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Three Moon Nights: Will the Real Supermoon Please Stand Up?

The first of this summer's three supersized full moons will rise in the sky this week — which raises a big question: How do you define a Supermoon?
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/ Source: Universe Today

'Tis the season once again, when rogue full moons nearing perigee seem to roam the summer skies at will, to the breathless exhortations of many an astronomical neophyte. We know … by now, you'd think that there'd be nothing new under the sun when it comes to the closest full moons of the year.

But love 'em or hate 'em, tales of the "Supermoon" are gracing ye olde Internet again, with hyperbole that's usually reserved for comets, meteor showers and celeb debauchery. Such tales promise the "biggest full moon ever!" ... just like last year, and the year before that, and the year before that.

How did this come to be?

First, here’s the lowdown on what’s coming up: The closest full moon of 2014 occurs next month on Aug. 10, at 2:11 p.m. ET. On that date, the moon reaches perigee, or its closest approach to the Earth at 221,765 miles (356,896) kilometers distant at 1:44 p.m. ET, less than an hour from full.

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Of course, the moon reaches perigee nearly as close once every anomalistic month (the time from perigee to perigee) of 27.55 days, and passes through full phase once every synodic period (the period from like phase to phase) with a long-term average of 29.53 days.

The August perigee of the Moon beats out the perigee of Jan. 1, 2014, by a scant 15 miles (25 kilometers) for the title of the closest perigee of the year, although the moon was at its new phase on that date, generating a lot less fanfare and hoopla. Perigee itself can vary from 221,450 to 230,150 miles (356,400 to 370,400 kilometers) distant.

But there’s more. Folks like to play fast and loose with the informal definitions when the Supermoon rolls around. If you consider a Supermoon to be a full moon falling within 24 hours of perigee, then we actually have a trio of Supermoons on tap for 2014 — with one of them coming this week on July 12, and on Sept. 9 as well.

What, then, is this lunacy?

Well, the term "Supermoon" first appeared in a 1979 astrology publication, and the name stuck. A more accurate astronomical term for a Supermoon is a perigee-syzygy full moon or a proxigean moon, but those phrases just don't fill the seats when it comes to Internet hype.

One of the more arcane aspects of that 1979 definition is its curious description of a Supermoon as a "full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit."

We prefer to think of a proxigean moon as a full moon within 24 hours of perigee. There. Simple. Done.

And let’s not forget, the full phase is but an instant in time when the moon passes an ecliptic longitude of 180 degrees opposite the sun. The moon never reaches 100 percent illumination, due to its 5.1 degree tilt to the ecliptic. When it does fall exactly opposite the sun, it passes into Earth’s shadow for a total lunar eclipse.

Slideshow: See the Supermoon of 2013

The truth is, the moon's apparent size does vary as it ranges from perigee to apogee, and a discerning eye can tell the difference. That's not taking into account the rising "moon illusion,” which is actually a variation of an optical effect known as the Ponzo Illusion. And besides, the moon is actually more distant when it's on the local horizon than overhead, to the tune of about one Earth radius.

Like the "mini-moon" and the blue moon, the Supermoon will probably continue to be a part of the informal astronomical lexicon. And we'll probably continue to get messages from friends/relatives/random people on Twitter about "the biggest full moon ever!"

Image: Supermoon and mini-moon
This graphic compares the size of the moon when it's closest to Earth, during a Supermoon full phase (left), with how a full moon would look when it's farthest from Earth (right).The Weather Channel

Will this summer's trio of full moons look bigger to you than any other time of year? It will be tough to tell visually. Perhaps photographs capturing the July, August and September full moons can tease out the very slight differences.

For those preferring not to buy in to the annual Supermoon hype, the names for the July, August and September full moons are the Buck, Sturgeon and Corn Moon, respectively. The September full moon near the time of the equinox is also popularly known as the Harvest Moon.

And in case you’re wondering, or just looking to mark your calendar for the next annual “largest full moon(s) of all time,” here’s our nifty table of Supermoons through 2020, based on the standard of a full moon falling within 24 hours of perigee.

So what do you say? Let ‘em come for the hype, and stay for the science. Let’s take back the Supermoon.

— David Dickinson, Universe Today

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe from Tampa Bay, Florida.

The full moon will look bigger and brighter than normal on Saturday, during the first in a string of three summer Supermoons. For what it's worth, the official moment of the full moon is 7:25 a.m. ET Saturday. Got pictures? Share ‘em with NBC News by using the hashtag #NBCSupermoon on Twitter or Instagram, or pass 'em along via the NBC FirstPerson photo-upload service.

This report was originally published by Universe Today on July 8 as "Would the Real Supermoon Please Stand Up?" Copyright 2014 Universe Today. Used with permission.