A rainbow is actually circular

The reason is that what is seen as a rainbow is actually light reflected by individual, spherical water droplets. Although each droplet reflects white light, it divides the white light into a spectrum, and the color perceived varies with the angle from which the droplet is viewed, so all droplets viewed from a specific angle are of the same color. The locus of droplets at the same angle relative to one's eye and the sun is a circle; since the Earth obscures the bottom part of the circle, the rainbow must always curve in a certain direction.