Daniel assumes pirates wore eye patches because they lost an eye.

That is probably part of the answer. But there is a more interesting theory.

When you move from bright light into darkness, your eyes need time to adjust. Full dark adaptation can take up to half an hour. Part of what happens involves rod cells at the back of the eye becoming more sensitive to dim light -- a process that takes time because it was undone by the bright light you were just in.

The theory is that some sailors kept one eye covered on deck so it stayed dark-adapted. Then when they went below -- into a dark hold or a gun deck -- they switched the patch to the other eye and immediately had useful night vision. No stumbling around waiting for their eyes to adjust.

The biology behind this is real. Keeping one eye away from bright light does help prese ...