Orekit third body fixtures

tests/orekit_comparison.rs

@@ -123,6 +123,40 @@ fn test_a_low_degree_field_agrees_with_orekit() {
 }
 
 #[test]
-fn test_a_full_field_agrees_with_orekit() {
-    gravity(70, 0.2);
+fn test_a_higher_degree_field_agrees_with_orekit() {
+    // Degree 20 rather than 70: the agreement is the same to within a couple of
+    // centimeters and the arc costs a fraction of the time in a debug build.
+    gravity(20, 0.2);
+}
+
+/// The Sun and the Moon over a point mass, from the same DE-440 ephemeris on both sides.
+///
+/// Isolated from the field on purpose: the third-body term is the difference of two
+/// nearly equal attractions, so it is the one most easily got wrong, and a disagreement
+/// here would otherwise hide under the gravity figure.
+#[test]
+fn test_the_sun_and_moon_agree_with_orekit() {
+    let Some(almanac) = almanac() else {
+        eprintln!("skipped: kernels not fetched, run scripts/fetch-kernels.sh");
+        return;
+    };
+    let path = Path::new("tests/fixtures/orekit/third-body-low-earth.json");
+    let arc = ReferenceArc::load(path).expect("the fixture is committed");
+    assert!(arc.setup.forces.iter().any(|f| f.contains("Moon")));
+    assert!(arc.setup.forces.iter().any(|f| f.contains("Sun")));
+
+    let (position, velocity) = compare(
+        &arc,
+        &[
+            TwoBody { body: Body::EARTH },
+            ThirdBody { body: Body::MOON },
+            ThirdBody { body: Body::SUN },
+        ],
+        Some(almanac),
+    );
+    assert!(
+        position < 0.1,
+        "worst position disagreement {position:.3e} m over the day"
+    );
+    println!("third body: {position:.3e} m, {velocity:.3e} m/s");
 }