Posted on by David Dickinson

Get Set For Super (Duper?) Moon 2 of 3 For 2014

The July 2014 Supermoon rising over the University of Texas at Austin Tower. Credit: Mark Ezell, used with permission.

You could be forgiven for thinking this summer that the “supermoon” is now a monthly occurrence. But this coming weekend’s Full Moon is indeed (we swear) the closest to Earth for 2014.

What’s going on here? Well, as we wrote one synodic month ago — the time it takes for the Moon to return to the same phase at 29.5 days — we’re currently in a cycle of supermoons this summer. That is, a supermoon as reckoned as when the Full Moon falls within 24 hours of perigee, a much handier definition than the nebulous “falls within 90% of its orbit” proposed and popularized by astrologers.

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A super-sized shot of the July 2014 supermoon. Credit: Russell Bateman.

The supermoons for 2014 fall on July 13th, August 10th and September 8th respectively. You could say that this weekend’s supermoon is act two in a three act movement, a sort of Empire Strikes Back to last month’s A New Hope.

Now for the specifics: Full Moon this weekend occurs on August 10th at 18:10 Universal Time (UT) or 2:10 PM EDT. The Moon will reach perigee or its closest point to the Earth at 17:44 UT/1:44 PM EDT just 26 minutes prior to Full, at 55.96 Earth radii distant or 356,896 kilometres away. This is just under 500 kilometres shy of the closest perigee that can occur at 356,400 kilometres distant. Perigee was closer to Full phase time-wise last year on June 23rd, 2013, but this value won’t be topped or tied again until November 25th, 2034. The Moon will be at the zenith and closest to the surface of the Earth at the moment it passes Full over the mid-Indian Ocean on Sunday evening nearing local midnight.

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The 99.8% Full Moon from July 2014. Credit: Stephen Rahn.

Now for a reality check: The August lunar perigee only beats out the January 1st approach of the Moon for the closest of 2014 by a scant 25 kilometres. Perigees routinely happen whether the Moon is Full or not, and they occur once every anomalistic month, which is the average span from perigee-to-perigee at 27.6 days. This difference between the anomalistic and synodic period causes the coincidence that is the supermoon to precess forward about a month a year. You can see our list of supermoon seasons out until 2020 here.

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A comparison of lunar distance (dark line) with phase (grey line) for 2014. Note that 0.5 denotes Full, while 0 denotes New phase. Credit: Darekk2, Wikimedia Commons graphic under a 3.0 Unported license.

And don’t forget, the Moon actually approaches you to the tune of about half of the radius of the Earth while it rises to the zenith, only to recede again as it sinks back down to the horizon. The rising Full Moon on the horizon  only appears larger mainly due to an illusion known as the Ponzo Effect.

The apparent size of the Moon varies about 14% in angular diameter from 29.3′ (known as an apogee “mini-Moon”) to 34.1′ at its most perigee “super-size” as seen from the Earth.

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The July 2014 supermoon on the rise. Credit: Brad Timerson @btimerson.

Astronomers prefer the use of the term Perigee Full Moon, but the supermoon meme has taken on a cyber-life of its own. Of course, we’ve gone on record before and stated that we prefer the more archaic term Proxigean Moon, but the supermoon seems here to stay.

And as with many Full Moon myths, this week’s supermoon will be implicated in everything from earthquakes to lost car keys to other terrestrial woes, though of course no such links exist. Coworkers/family members/strangers on Twitter will once again insist it was “the biggest ever,” and claim it took up “half the sky” as they unwittingly take part in an impromptu psychological perception test.

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The July supermoon shot through a blue filter… I wonder just how rare a “Super-Blue Moon” might be? Credit: Talia Landman @taliaeliana.

Fun fact: you could ring local the horizon with 633 supermoons!

And of course, many a website will recycle their supermoon posts, though of course not here at Universe Today, as we bake our science fresh daily.

So what can you expect? Well, a perigee Full Moon can make for higher than usual tides. New York City residents had the bad fortune of a Full Moon tidal surge in 2012 when Hurricane Sandy made landfall. Though there doesn’t seem to be a chance for a repeat of such an occurrence in 2014 in the Atlantic, super-typhoon Halong is churning towards the Japanese coastline for landfall this weekend…

The rising Waxing Gibbous Moon on the evening of August 9th. Credit: Stellarium.
The rising Waxing Gibbous Moon on the evening of August 9th. Credit: Stellarium.

Observationally, Full Moon is actually a lousy time for astronomical observations, causing many a deep sky astrophotographer to instead stay home and visit the family, while lurking astrophotography forums and debunking YouTube UFO videos.

Pro-tip: want your supermoon photo/video to go viral? Shoot the rising Moon just the evening prior when it’s waxing gibbous but nearly Full. Not only will it be more likely to be picked up while everyone is focused on supermoon lunacy, but you’ll also have the added bonus of catching the Moon silhouetted against a low-contrast dusk sky. We have a pre-supermoon rising video from a few years back that still trends with each synodic period!

Well, that’s it ‘til September, when it’ll be The Return (Revenge?) of the Supermoon. Be sure to send those pics in to Universe Today’s Flickr forum, you just might make the supermoon roundup!

Posted on by David Dickinson

Would the Real ‘SuperMoon’ Please Stand Up?

The perigee Full Moon of June 22nd, 2013. Credit: Russell Bateman (@RussellBateman1)

‘Tis the season once again, when rogue Full Moons nearing perigee seem roam the summer skies to the breathless exhortations of many an astronomical neophyte at will. We know… by now, you’d think that there’d be nothing new under the Sun (or in this case, the Moon) to write about the closest Full Moons of the year.

But love ‘em or hate ‘em, tales of the “Supermoon” will soon be gracing ye ole internet again, with hyperbole that’s usually reserved for comets, meteor showers, and celeb debauchery, all promising the “biggest Full Moon EVER…” just like last year, and the year be for that, and the year before that…

How did this come to be?

What’s happening this summer: First, here’s the lowdown on what’s coming up. The closest Full Moon of 2014 occurs next month on August 10th at 18:11 Universal Time (UT) or 1:44 PM EDT. On that date, the Moon reaches perigee or its closest approach to the Earth at 356,896 kilometres distant at 17:44, less than an hour from Full. 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 Full phase once every synodic period (the period from like phase to phase) with a long term average of 29.53 days.

Moon rise on the evening of July 11th, 2014 as seen from latitude 30 degrees north. Credit: Stellarium.
Moon rise on the evening of July 11th, 2014 as seen from latitude 30 degrees north. Credit: Stellarium.

And the August perigee of the Moon only beats out the January 1st, 2014 perigee out by a scant 25 kilometres for the title of the closest perigee of the year, although the Moon was at New phase on that date, with lots less fanfare and hoopla for that one. Perigee itself can vary from 356,400 to 370,400 kilometres distant.

But there’s more. If you consider a “Supermoon” as a Full Moon falling within 24 hours of perigee, (folks like to play fast and loose with the informal definitions when the Supermoon rolls around, as you’ll see) then we actually have a trio of Supermoons on tap for 2014, with one this week on July 12th and September 9th as well.

What, then, is this lunacy?

Well, as many an informative and helpful commenter from previous years has mentioned, the term Supermoon was actually coined by an astrologer. Yes, I know… the same precession-denialists that gave us such eyebrow raising terms as “occultation,” “trine” and the like. Don’t get us started. The term “Supermoon” is a more modern pop culture creation that 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 Proxigean Moon, but those just don’t seem to be able to “fill the seats” when it comes to internet hype.

One of the more arcane aspects set forth by the 1979 definition of a Supermoon is its curiously indistinct description as a “Full Moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit.” This is a strange demarcation, as it’s pretty vague as to the span of distance (perigee varies, due to the drag of the Sun on the Moon’s orbit in what’s known as the precession of the line of apsides) and time. The Moon and all celestial bodies move faster near perigee than apogee as per Kepler’s 2nd Law of planetary motion.

A photo essay comparing Full Moon sizes and appearance from one Supermoon to the next, spanning 2011-2012. Credit:
A photo essay comparing Full Moon sizes and appearance from one Supermoon to the next, spanning 2011-2012. Credit: Marion Haligowski/RadicalRetinscopy. Used with permission.

We very much prefer to think of a Proxigean Moon as defined by a “Full Moon within 24 hours of perigee”. There. Simple. Done.

And let’s not forget, Full phase is but an instant in time when the Moon passes an ecliptic longitude of 180 degrees opposite from the Sun. The Moon actually never reaches 100% illumination due to its 5.1 degree tilt to the ecliptic, as when it does fall exactly opposite to the Sun it also passes into the Earth’s shadow for a total lunar eclipse.

-Check out this animation of the changing size of the Moon and its tilt — known as libration and nutation, respectively — as seen from our Earthly perspective over the span of one lunation.

The truth is, the Moon does vary from 356,400 to 406,700 kilometres in its wonderfully complicated orbit about our fair world, and a discerning eye can tell the difference in its size from one lunation to the next. This means the apparent size of the Moon can vary from 29.3’ to 34.1’ — a difference of almost 5’ — from perigee to apogee. And 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 its on the local horizon than overhead, to the tune of about one Earth radius.

Like its bizarro cousin the “minimoon” and the Blue Moon (not the beer), the Supermoon will probably now forever be part of the informal astronomical lexicon. And just like recent years before 2014, astronomers will soon receive gushing platitudes during next month’s Full Moon from friends/relatives/random people on Twitter about how this was “the biggest Full Moon ever!!!”

Credit Stephen Rahn
The perigee Full Moon of May 5th, 2012. Credit: Stephen Rahn (@StephenRahn13)

Does the summer trio of Full Moons look bigger to you than any other time of year? It will be tough to tell the difference visually over the next three Full Moons. Perhaps a capture of the July, August and September Full Moons might just tease out the very slight difference between the three.

And 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. And of course, the September Full Moon near 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, as reckoned by our handy definition 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.