Add data test reference

Jorge · Jorge · commit cb10940dce02 · 2019-04-11T16:44:49.000+02:00
diff --git a/src/poliastro/tests/tests_twobody/test_propagation.py b/src/poliastro/tests/tests_twobody/test_propagation.py
@@ -56,6 +56,8 @@ def test_propagation(method):
 
 
 def test_gauss():
+    # These data was taken from problem 5-30 in Battin 1999:
+    # "An introduction to the Mathematics and Methods of Astrodynamics"
     r0 = [0.159321004, 0.579266185, 0.052359607] * u.AU
     v0 = [-9.303603251, 3.018641330, 1.536362143] * u.AU / u.year
     expected_r = [0.057594337, 0.605750797, 0.068345246] * u.AU
