Dear Tom,
Why do some full moons seem to be so huge and seem close enough to reach out and touch, while others, full and bright also, are small and high in the sky? The Aug. 28 full moon, just over the trees, was gigantic.
Doris Bowery, Hazel Crest
Dear Doris,
The phenomenon of the full moon appearing larger when it is near the horizon than when it is higher in the sky is such a pervasive optical illusion that it has a name: the “moon illusion.”
Penn State professor Craig Bohren, an expert in atmospheric optics, says it’s the oldest known optical illusion, dating back at least 1,000 years, possibly 2,000. It is a genuine optical illusion because the angle subtended by the moon’s diameter is always 0.52 degrees of arc, no matter where the moon is in the sky.
Your question is appropriate because the next full moon occurs this Wednesday (Sept. 26).
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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.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
