From df208d901e3a8149a39d1c1411e1e440db1e1ba9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Richard=20K=C3=B6rber?= <github@shredzone.org>
Date: Fri, 14 Apr 2023 13:49:42 +0200
Subject: [PATCH] Reduce error of MoonIllumination.getAngle()

---
 .../org/shredzone/commons/suncalc/MoonIllumination.java     | 6 +++++-
 .../org/shredzone/commons/suncalc/MoonIlluminationTest.java | 6 +++---
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/main/java/org/shredzone/commons/suncalc/MoonIllumination.java b/src/main/java/org/shredzone/commons/suncalc/MoonIllumination.java
index f8b42ed..9bf9f7a 100644
--- a/src/main/java/org/shredzone/commons/suncalc/MoonIllumination.java
+++ b/src/main/java/org/shredzone/commons/suncalc/MoonIllumination.java
@@ -69,11 +69,15 @@ public MoonIllumination execute() {
             Vector m = Moon.position(t);
             double phi = PI - acos(m.dot(s) / (m.getR() * s.getR()));
             Vector sunMoon = m.cross(s);
+            double angle = atan2(
+                    cos(s.getTheta()) * sin(s.getPhi() - m.getPhi()),
+                    sin(s.getTheta()) * cos(m.getTheta()) - cos(s.getTheta()) * sin(m.getTheta()) * cos(s.getPhi() - m.getPhi())
+            );
 
             return new MoonIllumination(
                             (1 + cos(phi)) / 2,
                             toDegrees(phi * signum(sunMoon.getTheta())),
-                            toDegrees(sunMoon.getTheta()));
+                            toDegrees(angle));
         }
     }
 
diff --git a/src/test/java/org/shredzone/commons/suncalc/MoonIlluminationTest.java b/src/test/java/org/shredzone/commons/suncalc/MoonIlluminationTest.java
index 96eb5bb..8617be4 100644
--- a/src/test/java/org/shredzone/commons/suncalc/MoonIlluminationTest.java
+++ b/src/test/java/org/shredzone/commons/suncalc/MoonIlluminationTest.java
@@ -35,7 +35,7 @@ public void testNewMoon() {
                         .execute();
         assertThat(mi.getFraction()).as("fraction").isCloseTo(0.0, ERROR);
         assertThat(mi.getPhase()).as("phase").isCloseTo(176.0, ERROR); // -180.0
-        assertThat(mi.getAngle()).as("angle").isCloseTo(1.8, ERROR);
+        assertThat(mi.getAngle()).as("angle").isCloseTo(2.0, ERROR);
         assertThat(mi.getClosestPhase()).as("MoonPhase.Phase").isEqualTo(Phase.NEW_MOON);
     }
 
@@ -59,7 +59,7 @@ public void testFullMoon() {
                         .execute();
         assertThat(mi.getFraction()).as("fraction").isCloseTo(1.0, ERROR);
         assertThat(mi.getPhase()).as("phase").isCloseTo(-3.2, ERROR); // 0.0
-        assertThat(mi.getAngle()).as("angle").isCloseTo(-7.0, ERROR);
+        assertThat(mi.getAngle()).as("angle").isCloseTo(-7.4, ERROR);
         assertThat(mi.getClosestPhase()).as("MoonPhase.Phase").isEqualTo(Phase.FULL_MOON);
     }
 
@@ -71,7 +71,7 @@ public void testWaningHalfMoon() {
                         .execute();
         assertThat(mi.getFraction()).as("fraction").isCloseTo(0.5, ERROR);
         assertThat(mi.getPhase()).as("phase").isCloseTo(90.0, ERROR);
-        assertThat(mi.getAngle()).as("angle").isCloseTo(68.1, ERROR);
+        assertThat(mi.getAngle()).as("angle").isCloseTo(68.7, ERROR);
         assertThat(mi.getClosestPhase()).as("MoonPhase.Phase").isEqualTo(Phase.LAST_QUARTER);
     }