system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I know how a rainbow is formed...But what determines the size of the arch or 'circumference' of it?
gpt
The main rainbow appears 42 degrees away from the point opposite the sun, i.e., 138 degrees away from the sun.  (The angle is determined by the way light from the sun is reflected and refracted inside water drops.)  The nearer the sun is to the horizon, the higher the arch will appear.