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use crate::consts::{DAS2R, DJ00, DJC};
/// The TIO locator s', positioning the Terrestrial Intermediate Origin
/// on the equator of the Celestial Intermediate Pole.
///
/// Given:
/// date1,date2 f64 TT as a 2-part Julian Date (Note 1)
///
/// Returned (function value):
/// f64 the TIO 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:
///
/// 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 TIO locator s' is obtained from polar motion observations by
/// numerical integration, and so is in essence unpredictable.
/// However, it is dominated by a secular drift of about
/// 47 microarcseconds per century, which is the approximation
/// evaluated by the present function.
///
/// Reference:
///
/// McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
/// IERS Technical Note No. 32, BKG (2004)
pub fn sp00(date1: f64, date2: f64) -> f64 {
// Interval between fundamental epoch J2000.0 and current date (JC).
let t = ((date1 - DJ00) + date2) / DJC;
// Approximate s'.
-47e-6 * t * DAS2R
}