did you know a rainbow is really a circle?

a primary rainbow lies around a circle centred opposite the Sun from your viewpoint. we usually see an arc because the ground blocks the lower part. from a high viewpoint, a full circle can sometimes appear.

sunlight bends into a raindrop, reflects inside, and bends again on leaving. different wavelengths emerge most strongly at slightly different angles. the red edge of the primary bow is around 42 degrees from the antisolar direction.

all viewing directions at a fixed angle from that axis form a cone. droplets along those directions send the right light into your eyes. the circular arrangement comes from angles, not from a physical hoop hanging in the rain.

move your head and the relevant droplets change. another observer sees their own bow, centred on their own viewpoint.

it is the angular radius of the red edge, measured from the direction exactly opposite the Sun. it is not the angle above the horizon, and the bow's position changes as the Sun moves.

viewing angle from antisolar direction ≈ 42° for primary red light

• the secondary bow involves two internal reflections. it appears outside the primary and has its colours in the opposite order.
• a higher Sun pushes more of the primary circle below the horizon. suitable water droplets and a clear sunlit path still have to be present.