If there is no landscape to interfere, rainbows are actually circular.

The brightest rainbows appear when the water drops in the sky are large, so if a big rainstorm is moving off in front of you, the rainbows will be more spectacular. The lower the sun is to the horizon the more arc you will see. And, if you could float upwards, the higher you got the more circular the rainbow would become, especially if the sun is directly behind you. Circular rainbows are seen all the time—not by floating people but by passengers flying in airplanes.