sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use super::{bpn2xy, pnm06a, s06};

/// The CIO locator s, positioning the Celestial Intermediate Origin on
/// the equator of the Celestial Intermediate Pole, using the IAU 2006
/// precession and IAU 2000A nutation models.
///
/// This function is part of the International Astronomical Union's
/// SOFA (Standards of Fundamental Astronomy) software collection.
///
/// Status:  support function.
///
/// Given:
///    date1,date2  f64    TT as a 2-part Julian Date (Note 1)
///
/// Returned (function value):
///                 f64    the CIO locator s in radians (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:
///
/// ```text
///           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 CIO locator s is the difference between the right ascensions
///    of the same point in two systems.  The two systems are the GCRS
///    and the CIP,CIO, and the point is the ascending node of the
///    CIP equator.  The CIO locator s remains a small fraction of
///    1 arcsecond throughout 1900-2100.
///
/// 3) The series used to compute s is in fact for s+XY/2, where X and Y
///    are the x and y components of the CIP unit vector;  this series is
///    more compact than a direct series for s would be.  The present
///    function uses the full IAU 2000A nutation model when predicting
///    the CIP position.
///
/// References:
///
///    Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
///    "Expressions for the Celestial Intermediate Pole and Celestial
///    Ephemeris Origin consistent with the IAU 2000A precession-
///    nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
///
///    n.b. The celestial ephemeris origin (CEO) was renamed "celestial
///         intermediate origin" (CIO) by IAU 2006 Resolution 2.
///
///    Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855
///
///    McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003),
///    IERS Technical Note No. 32, BKG
///
///    Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
pub fn s06a(date1: f64, date2: f64) -> f64 {
    /* Bias-precession-nutation-matrix, IAU 20006/2000A. */
    let rnpb = pnm06a(date1, date2);

    /* Extract the CIP coordinates. */
    let (x, y) = bpn2xy(&rnpb);

    /* Compute the CIO locator s, given the CIP coordinates. */
    s06(date1, date2, x, y)
}