Reminds me of the Big Dipper eye test. If you can spot the double star in Ursa Major constellation you’re supposed to have 20/20 vision. The two stars are Mizar (magnitude +2) and Alcor (magnitude +4). In urban environments this is a good combined test ;-)
On a personal note (which I hope many will relate), I’ll never forget the very first time I looked up at the night sky after getting my first pair of glasses. The sheer number of extra stars I could see combined with the clarity, made me fall in love with stargazing :-)
I read somewhere that in Roman times the test to become a soldier was to look at the Pleiades cluster and if you could see seven stars you would pass the eye test.
Of what use is a vision test where it's easy to memorize the answer? In English Pleiades is also known as the Seven Sisters, because of the ancient Greek myth. I can easily believe that Pleiades was also associated with 7 in general Roman culture.
It doesn't help that more than 7 stars can be visible. (https://en.wikipedia.org/wiki/Pleiades says "Its light is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions.") Muhammad was written to have been able to see 12.
If someone says there are 9 stars, is that a sign of good vision? Or simply a guess?
In modern times only 6 stars are usually visible to the naked eye, and there are proposals for why 7 was used in Ancient Greece; eg, stellar variability or a numerological leaning towards 7.
On a personal note (which I hope many will relate), I’ll never forget the very first time I looked up at the night sky after getting my first pair of glasses. The sheer number of extra stars I could see combined with the clarity, made me fall in love with stargazing :-)