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use crate::consts::{DAS2R, DJ00, DJC};
/// Mean obliquity of the ecliptic, IAU 2006 precession model.
///
/// This function is part of the International Astronomical Union's
/// SOFA (Standards of Fundamental Astronomy) software collection.
///
/// Status: canonical model.
///
/// Given:
/// date1,date2 double TT as a 2-part Julian Date (Note 1)
///
/// Returned (function value):
/// double obliquity of the ecliptic (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:
///
/// 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 result is the angle between the ecliptic and mean equator of
/// date date1+date2.
///
/// Reference:
///
/// Hilton, J. et al., 2006, Celest.Mech.Dyn.Astron. 94, 351
pub fn obl06(date1: f64, date2: f64) -> f64 {
let (t, eps0): (f64, f64);
/* Interval between fundamental date J2000.0 and given date (JC). */
t = ((date1 - DJ00) + date2) / DJC;
/* Mean obliquity */
eps0 = (84381.406
+ (-46.836769
+ (-0.0001831 + (0.00200340 + (-0.000000576 + (-0.0000000434) * t) * t) * t) * t)
* t)
* DAS2R;
eps0
}