diff --git a/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/predict/CelestialComputerTest.kt b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/predict/CelestialComputerTest.kt
new file mode 100644
index 00000000..aeb944ea
--- /dev/null
+++ b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/predict/CelestialComputerTest.kt
@@ -0,0 +1,55 @@
+package com.rtbishop.look4sat.core.domain.predict
+
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * The Moon's Greenwich Hour Angle must be a reduced angle: callers turn it into a
+ * sub-lunar longitude with `if (gha <= 180) -gha else 360 - gha`, which only
+ * yields a valid longitude when gha is within 0..360.
+ *
+ * Regression: the GMST polynomial feeding the hour angle is never reduced (about
+ * 3.5e6 degrees at present), so the old `if (gha < 0) gha += 360` guard never
+ * fired and every sub-lunar longitude landed millions of degrees off the map.
+ */
+class CelestialComputerTest {
+
+    private val observer = GeoPos(22.314066, 108.706575)
+
+    private fun subLunarLongitude(gha: Double) = if (gha <= 180.0) -gha else 360.0 - gha
+
+    @Test
+    fun `moon hour angle stays within one revolution`() {
+        // Sample a full synodic month at 37-minute steps so the sweep crosses
+        // every hour-angle quadrant many times over.
+        var timeMillis = 1786060800_000L // 2026-08-07T00:00:00Z
+        val stepMillis = 37 * 60 * 1000L
+        val endMillis = timeMillis + 30L * 86400_000L
+        var samples = 0
+        while (timeMillis < endMillis) {
+            val gha = CelestialComputer.getMoonPosition(observer, timeMillis).gha
+            assertTrue("gha=$gha out of 0..360 at $timeMillis", gha >= 0.0 && gha < 360.0)
+            val longitude = subLunarLongitude(gha)
+            assertTrue(
+                "sub-lunar longitude=$longitude out of -180..180 at $timeMillis",
+                longitude >= -180.0 && longitude <= 180.0
+            )
+            samples++
+            timeMillis += stepMillis
+        }
+        assertTrue("expected a meaningful sweep, got $samples samples", samples > 1000)
+    }
+
+    @Test
+    fun `moon hour angle advances westward over an hour`() {
+        // The Moon's hour angle grows by roughly 14.5 degrees per hour, so an
+        // hour apart the two readings must differ but stay reduced.
+        val base = 1786060800_000L
+        val first = CelestialComputer.getMoonPosition(observer, base).gha
+        val later = CelestialComputer.getMoonPosition(observer, base + 3_600_000L).gha
+        assertTrue("first=$first", first >= 0.0 && first < 360.0)
+        assertTrue("later=$later", later >= 0.0 && later < 360.0)
+        val delta = ((later - first) % 360.0 + 360.0) % 360.0
+        assertTrue("hourly advance was $delta degrees", delta in 10.0..20.0)
+    }
+}