That's Not Dick Cheney's Head; It's Just The Moon

Watch for a "moon illusion" after sunset tonight. You know... When the moon looks twice as big as usual? Like In that movie, "Moonstruck."

On Saturday night, June 30th, step outside at sunset and look around. You'll see a giant moon rising in the east. It looks like Earth's moon with the usual craters and seas, but something's wrong. This full moon is strangely inflated. It's huge!

You've just experienced the Moon Illusion.

Sky watchers have known for thousands of years that low-hanging moons look unnaturally big. Cameras don't see it, but human eyes do; it's a genuine illusion.

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. [More]

This weekend's full moon hangs lower in the sky than any other full moon of 2007, so the Moon Illusion is going to be strong. What makes the moon so low? Consider the following: The sun and full moon lie on opposite sides of the sky. They are like a see-saw: when one is high, the other is low. Because the summer solstice was just last week (June 21st), the sun is near its highest point in northern skies. The full moon is correspondingly low.

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. So why does your brain think one is bigger than the other? After all these years, scientists still aren't sure of the answer.

Here's hoping you have a pleasant "Moonstruck" Moment this evening!


Best bar bet in the world: Delilah didn't do it.
Judges 16:19-- And she made him (Samson) sleep upon her knees; and she called for a man, and she caused him to shave off the seven locks of his head.