Dear Tom,
On a jet plane trip, I looked out the window and saw a full-circle rainbow!
Rosemary Schwartel, Chicago

Dear Rosemary,
In theory, every rainbow is actually a full circle. The central point of a rainbow is opposite the sun’s position in the sky. More of the rainbow’s circle becomes visible as the sun approaches the horizon, and the largest portion of a rainbow’s circle can be seen at sunrise or sunset. When the sun is on the horizon, you can view 50 percent of the rainbow. The lower half of the rainbow will lie below your horizon and you cannot see it. But as you go higher, such as when you’re in an airplane or a very high building, a rainbow’s full circle can be seen. It’s also possible to produce a full-circle rainbow by spraying water from a garden hose when facing away from the sun.