Summer is a good time to spot giant moons very near the horizon. It's the season for the mysterious moon illusion.

 

 

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June 24, 2002: Sometimes you just can't believe your own eyes.

 

Tonight is one of those times. Go outside around 9:00 p.m. and look east. Rising above the horizon you might see a giant moon. Bright, very likely pink or orange, and swollen. The moon is real, but its surprising diameter is an illusion--the "moon illusion."

The moon illusion is a well-known trick of the eye: a low-hanging moon looks unusually big. The moon is really the same size (0.5 degrees wide) no matter where it appears in the sky--photographs prove it--but the human eye and brain tell us differently.

Above: A time-lapse sequence of the moon rising over Seattle. To the camera, the moon appears to be the same size no matter what its location on the sky. Credit and copyright: Shay Stephens.

 

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Summer is a good time to experience the moon illusion because the full moon never gets very far above the horizon. (The Sun and the full moon are on opposite sides of the sky. So when the Sun rises high--as it does during summer--the full moon hangs low.) Tonight's full moon, the first of northern summer, will hug the horizon long after moonrise.

Low-hanging moons often glow pink or orange. That's no illusion. Dust, smoke and pollution scatter moonlight and make the moon itself look colorful for the same reason that sunsets often glow vivid shades of red. Smoke and ash from summertime wildfires in North America will add an extra dash of color to the moon this week.

When you look at the Moon, rays of moonlight converge and form an image about 0.15 mm wide in the back of your eye. High moons and low moons make the same sized spot. Why, then, does the brain report a larger moon near the horizon? Scientists aren't sure.

 

The best-known explanation is the "Ponzo illusion." In 1913 Mario Ponzo drew two identical bars across a pair of converging lines--like the railroad tracks pictured here. The upper yellow bar appears much larger because it spans a greater apparent distance between the rails. In fact, the two bars are the same width.

Left: The Ponzo illusion. Image credit: Tony Phillips.

When the Moon is rising or setting, distant trees and houses might play the role of Ponzo's converging tracks--they make the moon seem bigger than it is. If so, the moon illusion is simply Ponzo's illusion turned upside down, with the wide bar at the bottom and the narrow bar at the top.

But there's a problem. Carl Wenning, a physics professor at Illinois State University, notes that airline pilots flying at very high altitudes sometimes experience the moon illusion without any objects in the foreground. What tricks their eyes?

Wenning and others believe that the sky itself might be the answer. Humans perceive the sky as a flattened dome. The zenith seems nearby and the horizon far away. (Birds and airplanes reinforce this notion. Birds flying overhead are closer than birds on the horizon.) When the moon is low, we judge it to be far away. Something far away must be really big to span half-a-degree across the sky, so our brain inflates the moon accordingly.

There are other explanations, too. It hardly matters to sky watchers which is correct, though. The moon illusion is real and extraordinary.

 

Moonrise over Selected US Cities

City Time Zone June 24 June 25 June 26 New York, NY    EDT   8:41 p.m.   9:37 p.m.   10:24 p.m. San Diego, CA    PDT   8:14 p.m.   9:10 p.m.   9:58 p.m. Washington, DC    EDT   8:47 p.m.   9:43 p.m.   10:31 p.m. Honolulu, HI   HDT   8:35 p.m.   9:31 p.m.   10:21 p.m. Chicago, IL   CDT   8:42 p.m.   9:38 p.m.   10:24 p.m. Houston, TX   CDT   8:35 p.m.   9:32 p.m.   10:22 p.m. Denver, CO   MDT   8:46 p.m.   9:42 p.m.   10:29 p.m. Miami, FL    EDT   8:22 p.m.   9:19 p.m.   10:10 p.m. Seattle, WA   PDT   9:32 p.m.   10:26 p.m.   11:09 p.m. If your city does not appear in the list, click here for more data from the US Naval Observatory.

Step outside tonight (June 24th) and see for yourself. The best time would be when the moon is very close to the horizon--between 8:30 and 9:00 p.m. for most cities in the United States. Try looking from a spot where the eastern horizon is reasonably unobstructed; trees and buildings can get in the way of the rising moon. On the other hand, trees and buildings might also help trigger the illusion, so a bare horizon is not necessarily best.

A fun activity: Look at the moon directly and then through an aperture of some sort. For example, 'pinch' the moon between your thumb and forefinger or view it through a tube, which hides the foreground terrain. Can you make the optical illusion vanish?

If you do make it disappear, you'll soon want it back. The moon illusion is too lovely to miss.

 

 

Editor's Note: If you miss the moon illusion on June 24th, try again on June 25th or 26th -- two nights with nearly-full moons. Or wait until July 23rd when the second full moon of summer arrives.

more information

 

The Moon Illusion: An Interactive Demonstration -- from sandlotscience.com

The Inconstant Moon -- an essential reference for moon watchers.

New Thoughts on Understanding the Moon Illusion -- by Carl J. Wenning, Physics Department, Illinois State University

Experiment in Perception -- (UnMuseum) more about the Ponzo Illusion and the Moon

Oculomotor micropsia and the moon illusion -- Prof. Don McCready, Psychology Department University of Wisconsin-Whitewater, explains how oculomotor micropsia might trigger the moon illusion.

Father-Son Scientists Confirm Why Horizon Moon Appears Larger -- IBM Research News

Moonrise over Seattle -- (APOD) Is the Moon larger when near the horizon? No -- as shown above, the Moon appears to be very nearly the same size no matter its location on the sky.

 

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