Fix #928: check .center when subtracting positions

CHANGELOG.rst

@@ -15,6 +15,11 @@ Next version
 * For convenience, geoids like :data:`~skyfield.toposlib.wgs84` have a
   new attribute :data:`~skyfield.toposlib.Geoid.polar_radius`.
 
+* You can no longer subtract two positions unless they have the same
+  ``.center``.  Otherwise, a ``ValueError`` is raised.  This check has
+  always been performed when you subtract vector functions, but it was
+  missing from the position subtraction routine.
+
 v1.47 — 2024 January 13
 -----------------------
 

skyfield/positionlib.py

@@ -168,6 +168,11 @@ def __repr__(self):
 
     def __sub__(self, body):
         """Subtract two ICRF vectors to produce a third."""
+        if self.center != body.center:
+            raise ValueError(
+                "you can only subtract two vectors"
+                " if they both start at the same center"
+            )
         p = self.position.au - body.position.au
         if self.velocity is None or body.velocity is None:
             v = None

skyfield/tests/test_positions.py

@@ -20,6 +20,10 @@ def test_subtraction():
     assert tuple(p.position.au) == (9, 18, 27)
     assert tuple(p.velocity.au_per_d) == (36, 45, 54)
 
+    p1.center = 1
+    with assert_raises(ValueError):
+        p0 - p1
+
 def test_separation_from_on_scalar():
     p0 = ICRF((1, 0, 0))
     p1 = ICRF((0, 1, 0))