Astronomy Buff

Tonight’s Full Moon Will Look Huge

by Tony on June 30th, 2007

Photo Credit: Paul Moody

(But it won’t REALLY be any bigger than any other full moon)

Right around the time of the summer solstice, the full moon’s are always a little interesting. There’s this illusions that happens, most noticeable near the summer solstice, when the moon looks very large as it rises over the eastern horizon.

The moon isn’t REALLY bigger, but for reasons no one quite understands, our brains trick us into thinking it’s bigger. There are many optical illusions out there, many of them attributable to our brains using some nearby reference point from which to (incorrectly) infer sizes, but this particular illusion is different in that our brains are fooled by it even if there are no such reference points.

For example, airline pilots, 30,000 feet in the air have reported seeing a large apparent moon, but there’s no reference points at that altitude to fool us.

One theory for this is that it’s got nothing to do with reference points or Ponzi illusions, rather the shape of the sky is causing it:

Illustration Credit: NASA

What’s the reason the effect is larger during the summer solstice? From our buddies at NASA:

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.

So, go out tonight at moonrise (this NASA press release has some moonrise times for many major cities) and see if you can fool your brain.

A fun activity: Look at the moon directly and then through a narrow opening of some kind. For example, ‘pinch’ the moon between your thumb and forefinger or view it through a cardboard tube, which hides the foreground terrain. Can you make the optical illusion vanish?

Note to werewolves: not a good night for you, please stay indoors. Thank your for your cooperation.

Photo Credit: Elfleda
(Oops, wrong full moon… sorry, my bad)

Technorati Tags: full moon, optical illusions, summer solstice

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POSTED IN: observing the night sky