Moon To Play Tricks On Your Eyes Tonight

Moon To Play Tricks On Your Eyes Tonight

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Story Published: Jun 24, 2002 at 12:42 PM PDT

Story Updated: Aug 31, 2006 at 12:43 AM PDT

By Scott Sistek

SEATTLE - Have you ever noticed a full moon rising in summertime just after sunset that seems about twice as large as normal and thought: "I can't believe my eyes!"

Well, you're right. You can't believe your eyes.

Head outside tonight (June 24) around 9:30 p.m., or 10:30 p.m. Tuesday and take a peek at the rising full moon in the East. It'll look huge -- maybe twice as large as normal. (It might also look pink or orange, but that's from the dust and pollution particles scattering the light into different shades.)

But you know darn well the moon really isn't any larger tonight that it usually is, and it's not any closer to Earth. In fact, it's still in its same spot as always, circling the Earth at its same usual distance.

To prove it, take a photo of it. The moon will look the same size as always.

So why does it look bigger to us?

Scientists aren't 100 percent sure.

One explanation, which comes to us from science.nasa.gov, 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.

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."

Check that link above for an illustration of how that works.

But that explanation may not totally fly. Some pilots up in the sky report the large moon without horizons to trick the eye.

There are other explanations out there -- including others that believe that the sky itself might be the answer. Those explanations range from how we perceive the size of objects relative to how far away we think they are, to the atmosphere magnifying the image. Again, check the Web link for more information.

Why does it only happen in summertime? This is the best time of the year for the effect, because the sun and full moon are on exact opposite ends of the sky. So, during summer solstice, when the sun takes its highest-altitude path across the sky, the full moon takes the lowest-altitude path (roughly 20 degrees above the horizon), so it'll stay low to the ground most of the night.

If you miss it this month, you can catch it again in late July when we have our next full moon (weather permitting.)

For More Information:

science.nasa.gov

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