Add moon phase API

paper-api/src/main/java/io/papermc/paper/world/MoonPhase.java

@@ -0,0 +1,35 @@
+package io.papermc.paper.world;
+
+import java.util.HashMap;
+import java.util.Map;
+import org.jspecify.annotations.NullMarked;
+
+@NullMarked
+public enum MoonPhase {
+    FULL_MOON(0L),
+    WANING_GIBBOUS(1L),
+    LAST_QUARTER(2L),
+    WANING_CRESCENT(3L),
+    NEW_MOON(4L),
+    WAXING_CRESCENT(5L),
+    FIRST_QUARTER(6L),
+    WAXING_GIBBOUS(7L);
+
+    private final long day;
+
+    MoonPhase(final long day) {
+        this.day = day;
+    }
+
+    private static final Map<Long, MoonPhase> BY_DAY = new HashMap<>();
+
+    static {
+        for (final MoonPhase phase : values()) {
+            BY_DAY.put(phase.day, phase);
+        }
+    }
+
+    public static MoonPhase getPhase(final long day) {
+        return BY_DAY.get(day % 8L);
+    }
+}

paper-api/src/main/java/org/bukkit/RegionAccessor.java

@@ -445,4 +445,12 @@ public interface RegionAccessor {
      */
     @NotNull
     public <T extends Entity> T addEntity(@NotNull T entity);
+
+    // Paper start
+    /**
+     * @return the current moon phase at the current time in the world
+     */
+    @NotNull
+    io.papermc.paper.world.MoonPhase getMoonPhase();
+    // Paper end
 }