Do not attempt to refresh the IERS-A table if downloading is disabled

astropy/utils/iers/iers.py

@@ -883,6 +883,11 @@ def _refresh_table_as_needed(self, mjd):
           In other words the IERS-A table was created by IERS long enough
           ago that it can be considered stale for predictions.
         """
+        # If downloading is disabled, bail out silently.
+        # _check_interpolate_indices() will error later if appropriate.
+        if not conf.auto_download:
+            return
+
         # Pass in initial to np.max to allow things to work for empty mjd.
         max_input_mjd = np.max(mjd, initial=50000)
         now_mjd = self.time_now.mjd

astropy/utils/iers/tests/test_iers.py

@@ -341,12 +341,19 @@ def test_simple(self):
                 dat.ut1_utc(Time(60000, format="mjd").jd)
             assert len(warns) == 1
 
-            # Warning only if we are getting return status
+            # Warning only (i.e., no exception) if we are getting return status
             with pytest.warns(
                 iers.IERSStaleWarning, match="IERS_Auto predictive values are older"
-            ) as warns:
+            ):
                 dat.ut1_utc(Time(60000, format="mjd").jd, return_status=True)
-            assert len(warns) == 1
+
+            # Confirm that there is an interpolation error when downloading is disabled
+            with iers.conf.set_temp("auto_download", False):
+                with pytest.raises(
+                    ValueError,
+                    match="interpolating from IERS_Auto using predictive values that are more",
+                ):
+                    dat.ut1_utc(Time(60000, format="mjd").jd)
 
             # Now set auto_max_age = None which says that we don't care how old the
             # available IERS-A file is.  There should be no warnings or exceptions.

docs/changes/utils/17387.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed a bug where an old IERS-A table with stale predictive values could trigger
+the download of a new IERS-A table even if automatic downloading was disabled.