Seen from the sky

A rainbow is really a circle

Go high enough that the ground is no longer in the way, and you can see a rainbow that continues all the way around into a full circle. People sometimes see one from an airplane window or a mountaintop. The center of the circle is always the shadow point. From a plane, you may even see the plane’s shadow sitting right there.

Why?A rainbow is round because of the cone. Look along a cone from its tip and its rim is a circle. On the ground, the ground simply hides the bottom of that circle.

Learn moreEven in a backyard, if you spray a hose with the Sun at your back and the spray spreads wide in front of and below you, you can see a rainbow that is nearly a full circle.

Width of a full-circle rainbow (angle)

84°

calculated from the refractive index of water (IAPWS, 20 °C) and the laws of refraction and reflection