Dear Tom,
Your column about rainbows sparked a lively argument with a friend. He says rainbows are shaped like arches like the Gateway Arch in St. Louis. I say they are circular.
— Susan Taylor
Dear Susan,
The arc of a rainbow is the same for all rainbows, and the arc is always circular. Triton College astronomer Dan Joyce explains, “Most of the time, because we see only a fraction of the full rainbow, it is tempting to perceive what we see as a parabola or some other non-spherical conic section, but it’s a circle.” Because a rainbow always appears at an angle of 138 degrees from the sun, it is seen on the opposite side of the sky from the sun. The best view of a rainbow’s spectacular color display is afforded not from the ground, however, but from high-flying aircraft. Occupants of jet planes can sometimes see a rainbow’s full circular sweep from top to bottom.
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