Fix: add frame tie to mean-equator-equinox frame

A question in #1048 led me to notice this oversight.

Author: brandon-rhodes

CHANGELOG.rst

@@ -18,6 +18,10 @@ Unreleased future version
   would loop forever.
   `#1046 <https://github.com/skyfielders/python-skyfield/issues/1046>`_
 
+* Fix: the new :class:`~skyfield.framelib.mean_equator_and_equinox_of_date`
+  reference frame was off by nearly 0.02 arcseconds because it applied
+  precession but forgot to apply the frame tie between ICRS and J2000.
+
 v1.51 — 2025 February 23
 ------------------------
 

skyfield/framelib.py

@@ -60,7 +60,7 @@ class mean_equator_and_equinox_of_date(object):
     """
     @staticmethod
     def rotation_at(t):
-        return t.P
+        return mxm(t.P, ICRS_to_J2000)
 
 class true_equator_and_equinox_of_date(object):
     """The dynamical frame of Earth’s true equator and true equinox of date.