According to one report it is not because they have a damaged eye like Lord Nelson.

When you cover one eye with a patch the pupil opens up which means if you are in a dark room and take the patch off, that eye is immediately accustomed to the dark and you can see really well.

The human eye can adapt rapidly from low light or night conditions to daylight conditions, but it takes as much as 25 minutes for the eye to adjust from daylight conditions to low light levels resembling night.

So When pirates "entered" ships it meant when they went below deck they just lifted the patch and could see the insides of the ship.

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The answer is actually not very difficult but wikipedia adds some fun to the answer.The anser is an "Erg". The erg is a unit of energy equal to 10−7 joules (100 nJ). Its name is derived from ergon (ἔργον), a Greek word meaning 'work' or 'task'.But how big is an Erg really?An erg is about the amount of work done (or energy consumed) by one common housefly performing a "press-up", the leg-bending dip that brings its mouth to the surface on which it stands and back up.

The word tarpaulin originated as a compound of the words tar and palling, referring to a tarred canvas pall used to cover objects on ships. Sailors often tarred their own overclothes in the same manner as the sheets or palls. By association, sailors became known as "jack tars".

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There is nothing like a little trivia to brighten up ones day. Useless information can be priceless.

Everybody - well almost - everybody knows the Pink Panther and many have watched the cartoon series. The original Film from 1963 is about a diamond with a flaw that is supposed to look like a leaping panther. The first cartoon was in 1964 called the Pink Phink.