Fix #161 by reordering names in vector subtraction

CHANGELOG.rst

@@ -3,6 +3,9 @@ Changelog
 
 .. currentmodule:: skyfield.positionlib
 
+* Issues fixed:
+  `#161 <https://github.com/skyfielders/python-skyfield/issues/161>`_
+
 1.1 — 2018 January 14
 ---------------------
 

skyfield/jpllib.py

@@ -188,6 +188,8 @@ def __init__(self, ephemeris, spk_segment):
         self.ephemeris = ephemeris
         self.center = spk_segment.center
         self.target = spk_segment.target
+        # self.center_name = _format_code_and_name(self.center)
+        # self.target_name = _format_code_and_name(self.target)
         self.spk_segment = spk_segment
 
     def __str__(self):

skyfield/tests/test_vectors.py

@@ -23,7 +23,6 @@ def test_vectors():
 
     planets = load('de421.bsp')
     earth = planets['earth']
-    moon = planets['moon']
     mars = planets['mars']
 
     v = earth
@@ -33,6 +32,9 @@ def test_vectors():
  + Segment 'de421.bsp' 0 SOLAR SYSTEM BARYCENTER -> 3 EARTH BARYCENTER
  + Segment 'de421.bsp' 3 EARTH BARYCENTER -> 399 EARTH"""
 
+    assert repr(v) == "\
+<VectorSum of 2 vectors 0 SOLAR SYSTEM BARYCENTER -> 399 EARTH>"
+
     assert str(v.at(t)) == "\
 <Barycentric position and velocity at date t center=0 target=399>"
 
@@ -45,5 +47,8 @@ def test_vectors():
  + Segment 'de421.bsp' 0 SOLAR SYSTEM BARYCENTER -> 3 EARTH BARYCENTER
  + Segment 'de421.bsp' 3 EARTH BARYCENTER -> 399 EARTH"""
 
+    assert repr(v) == "\
+<VectorSum of 4 vectors 499 MARS -> 399 EARTH>"
+
     assert str(v.at(t)) == "\
 <Geometric position and velocity at date t center=499 target=399>"

skyfield/vectorlib.py

@@ -47,7 +47,7 @@ def __sub__(self, other):
         othern = getattr(other, 'negatives', ())
 
         return VectorSum(other.target, self.target,
-                         self.target_name, other.target_name,
+                         other.target_name, self.target_name,
                          selfp + othern, selfn + otherp)
 
     @raise_error_for_deprecated_time_arguments