diff --git a/astroplan/constraints.py b/astroplan/constraints.py
index 92c536f6..1919679d 100644
--- a/astroplan/constraints.py
+++ b/astroplan/constraints.py
@@ -27,6 +27,7 @@
 from .utils import time_grid_from_range
 from .target import get_skycoord
 from .exceptions import MissingConstraintWarning
+from .observer import _make_cache_key
 
 __all__ = ["AltitudeConstraint", "AirmassConstraint", "AtNightConstraint",
            "is_observable", "is_always_observable", "time_grid_from_range",
@@ -44,47 +45,6 @@
 )
 
 
-def _make_cache_key(times, targets):
-    """
-    Make a unique key to reference this combination of ``times`` and ``targets``.
-
-    Often, we wish to store expensive calculations for a combination of
-    ``targets`` and ``times`` in a cache on an ``observer``` object. This
-    routine will provide an appropriate, hashable, key to store these
-    calculations in a dictionary.
-
-    Parameters
-    ----------
-    times : `~astropy.time.Time`
-        Array of times on which to test the constraint.
-    targets : `~astropy.coordinates.SkyCoord`
-        Target or list of targets.
-
-    Returns
-    -------
-    cache_key : tuple
-        A hashable tuple for use as a cache key
-    """
-    # make a tuple from times
-    try:
-        timekey = tuple(times.jd) + times.shape
-    except BaseException:        # must be scalar
-        timekey = (times.jd,)
-    # make hashable thing from targets coords
-    try:
-        if hasattr(targets, 'frame'):
-            # treat as a SkyCoord object. Accessing the longitude
-            # attribute of the frame data should be unique and is
-            # quicker than accessing the ra attribute.
-            targkey = tuple(targets.frame.data.lon.value.ravel()) + targets.shape
-        else:
-            # assume targets is a string.
-            targkey = (targets,)
-    except BaseException:
-        targkey = (targets.frame.data.lon,)
-    return timekey + targkey
-
-
 def _get_altaz(times, observer, targets, force_zero_pressure=False):
     """
     Calculate alt/az for ``target`` at times linearly spaced between
diff --git a/astroplan/observer.py b/astroplan/observer.py
index ef2508bc..d742b43b 100644
--- a/astroplan/observer.py
+++ b/astroplan/observer.py
@@ -19,7 +19,6 @@
 # Package
 from .exceptions import TargetNeverUpWarning, TargetAlwaysUpWarning
 from .moon import moon_illumination, moon_phase_angle
-from .observer import _make_cache_key
 from .target import get_skycoord, SunFlag, MoonFlag
 
 
@@ -28,6 +27,47 @@
 MAGIC_TIME = Time(-999, format='jd')
 
 
+def _make_cache_key(times, targets):
+    """
+    Make a unique key to reference this combination of ``times`` and ``targets``.
+
+    Often, we wish to store expensive calculations for a combination of
+    ``targets`` and ``times`` in a cache on an ``observer``` object. This
+    routine will provide an appropriate, hashable, key to store these
+    calculations in a dictionary.
+
+    Parameters
+    ----------
+    times : `~astropy.time.Time`
+        Array of times on which to test the constraint.
+    targets : `~astropy.coordinates.SkyCoord`
+        Target or list of targets.
+
+    Returns
+    -------
+    cache_key : tuple
+        A hashable tuple for use as a cache key
+    """
+    # make a tuple from times
+    try:
+        timekey = tuple(times.jd) + times.shape
+    except BaseException:        # must be scalar
+        timekey = (times.jd,)
+    # make hashable thing from targets coords
+    try:
+        if hasattr(targets, 'frame'):
+            # treat as a SkyCoord object. Accessing the longitude
+            # attribute of the frame data should be unique and is
+            # quicker than accessing the ra attribute.
+            targkey = tuple(targets.frame.data.lon.value.ravel()) + targets.shape
+        else:
+            # assume targets is a string.
+            targkey = (targets,)
+    except BaseException:
+        targkey = (targets.frame.data.lon,)
+    return timekey + targkey
+
+
 # Handle deprecated MAGIC_TIME variable
 def deprecation_wrap_module(mod, deprecated):
     """Return a wrapped object that warns about deprecated accesses"""