@mdboom - I tried `self.__class__._validate_angles(value)` first but I got an error stating that I should pass on a Latitude class object. However, looking at it with fresh eyes, I see that this was because `_validate_angles` does not return anything...  If I add `return self` at the end, it works.

Before I just go ahead, an alternative would be to give `_validate_angles` an optional argument of *which* angles to check (which would default to self). Would you prefer that approach? It does mean that if we ever decide to allow different ranges (e.g., 0 to 180), then things would more easily continue to work.