Our eyesight is an important part of our health. So, why did pirates wear eyepatches? 

I have come across a couple of explanations for sailors, and not just pirates, to wear eye patches. 

Using the sextant: As stated in another article here, sailors use sextants, time pieces and navigational charts to find their location on the ocean. When using a sextant, two objects are always sought; the horizon and the brightest light in the sky, be it day or night. 

In the daytime, that would be the sun. But staring into the sun can cause blindness…so… this is why it's important not to state at the sun. (modern sextants have built-in sunlight filters). 

The other cause: Who do you think can see better; 

or someone moving from light into darkness?  

I am told that it takes longer for our eyes to adjust to darkness when coming out of the bright light. 

I've read that mariners wore an eye patch so, they could have one ‘day-eye’ and one ‘night-eye’. 

When they needed to go down into the hold, (very dark), day or night, they would switch the patch to the other eye for night-vision. Old ships, like old lighthouses, had no electricity, and used oil lamps for light. 

It could be dangerous to take an oil lamp into the 'hold' (the belly) of a wooden ship, filled with cargo that could burn. 

About 1840, someone imagined the idea of putting prisms (photo) in the deck of the ship 

that scattered light through the hold of the ship. 

Eric J. Rose 

middlegrademysteries.com 

photo: https://insightsaboutlightandglass.com/deck-prism/