Dear Tom,
I have to ask: Why does the moon sometimes look so big?
Sarah Newman
Dear Sarah,
Astronomer Dan Joyce of the Cernan Earth and Space Center at Triton College tells us the moon’s diameter is always 0.52 degrees of arc, no matter where the moon is positioned in the sky. Telescopic measurements confirm it, and that’s the moon’s actual size.
The moon’s apparent size, however, is a different matter: The moon looks larger when it is low in the sky and near the horizon, but that larger-appearing “horizon moon” is entirely an illusion. It’s such a compelling illusion, though, that it has a name: the “moon illusion.”
It is believed the illusion results from the varying ways our minds relate the size and distance of the moon when it is viewed by itself (overhead) or against a backdrop of other objects (at the horizon).
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
