Which sentence is capitalized correctly?

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.