The first mention in the Bible of a Rainbow is in Genesis 9:13-17.
            "I do set My Bow in the cloud, and it shall be for a token of a Covenant between Me and the Earth."
                   
                      A Covenant that God would not destroy this earth again by a Flood. It was only semi-circular, like the ones after a rain.

      The Rainbow Ezekiel and John saw around the Throne of God was circular.