rfa 0.5.9

A port ERFA to Rust.
Documentation 
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use crate::rfam::*;
use super::nut00a::*;

///  IAU 2000A nutation with adjustments to match the IAU 2006
///  precession.
///
///  Given:
///   * date1,date2 TT as a 2-part Julian Date (Note 1)
///
///  Returned:
///   * dpsi,deps nutation, luni-solar + planetary (Note 2)
///
///  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 nutation components in longitude and obliquity are in radians
///     and with respect to the mean equinox and ecliptic of date,
///     IAU 2006 precession model (Hilton et al. 2006, Capitaine et al.
///     2005).
///
///  3) The function first computes the IAU 2000A nutation, then applies
///     adjustments for (i) the consequences of the change in obliquity
///     from the IAU 1980 ecliptic to the IAU 2006 ecliptic and (ii) the
///     secular variation in the Earth's dynamical form factor J2.
///
///  4) The present function provides classical nutation, complementing
///     the IAU 2000 frame bias and IAU 2006 precession.  It delivers a
///     pole which is at current epochs accurate to a few tens of
///     microarcseconds, apart from the free core nutation.
///
/// # Called:
///     nut00a    nutation, IAU 2000A
///
/// # References:
///   * Chapront, J., Chapront-Touze, M. & Francou, G. 2002,
///     Astron.Astrophys. 387, 700
///   * Lieske, J.H., Lederle, T., Fricke, W. & Morando, B. 1977,
///     Astron.Astrophys. 58, 1-16
///   * Mathews, P.M., Herring, T.A., Buffet, B.A. 2002, J.Geophys.Res.
///     107, B4.  The MHB_2000 code itself was obtained on 9th September
///     2002 from ftp//maia.usno.navy.mil/conv2000/chapter5/IAU2000A.
///   * Simon, J.-L., Bretagnon, P., Chapront, J., Chapront-Touze, M.,
///     Francou, G., Laskar, J. 1994, Astron.Astrophys. 282, 663-683
///   * Souchay, J., Loysel, B., Kinoshita, H., Folgueira, M. 1999,
///     Astron.Astrophys.Supp.Ser. 135, 111
///   * Wallace, P.T., "Software for Implementing the IAU 2000
///     Resolutions", in IERS Workshop 5.1 (2002)
///
///  This revision:  2021 May 11
pub fn nut06a(date1: f64, date2: f64, dpsi: &mut f64, deps: &mut f64)
{

 /* Interval between fundamental date J2000.0 and given date (JC). */
    let t = ((date1 - URSA_DJ00) + date2) / URSA_DJC;
 
 /* Factor correcting for secular variation of J2. */
    let fj2 = -2.7774e-6 * t;
 
 let mut dp =0.0; let mut de = 0.0;
 /* Obtain IAU 2000A nutation. */
    nut00a(date1, date2, &mut dp, &mut de);
 
 /* Apply P03 adjustments (Wallace & Capitaine, 2006, Eqs.5). */
    *dpsi = dp + dp * (0.4697e-6 + fj2);
    *deps = de + de * fj2;
 
 /* Finished. */
 
 }