Update math for higher orders as well - cancels out for constant bin …

…width

specutils/analysis/moment.py

@@ -64,25 +64,25 @@ def _compute_moment(spectrum, regions=None, order=0, axis=-1):
     if order is None or order < 0:
         return None
 
+    dx = np.abs(np.diff(spectral_axis.bin_edges))
+    m0 = np.sum(flux * dx, axis=axis)
     if order == 0:
-        dx = np.abs(np.diff(spectral_axis.bin_edges))
-        return np.sum(flux * dx, axis=axis)
+        return m0
 
     dispersion = spectral_axis
     if len(flux.shape) > len(spectral_axis.shape):
         _shape = flux.shape[:-1] + (1,)
         dispersion = np.tile(spectral_axis, _shape)
 
     if order == 1:
-        return np.sum(flux * dispersion, axis=axis) / np.sum(flux, axis=axis)
+        return np.sum(flux * dispersion * dx, axis=axis) / m0
 
     if order > 1:
-        m0 = np.sum(flux, axis=axis)
 
         # By setting keepdims to True, the axes which are reduced are
         # left in the result as dimensions with size one. This means
         # that we can broadcast m1 correctly against dispersion.
-        m1 = (np.sum(flux * spectral_axis, axis=axis, keepdims=True)
-              / np.sum(flux, axis=axis, keepdims=True))
+        m1 = (np.sum(flux * dispersion * dx, axis=axis, keepdims=True)
+              / np.sum(flux * dx, axis=axis, keepdims=True))
 
-        return np.sum(flux * (dispersion - m1) ** order, axis=axis) / m0
+        return np.sum(flux * dx * (dispersion - m1) ** order, axis=axis) / m0

specutils/tests/test_analysis.py

@@ -1164,8 +1164,8 @@ def test_moment_cube_order_2():
     assert moment_2.shape == (10, 10000)
     assert moment_2.unit.is_equivalent(u.GHz**2)
     # check assorted values.
-    assert quantity_allclose(moment_2[0][0], 2.019e-28*u.GHz**2, rtol=0.01)
-    assert quantity_allclose(moment_2[1][0], 2.019e-28*u.GHz**2, rtol=0.01)
+    assert quantity_allclose(moment_2[0][0], 8.078e-28*u.GHz**2, rtol=0.01)
+    assert quantity_allclose(moment_2[1][0], 8.078e-28*u.GHz**2, rtol=0.01)
     assert quantity_allclose(moment_2[0][3], 2.019e-28*u.GHz**2, rtol=0.01)