Spin ambiguity
Raoul, you might know the answer to this. Is there any way to tell from a photograph of an oblique spiral galaxy (one whose axis of rotation is neither perpendicular nor parallel to the line of sight) whether we are looking at the top or bottom? We can define "top" using the Right Hand Rule, the direction the orthogonal spin vector (thumb) points when the finger tips point in the spin direction (i.e., the spiral arms trail back in the opposite direction, away from the fingertips).