Fix #355 by swapping sign of subtracted velocity

CHANGELOG.rst

@@ -10,6 +10,10 @@ Changelog
   method that returns their raw ITRF coordinates.
   `#354 <https://github.com/skyfielders/python-skyfield/issues/354>`_
 
+* Fixed the sign of the velocity vector when two vectors are directly
+  geometrically subtracted.
+  `#355 <https://github.com/skyfielders/python-skyfield/issues/355>`_
+
 1.18 — 2020 March 26
 --------------------
 

skyfield/positionlib.py

@@ -99,7 +99,7 @@ def __sub__(self, body):
         if self.velocity is None or body.velocity is None:
             v = None
         else:
-            v = body.velocity.au_per_d - self.velocity.au_per_d
+            v = self.velocity.au_per_d - body.velocity.au_per_d
         return ICRF(p, v, self.t)
 
     def __getitem__(self, i):

skyfield/tests/test_positions.py

@@ -5,6 +5,13 @@
 from skyfield.positionlib import ICRF
 from skyfield.starlib import Star
 
+def test_subtraction():
+    p0 = ICRF((10,20,30), (40,50,60))
+    p1 = ICRF((1,2,3), (4,5,6))
+    p = p0 - p1
+    assert tuple(p.position.au) == (9, 18, 27)
+    assert tuple(p.velocity.au_per_d) == (36, 45, 54)
+
 def test_J2000_ecliptic_coordinates_with_and_without_a_time_array():
     p0 = ICRF((1,0,0))
     p1 = ICRF((0,1,0))