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use crate::rfam::*;
/// Mean obliquity of the ecliptic, IAU 1980 model.
///
/// Given:
/// * date1,date2 double TT as a 2-part Julian Date (Note 1)
///
/// Returned (function value):
/// * 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:
/// * Explanatory Supplement to the Astronomical Almanac,
/// P. Kenneth Seidelmann (ed), University Science Books (1992),
/// Expression 3.222-1 (p114).
///
/// This revision: 2021 May 11
pub fn obl80(date1: f64, date2: f64)->f64
{
/* Interval between fundamental epoch J2000.0 and given date (JC). */
let t = ((date1 - URSA_DJ00) + date2) / URSA_DJC;
/* Mean obliquity of date. */
URSA_DAS2R * (84381.448 +
(-46.8150 +
(-0.00059 +
( 0.001813) * t) * t) * t)
/* Finished. */
}