Merge pull request #1583 from valdaarhun/ephem_epochs_calculation

Bug fix: Fixes #1563

src/poliastro/twobody/sampling.py

@@ -147,7 +147,7 @@ def sample(self, orbit):
         delta_ts = (
             np.unwrap(delta_ts, period=orbit.period.to_value(u.s)) << u.s
         )
-        epochs = orbit.epoch + (orbit.t_p + delta_ts)
+        epochs = orbit.epoch + (delta_ts - orbit.t_p)
 
         n = nu_values.shape[0]
         rr, vv = coe2rv_many(

tests/tests_twobody/test_orbit.py

@@ -1130,6 +1130,23 @@ def test_from_ephem_has_expected_properties():
     assert_quantity_allclose(ss.v, expected_v)
 
 
+def test_to_ephem_samples_correct_epochs_and_coordinates():
+    epoch = Time("2022-2-3 1:42:0")
+    orbit = Orbit.from_vectors(
+        Earth,
+        r=[6045.0, 3490.0, -2500.0] * u.km,
+        v=[-3.457, 6.618, 2.533] * u.km / u.s,
+        epoch=epoch,
+    )
+
+    ephem = orbit.to_ephem()
+    assert ephem.epochs[0].value == orbit.epoch
+
+    ephem_initial_coordinates = [coord[0] for coord in ephem.sample(epoch).xyz]
+
+    assert_quantity_allclose(ephem_initial_coordinates, orbit.r)
+
+
 def test_from_vectors_wrong_dimensions_fails():
     bad_r = [[1000, 0, 0]] * u.km
     bad_v = [[[0, 10, 0]]] * u.km / u.s