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

}