`assert_array_almost_equal` doesn't work on Quantities, because for arrays *a* and *b*, it forcibly downcasts the *b* to a raw array.  When it subtracts *b* from *a* to get a difference, `Quantity` complains, for obvious reasons.

On second thought, I probably should do `c.ra.value`, `c2.ra.value` here, as long as we pass `assert_array_almost_equal` raw arrays, we're fine.