Andromeda’s collective glow reaches the naked eye, but an Earth-Sun analogue there would combine a ten-billion-to-one contrast with a 1.3-microarcsecond separation.
Andromeda can be found without a telescope under a sufficiently dark sky, even though its light has travelled about 2.5 million years. That sounds like an argument for astronomical eyesight. It is re… [+10850 chars]