1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
use crate::consts::{DAS2R, DJ00, DJC};
/// Mean obliquity of the ecliptic, IAU 1980 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:
///
/// Explanatory Supplement to the Astronomical Almanac,
/// P. Kenneth Seidelmann (ed), University Science Books (1992),
/// Expression 3.222-1 (p114).
pub fn obl80(date1: f64, date2: f64) -> f64 {
/* Interval between fundamental epoch J2000.0 and given date (JC). */
let t = ((date1 - DJ00) + date2) / DJC;
/* Mean obliquity of date. */
DAS2R * (84381.448 + (-46.8150 + (-0.00059 + (0.001813) * t) * t) * t)
}