--When we see
rainbows we see them as arcs when in truth they are formed as
circles. There is a point called the antisolar point that the
rainbow seems to form around. From the picture we see the
antisolar point formed as a line from the observers head by the sun
shining from behind.
--We don't see the full circle of the rainbow because the horizon gets in the way. As the sun goes down we are able to see more of the rainbow, and the higher the sun is in the sky the smaller the arc seems.
--Sometimes people at high elevations, like in planes, see rainbows as full circles because they do not have the horizon to block their view.