Module obliquity 

Mean obliquity of the ecliptic.

The obliquity is the angle between Earth’s equatorial plane and the ecliptic (the plane of Earth’s orbit around the Sun). It’s approximately 23.4° and decreases slowly due to gravitational perturbations from other planets.

This module provides two IAU models:

Function	Model	J2000.0 Value	Polynomial Order
iau_2006_mean_obliquity	IAU 2006	84381.406″	5th order
iau_1980_mean_obliquity	IAU 1980	84381.448″	3rd order

Both return the mean obliquity — the smoothly varying component without short-period nutation oscillations. For the true obliquity (mean + nutation in obliquity), add NutationResult::delta_eps.

Time Argument

Both functions accept a two-part Julian Date in TDB. Split as (jd1, jd2) where typically jd1 = 2451545.0 (J2000.0) and jd2 is days from that epoch.

Example

use celestial_core::obliquity::iau_2006_mean_obliquity;
use celestial_core::constants::J2000_JD;

// At J2000.0
let eps = iau_2006_mean_obliquity(J2000_JD, 0.0);
let eps_deg = eps.to_degrees();
assert!((eps_deg - 23.4392794).abs() < 1e-6);

Functions

iau_1980_mean_obliquity

Mean obliquity of the ecliptic using the IAU 1980 model.

iau_2006_mean_obliquity

Mean obliquity of the ecliptic using the IAU 2006 precession model.