From 17bd6fe71088011aabf440ec3192a18484b17587 Mon Sep 17 00:00:00 2001
From: AngusGMorrison <github@angus-morrison.com>
Date: Tue, 2 Jan 2024 13:11:14 +0000
Subject: [PATCH] Calculate ERA using IAU 2000

---
 crates/lox_core/src/earth.rs                |  1 +
 crates/lox_core/src/earth/rotation_angle.rs | 56 +++++++++++++++++++++
 crates/lox_core/src/time/intervals.rs       |  2 +
 3 files changed, 59 insertions(+)
 create mode 100644 crates/lox_core/src/earth/rotation_angle.rs

diff --git a/crates/lox_core/src/earth.rs b/crates/lox_core/src/earth.rs
index a429d4a0..f739fb1b 100644
--- a/crates/lox_core/src/earth.rs
+++ b/crates/lox_core/src/earth.rs
@@ -13,3 +13,4 @@ mod cio;
 mod cip;
 mod coordinate_transformations;
 mod nutation;
+mod rotation_angle;
diff --git a/crates/lox_core/src/earth/rotation_angle.rs b/crates/lox_core/src/earth/rotation_angle.rs
new file mode 100644
index 00000000..cbd1eefa
--- /dev/null
+++ b/crates/lox_core/src/earth/rotation_angle.rs
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2024. Helge Eichhorn and the LOX contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+//! Module rotation_angle exposes functions for calculating the Earth Rotation Angle (ERA).
+
+use crate::bodies::Earth;
+use crate::time::intervals::UT1DaysSinceJ2000;
+use crate::types::Radians;
+use std::f64::consts::TAU;
+
+impl Earth {
+    /// Computes the Earth Rotation Angle (ERA) in radians using the IAU 2000 model.
+    pub fn rotation_angle_00(t: UT1DaysSinceJ2000) -> Radians {
+        let f = t.rem_euclid(1.0); // fractional part of t
+        TAU * (f + 0.7790572732640 + 0.00273781191135448 * t) % TAU
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use float_eq::assert_float_eq;
+
+    #[test]
+    fn test_rotation_angle_00() {
+        struct TestCase {
+            t: UT1DaysSinceJ2000,
+            expected: Radians,
+        }
+
+        [
+            TestCase {
+                t: -123.45,
+                expected: 6.227104062035152,
+            },
+            TestCase {
+                t: 0.0,
+                expected: 4.894961212823756,
+            },
+            TestCase {
+                t: 123.45,
+                expected: 3.562818363612361,
+            },
+        ]
+        .iter()
+        .for_each(|tc| {
+            let actual = Earth::rotation_angle_00(tc.t);
+            assert_float_eq!(tc.expected, actual, rel <= 1e-9);
+        });
+    }
+}
diff --git a/crates/lox_core/src/time/intervals.rs b/crates/lox_core/src/time/intervals.rs
index dd9c6d6c..ae3817eb 100644
--- a/crates/lox_core/src/time/intervals.rs
+++ b/crates/lox_core/src/time/intervals.rs
@@ -13,6 +13,8 @@ pub fn tdb_julian_centuries_since_j2000(epoch: Epoch) -> TDBJulianCenturiesSince
     }
 }
 
+pub type UT1DaysSinceJ2000 = f64;
+
 #[cfg(test)]
 mod epoch_tests {
     use float_eq::assert_float_eq;