Which sentence is capitalized correctly?

On a cloudless night, I studied pisces, a constellation in the north sky.

on a Cloudless Night, i studied Pisces, a constellation in the North Sky.

On a cloudless night, I studied Pisces, a constellation in the North sky.

on a cloudless night, I studied pisces, a constellation in the north sky.

Answer : 

On a cloudless night, I studied Pisces, a constellation in the North sky.

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