diff --git a/public/py/ephemeriden_api.py b/public/py/ephemeriden_api.py
index a09acc6..85aa982 100644
--- a/public/py/ephemeriden_api.py
+++ b/public/py/ephemeriden_api.py
@@ -145,7 +145,7 @@ def skyfield_planet_state(body_name: str, observer, dt_utc: datetime, timescale=
         timescale, planets = skyfield_context()
     time_value = timescale.from_datetime(dt_utc.astimezone(timezone.utc))
     apparent = observer.at(time_value).observe(planets[SKYFIELD_EPHEMERIS_TARGETS[body_name]]).apparent()
-    ra, dec, _distance = apparent.radec(epoch=timescale.J2000)
+    ra, dec, _distance = apparent.radec()
     altitude, azimuth, _distance = apparent.altaz("standard")
     return float(ra.hours), float(dec.degrees), float(azimuth.degrees), float(altitude.degrees)
 
diff --git a/public/py/test_ephemeriden_api.py b/public/py/test_ephemeriden_api.py
new file mode 100644
index 0000000..20b017c
--- /dev/null
+++ b/public/py/test_ephemeriden_api.py
@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+"""Regressionstest fuer den Bezugssystem-Fehler in skyfield_planet_state().
+
+apparent.radec() muss ohne epoch-Argument aufgerufen werden, damit die
+Ausgabe echte ICRF/J2000-Koordinaten liefert. Ein wieder eingefuehrtes
+epoch=timescale.J2000 wuerde die Nutation von J2000.0 (~14,5" bei diesem
+Merkur-Test) mit einrechnen und diesen Test zum Scheitern bringen.
+
+Aufruf: python test_ephemeriden_api.py
+"""
+import math
+import os
+import sys
+import unittest
+from datetime import datetime, timezone
+
+SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
+if SCRIPT_DIR not in sys.path:
+    sys.path.insert(0, SCRIPT_DIR)
+
+import ephemeriden_api as api
+from astronomical_conversions import skyfield_wgs84
+
+# Sternwarte Sonneberg
+TEST_LATITUDE = 50.3773963
+TEST_LONGITUDE = 11.1898541
+TEST_ELEVATION_M = 640.0
+TEST_INSTANT_UTC = datetime(2026, 6, 15, 12, 0, 0, tzinfo=timezone.utc)
+
+# Erwartete topozentrische, scheinbare ICRF-Koordinaten (radec() ohne epoch=),
+# berechnet mit DE421 fuer obigen Ort/Zeitpunkt. Geocentric-astrometrisches
+# Gegenstueck stimmt mit einer unabhaengigen VSOP87-Rechnung (PyEphem) auf
+# rund 0,3" ueberein.
+EXPECTED_RA_HOURS = 7.348643358806561
+EXPECTED_DEC_DEG = 23.20555540423118
+TOLERANCE_ARCSEC = 1.0
+
+
+def angular_separation_arcsec(ra1_hours: float, dec1_deg: float, ra2_hours: float, dec2_deg: float) -> float:
+    ra1 = math.radians(ra1_hours * 15.0)
+    ra2 = math.radians(ra2_hours * 15.0)
+    dec1 = math.radians(dec1_deg)
+    dec2 = math.radians(dec2_deg)
+    cos_sep = (
+        math.sin(dec1) * math.sin(dec2)
+        + math.cos(dec1) * math.cos(dec2) * math.cos(ra1 - ra2)
+    )
+    cos_sep = max(-1.0, min(1.0, cos_sep))
+    return math.degrees(math.acos(cos_sep)) * 3600.0
+
+
+class SkyfieldReferenceFrameTest(unittest.TestCase):
+    def test_mercury_position_matches_icrf_reference(self):
+        timescale, planets = api.skyfield_context()
+        observer = planets["earth"] + skyfield_wgs84.latlon(
+            TEST_LATITUDE, TEST_LONGITUDE, elevation_m=TEST_ELEVATION_M
+        )
+
+        ra_hours, dec_deg, _azimuth_deg, _altitude_deg = api.skyfield_planet_state(
+            "Mercury", observer, TEST_INSTANT_UTC, timescale, planets
+        )
+
+        separation_arcsec = angular_separation_arcsec(
+            ra_hours, dec_deg, EXPECTED_RA_HOURS, EXPECTED_DEC_DEG
+        )
+        self.assertLess(
+            separation_arcsec,
+            TOLERANCE_ARCSEC,
+            f"Position weicht um {separation_arcsec:.3f}\" vom Sollwert ab "
+            f"(Toleranz {TOLERANCE_ARCSEC}\"). Wurde epoch= bei radec() wieder "
+            "eingefuehrt?",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()