sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use crate::consts::{DAS2R, DJ00, DJC};

///  Precession-rate part of the IAU 2000 precession-nutation models
///  (part of MHB2000).
///
///  Given:
///     date1,date2    f64      TT as a 2-part Julian Date (Note 1)
///
///  Returned:
///     (dpsipr,depspr) (f64,f64) precession corrections (Notes 2,3)
///
///  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 precession adjustments are expressed as "nutation
///     components", corrections in longitude and obliquity with respect
///     to the J2000.0 equinox and ecliptic.
///
///  3) Although the precession adjustments are stated to be with respect
///     to Lieske et al. (1977), the MHB2000 model does not specify which
///     set of Euler angles are to be used and how the adjustments are to
///     be applied.  The most literal and straightforward procedure is to
///     adopt the 4-rotation epsilon_0, psi_A, omega_A, xi_A option, and
///     to add dpsipr to psi_A and depspr to both omega_A and eps_A.
///
///  4) This is an implementation of one aspect of the IAU 2000A nutation
///     model, formally adopted by the IAU General Assembly in 2000,
///     namely MHB2000 (Mathews et al. 2002).
///
///  References:
///
///     Lieske, J.H., Lederle, T., Fricke, W. & Morando, B., "Expressions
///     for the precession quantities based upon the IAU (1976) System of
///     Astronomical Constants", Astron.Astrophys., 58, 1-16 (1977)
///
///     Mathews, P.M., Herring, T.A., Buffet, B.A., "Modeling of nutation
///     and precession   New nutation series for nonrigid Earth and
///     insights into the Earth's interior", J.Geophys.Res., 107, B4,
///     2002.
///
///     Wallace, P.T., "Software for Implementing the IAU 2000
///     Resolutions", in IERS Workshop 5.1 (2002).
pub fn pr00(date1: f64, date2: f64) -> (f64, f64) {
    /* Precession and obliquity corrections (radians per century) */
    const PRECOR: f64 = -0.29965 * DAS2R;
    const OBLCOR: f64 = -0.02524 * DAS2R;

    /* Interval between fundamental epoch J2000.0 and given date (JC). */
    let t = ((date1 - DJ00) + date2) / DJC;

    /* Precession rate contributions with respect to IAU 1976/80. */
    let dpsipr = PRECOR * t;
    let depspr = OBLCOR * t;

    (dpsipr, depspr)
}