Dear Tom,
Your column about rainbows sparked a lively argument with a friend.
He says rainbows are shaped like arches like the St. Louis Arch.
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 entire full circular sweep from
top to bottom.
