mean longitude has been omitted.

      3) Comparisons with ELP/MPP02 over the interval 1950-2100 gave RMS
         errors of 2.9 arcsec in geocentric direction, 6.1 km in position
         and 36 mm/s in velocity.  The worst case errors were 18.3 arcsec
         in geocentric direction, 31.7 km in position and 172 mm/s in
         velocity.

      4) The original algorithm is expressed in terms of "dynamical time",
         which can either be TDB or TT without any significant change in
         accuracy.  UT cannot be used without incurring significant errors
         (30 arcsec in the present era) due to the Moon's 0.5 arcsec/sec
         movement.

      5) The result is with respect to the GCRS (the same as J2000.0 mean
         equator and equinox to within 23 mas).

      6) Velocity is obtained by a complete analytical differentiation
         of the Meeus model.

      7) The Meeus algorithm generates position and velocity in mean
         ecliptic coordinates of date, which the present function then
         rotates into GCRS.  Because the ecliptic system is precessing,
         there is a coupling between this spin (about 1.4 degrees per
         century) and the Moon position that produces a small velocity
         contribution.  In the present function this effect is neglected
         as it corresponds to a maximum difference of less than 3 mm/s and
         increases the RMS error by only 0.4%.

      References:

         Meeus, J., Astronomical Algorithms, 2nd edition, Willmann-Bell,
         1998, p337.

         Simon, J.L., Bretagnon, P., Chapront, J., Chapront-Touze, M.,
         Francou, G. & Laskar, J., Astron.Astrophys., 1994, 282, 663

      Defined in erfam.h:
         ERFA_DAU           astronomical unit (m)
         ERFA_DJC           days per Julian century
         ERFA_DJ00          reference epoch (J2000.0), Julian Date
         ERFA_DD2R          degrees to radians

      Called:
         eraS2pv      spherical coordinates to pv-vector
         eraPfw06     bias-precession F-W angles, IAU 2006
         eraIr        initialize r-matrix to identity
         eraRz        rotate around Z-axis
         eraRx        rotate around X-axis
         eraRxpv      product of r-matrix and pv-vector

      This revision:  2021 May 11

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