The constellation The Pleiades
This group of stars, also called "The Seven Sisters", is visible from November to mid-March. In ancient times this constellation was considered to be an eye test. Each of the 7 stars visible with the naked eye has a slightly different brightness, so if you could see all seven stars you were said to have "perfect" vision. If you could see 6, your vision wasn't quite perfect. If you could see 5 then your vision was slightly worse than "not quite perfect, and so on.
These days, that test doesn't work as well because of all the light pollution around where most of us live. If, however, you're out somewhere away from our light-polluted skies, you can test your vision just like those in ancient times did!