It's called a "clear view screen" and it's sort of a high-efficiency windshield wiper (it spins at high speed) ensuring a clean view for the captain or pilot regardless of how much spray or ice or rain happens. Ships generally also have regular windscreen wipers on the other windows.
You are misinformed, some windows are round, eg the rose window in Notre-Dame Cathedral or ships portholes.
The round windows in staterooms or cabins on board ships are called port holes. These round windows would be kept closed when the ship is at sea, but could be opened when the ship is at anchor or docked in port.
maru means round in japanese ships
a Spiech. It is a piece of glass that spins to keep moisture and water from building up on it.
Portholes are small round windows in the huls of ships that let in light and air. Castles are defensive structures common in the middle ages. Both can and will exist without you.
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By the ships disappearing below the horizon
Ships can circle the world by water and end in the spot where they began, proving the Earth must be round to the contrary belief that it was flat.
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A porthole (generally the round ones)
by shortening the sailing time from Europe to Asia, ships do not have to sail round Africa
It was a way of getting round for them as they did not have aeroplanes cars ect...