Fix Moon parallactic angle calculation

src/doc/docs/migration.md

@@ -2,6 +2,9 @@
 
 This document will help you migrate your code to the latest _suncalc_ version.
 
+## Version 2.13
+* Due to a very old bug, `MoonPosition.getParallacticAngle()` gave completely wrong results. If you used that method in your code or tests, prepare to get totally different values now.
+
 ## Version 2.10
 * Due to a very old bug, setting the `height()` had almost no impact on the result, which is obviously wrong (on sunset, a plane in the sky is still illuminated while the sun has just gone at the viewer's position). This bug has been fixed. If you are using `height()`, you will get correct results now. They may deviate by several minutes from the results of earlier versions.
 

src/main/java/org/shredzone/commons/suncalc/MoonIllumination.java

@@ -102,7 +102,7 @@ public double getPhase() {
      * <p>
      * By subtracting {@link MoonPosition#getParallacticAngle()} from {@link #getAngle()},
      * one can get the zenith angle of the moons bright limb (anticlockwise). The zenith
-     * angle can be used do draw the moon shape from the observers perspective (e.g. the
+     * angle can be used do draw the moon shape from the observer's perspective (e.g. the
      * moon lying on its back).
      */
     public double getAngle() {

src/main/java/org/shredzone/commons/suncalc/MoonPosition.java

@@ -81,7 +81,7 @@ public MoonPosition execute() {
 
             double hRef = refraction(horizontal.getTheta());
 
-            double pa = atan2(sin(h), tan(phi) * cos(mc.getTheta())) - sin(mc.getTheta()) * cos(h);
+            double pa = atan2(sin(h), tan(phi) * cos(mc.getTheta()) - sin(mc.getTheta()) * cos(h));
 
             return new MoonPosition(horizontal.getPhi(), horizontal.getTheta() + hRef, mc.getR(), pa);
         }

src/test/java/org/shredzone/commons/suncalc/MoonPositionTest.java

@@ -36,6 +36,7 @@ public void testCologne() {
                         .execute();
         assertThat(mp1.getAzimuth()).as("azimuth").isCloseTo(304.8, ERROR);
         assertThat(mp1.getAltitude()).as("altitude").isCloseTo(-39.6, ERROR);
+        assertThat(mp1.getParallacticAngle()).as("pa").isCloseTo(32.0, ERROR);
 
         MoonPosition mp2 = MoonPosition.compute()
                         .on(2017, 7, 12, 3, 51, 0)
@@ -45,6 +46,7 @@ public void testCologne() {
         assertThat(mp2.getAzimuth()).as("azimuth").isCloseTo(179.9, ERROR);
         assertThat(mp2.getAltitude()).as("altitude").isCloseTo(25.3, ERROR);
         assertThat(mp2.getDistance()).as("distance").isCloseTo(394709.0, DISTANCE_ERROR);
+        assertThat(mp2.getParallacticAngle()).as("pa").isCloseTo(0.0, ERROR);
     }
 
     @Test
@@ -56,6 +58,7 @@ public void testAlert() {
                         .execute();
         assertThat(mp1.getAzimuth()).as("azimuth").isCloseTo(257.5, ERROR);
         assertThat(mp1.getAltitude()).as("altitude").isCloseTo(-10.9, ERROR);
+        assertThat(mp1.getParallacticAngle()).as("pa").isCloseTo(7.5, ERROR);
 
         MoonPosition mp2 = MoonPosition.compute()
                         .on(2017, 7, 12, 2, 37, 0)
@@ -65,6 +68,7 @@ public void testAlert() {
         assertThat(mp2.getAzimuth()).as("azimuth").isCloseTo(179.8, ERROR);
         assertThat(mp2.getAltitude()).as("altitude").isCloseTo(-5.7, ERROR);
         assertThat(mp2.getDistance()).as("distance").isCloseTo(393609.0, DISTANCE_ERROR);
+        assertThat(mp2.getParallacticAngle()).as("pa").isCloseTo(0.0, ERROR);
     }
 
     @Test
@@ -76,6 +80,7 @@ public void testWellington() {
                         .execute();
         assertThat(mp1.getAzimuth()).as("azimuth").isCloseTo(311.3, ERROR);
         assertThat(mp1.getAltitude()).as("altitude").isCloseTo(55.1, ERROR);
+        assertThat(mp1.getParallacticAngle()).as("pa").isCloseTo(144.2, ERROR);
 
         MoonPosition mp2 = MoonPosition.compute()
                         .on(2017, 7, 12, 2, 17, 0)
@@ -85,6 +90,7 @@ public void testWellington() {
         assertThat(mp2.getAzimuth()).as("azimuth").isCloseTo(0.5, ERROR);
         assertThat(mp2.getAltitude()).as("altitude").isCloseTo(63.9, ERROR);
         assertThat(mp2.getDistance()).as("distance").isCloseTo(396272.0, DISTANCE_ERROR);
+        assertThat(mp2.getParallacticAngle()).as("pa").isCloseTo(-179.6, ERROR);
     }
 
     @Test
@@ -96,6 +102,7 @@ public void testPuertoWilliams() {
                         .execute();
         assertThat(mp1.getAzimuth()).as("azimuth").isCloseTo(199.4, ERROR);
         assertThat(mp1.getAltitude()).as("altitude").isCloseTo(-52.7, ERROR);
+        assertThat(mp1.getParallacticAngle()).as("pa").isCloseTo(168.3, ERROR);
 
         MoonPosition mp2 = MoonPosition.compute()
                         .on(2017, 2, 7, 23, 4, 0)
@@ -105,6 +112,7 @@ public void testPuertoWilliams() {
         assertThat(mp2.getAzimuth()).as("azimuth").isCloseTo(0.1, ERROR);
         assertThat(mp2.getAltitude()).as("altitude").isCloseTo(16.3, ERROR);
         assertThat(mp2.getDistance()).as("distance").isCloseTo(369731.0, DISTANCE_ERROR);
+        assertThat(mp2.getParallacticAngle()).as("pa").isCloseTo(-179.9, ERROR);
     }
 
     @Test
@@ -116,6 +124,7 @@ public void testSingapore() {
                         .execute();
         assertThat(mp1.getAzimuth()).as("azimuth").isCloseTo(240.6, ERROR);
         assertThat(mp1.getAltitude()).as("altitude").isCloseTo(57.1, ERROR);
+        assertThat(mp1.getParallacticAngle()).as("pa").isCloseTo(64.0, ERROR);
 
         MoonPosition mp2 = MoonPosition.compute()
                         .on(2017, 7, 12, 3, 11, 0)
@@ -125,6 +134,7 @@ public void testSingapore() {
         assertThat(mp2.getAzimuth()).as("azimuth").isCloseTo(180.0, ERROR);
         assertThat(mp2.getAltitude()).as("altitude").isCloseTo(74.1, ERROR);
         assertThat(mp2.getDistance()).as("distance").isCloseTo(395621.0, DISTANCE_ERROR);
+        assertThat(mp2.getParallacticAngle()).as("pa").isCloseTo(0.0, ERROR);
     }
 
 }