feat(lox-earth): add mean obliquity of the ecliptic

Author: helgee

crates/lox-earth/src/ecliptic.rs

@@ -0,0 +1,64 @@
+// SPDX-FileCopyrightText: 2025 Helge Eichhorn <[email protected]>
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use fast_polynomial::poly_array;
+use lox_test_utils::ApproxEq;
+use lox_time::{Time, julian_dates::JulianDate, time_scales::Tt};
+use lox_units::Angle;
+
+/// Mean obliquity of the ecliptic.
+#[derive(Debug, Clone, Copy, PartialEq, PartialOrd, ApproxEq)]
+pub struct MeanObliquity(pub Angle);
+
+impl MeanObliquity {
+    /// Returns the mean obliquity of the ecliptic based on the IAU1980 precession model.
+    pub fn iau1980(time: Time<Tt>) -> Self {
+        let t = time.centuries_since_j2000();
+
+        Self(Angle::arcseconds(poly_array(
+            t,
+            &[84381.448, -46.8150, -0.00059, 0.001813],
+        )))
+    }
+
+    /// Returns the mean obliquity of the ecliptic based on the IAU2006 precession model.
+    pub fn iau2006(time: Time<Tt>) -> Self {
+        let t = time.centuries_since_j2000();
+
+        Self(Angle::arcseconds(poly_array(
+            t,
+            &[
+                84381.406,
+                -46.836769,
+                -0.0001831,
+                0.00200340,
+                -0.000000576,
+                -0.0000000434,
+            ],
+        )))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use lox_test_utils::assert_approx_eq;
+
+    use super::*;
+
+    #[test]
+    fn test_mean_obliquity_iau1980() {
+        let time = Time::from_two_part_julian_date(Tt, 2400000.5, 54388.0);
+        let exp = MeanObliquity(Angle::new(0.409_075_134_764_381_6));
+        let act = MeanObliquity::iau1980(time);
+        assert_approx_eq!(act, exp, rtol <= 1e-14);
+    }
+
+    #[test]
+    fn test_mean_obliquity_iau2006() {
+        let time = Time::from_two_part_julian_date(Tt, 2400000.5, 54388.0);
+        let exp = MeanObliquity(Angle::new(0.409_074_922_938_725_8));
+        let act = MeanObliquity::iau2006(time);
+        assert_approx_eq!(act, exp, rtol <= 1e-14);
+    }
+}

crates/lox-earth/src/lib.rs

@@ -7,6 +7,7 @@
 pub mod cio;
 pub mod cip;
 pub mod coordinate_transformations;
+pub mod ecliptic;
 pub mod eop;
 pub mod ephemeris;
 pub mod itrf;