If you look at this rainbow, its angular width is about 1 degree, the angle subtended by my little finger. Also note that it is about 90 degrees to the horizon. This happens later on in the day when the Sun is nearing the horizon. A rainbow is a full circle in theory. Its centre is diagonally opposite to the Sun, following a line down from the Sun through you. The centre is thus called the anti-solar point. The lower the Sun, the closer the anti-solar point is to the horizon and the more of the rainbow is above the horizon. I suppose that in theory half of a rainbow should be above the horizon at sunset over the sea and the angle to the horizon at that point would be exactly 90 degrees.
