If the full moon looks like it's a little larger than usual this weekend, that's because it is -- or at least, because it's a little closer than usual.
Saturday night marks the year's "Supermoon" -- the name given to the full moon that best coincides with the moon's closest pass to Earth in any given year.
The moon's orbit around the Earth is slightly elliptical which means there is about a 27,000 mile variance between the moon's closest point (called "perigee" ) and furthest point (called "apogee") each month. The "supermoon" is the name given each year to the full moon that happens closest to the perigee.
The moon will appear about 14 percent larger than when it's at apogee, but it'll be hard to notice with the naked eye.
What you might notice instead is that the moon could look artificially super large when it is on the horizon as it's setting and rising just before and after sunset. That's due to the mysterious moon illusion where full moons near the horizon apparently trick the human eye into making it look larger than it is -- an effect that will be enhanced this weekend. Scientists have yet to explain how the illusion works, but suffice to say "object on horizon may appear closer than it really is."
The supermoon will bring unusually high tides because of its closeness and its alignment with the sun and Earth, but the effect will be modest, says Geoff Chester of the U.S. Naval Observatory.
The forecast is looking like it'll cooperate for Seattle with mostly clear skies expected by sunset Saturday evening -- the best time for the show will be just after sunset when the moon rises. Clouds will move in late Saturday night to where we might miss a similar show Sunday morning, but perhaps take a peek just in case.
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Does the Supermoon make people act differently?
The last supermoon, on March 19, 2011, was about 240 miles closer than this year's will be. Next year's will be a bit farther away than this year's.
But no matter how far away a full moon is, it's not going to make people kill themselves or others, commit other crimes, get admitted to a psychiatric hospital or do anything else that popular belief suggests, a psychologist says.
Studies that have tried to document such connections have found "pretty much a big mound of nothing, as far as I can tell," said Scott Lilienfeld of Emory University.
Lilienfeld, an author of "50 Great Myths of Popular Psychology," said the notion of full moons causing bizarre behavior ranks among the top 10 myths because "it's so widely held and it's held with such conviction."
Why do people cling to the idea?
Lilienfeld said a key reason could be the way people pay attention to things. If something unusual happens to occur during a full moon, people who believe the myth take note and remember, even telling other people because it confirms their ideas. But when another full moon appears and nothing out of the ordinary occurs, "they're not very likely to remember" or point it out to others.
So in the end, he said, all they remember are the coincidences.
Associated Press Science Reporter Malcom Ritter contributed to this report