Question

why pirates cover their one eye

Answer 1

Some assume pirates wore eye patches to cover a missing eye or an eye that was wounded in battle, but in fact, an eye patch was more likely to be used to condition the eye so the pirate could fight in the dark.

 

It takes an average human eye about 25 minutes to fully adapt from bright sunlight to seeing in complete darkness—if a pirate was fighting on deck in the sunlight, then had to continue the fight under the deck where it is usually pretty dark, it could take too long for their eyes to adjust and for the pirate to be able to see.  The eye patch could be used to prepare one eye to see in the dark, so when they would go below deck they could swap the eye patch from one eye to the other and see with the eye that has already adjusted to low light conditions. This would allow them to instantly see in the dark.

 

Mythbusters confirmed that this use of eye patches among pirates was plausible, but there is no recorded historical precedent for this fact. So for now, keep on wearing that pirate-y eye patch—you're actually looking like one smart pirate when you do!