November 24, 2017

Astro-Event: A Proxigean (Or do you say “Super?”) Full Moon.

Behold last year’s 2011 Super-Moon! (Photo by Author).

Put away the telescope; this week’s astro-event requires nothing more than a properly functioning set of eyes. Last year, the Internet was abuzz with tales of the “Super-Moon,” one of the closest perigee Full Moons of the century. Well, this May, we’ll witness one that’s even closer… in a sense. Let me explain before the wackiness that is the modern media runs away with the story as only they can… trust me, the tale is a fascinating look into the wonderfully bizarre and complex motion of our very own satellite.

First, let’s clear one thing up; the Super-Moon won’t cause weather disruptions, Armageddon, zombies to arise, the 8-track to return, or any such (insert Woo here) non-sense. I know, there was “that one study” by researchers in Taiwan that suggested a statistical link between earthquakes and perigee Moons, but as the USGS will tell you, the signal present in the data is weak, at best. And remember, predicting an earthquake after it happens doesn’t count. Nor does saying that an earthquake will occur somewhere on the planet, each and everyday, which of course it does.

Now, back to the science. On May 6th at 3:36 AM UTC (The night of May 5th at 11:36 PM EDT) the Moon will reach Full only 2 minutes after it passes perigee at a distance of 356,953 km from the Earth. Rising in the constellation Libra at local sunset, this is the closest that perigee will occur to a Full Moon for this century. The second runner up for this title is a 3 minute spread on Jan 21st, 2023.

In fact, you’ll have to wait until the Proxigean Moon of June 16th, 2296 for a closer margin, according to calculations run by Mr Ed Kotapish, a long wait indeed!

Now, keep in mind two things; 1. The technical period of Full Moon is just an instant when it passes 180° degrees in celestial longitude (right ascension) from the Sun (A fancy way to say “it’s opposite in the sky”) and 2. The Moon passes through perigee every 29.5 (on average) day lunation, some of which are closer, in fact. The perigee of the Moon can actually vary from 356,400 to 370,400 km! Play with this nifty lunar perigee & apogee calculator if you don’t believe us; hey, this is what we do for fun! As you can see, we’re still 553 km off from the minimum value, and last year’s “Super-Moon” had a larger 59 minute difference from Full to Perigee and was only 376 km closer than this year’s.

So, why won’t the Moon just stay put in a simple orbit? To study the motion of the Moon is to know celestial mechanics; it’s a great break that the cosmos gave us to have a large natural satellite to cut our mathematical teeth on as we ventured out into the universe. In addition to simply traveling around the Earth in an elliptical path, the Moon’s orbit also precesses, or rotates about its fixed position. Among many of the many secondary motions acting on the Moon is the gravitational force of the Sun, which causes the eccentricity, or the value of the Moon’s orbital deviation from true circular to vary. Two of these values are known as the Equation of the Center and its opposing force, known as Evection. Evection causes a +/- 1.274° degree (over 2 times as large as the Full Moon itself) variation in ecliptic longitude over a span of 31.8 days. First noticed by Ptolemy, evection has to be taken into account to predict the true position of the Moon. (2.5 lunar diameters would be a pretty big goof, especially if you are sending a spacecraft there!) This stems from the variation in eccentricity over a span of roughly six months, which explains why all lunar perigees are not equal in distance from the Earth.

Got all that? This simply means that this week’s Full Moon will be the closest for 2012 and visually the largest, at 33° 45’ across. This is pretty close to the ideal largest Moon possible of 34° 06’ in size… and although that seems large, the Full Moon is on average about 31° 42’ across. You could stack over 700 of these all the way around the horizon!

So, will the “Super-Moon” affect anything? Well, expect larger than usual variations in ocean tides for coastal areas. And although the Moon may appear larger rising, though this is actually a well documented optical illusion. And to be really pedantic, you can refer to it as the Full Flower, Milk, Hare and/or Full Corn Planting Moon, in addition to its informal title of Super or Perigee (we like the obscure name of Proxigean) Moon of the year… next year’s closest Full Moon will beat out this one by a mere 57 km on August 10th, 2014, but will have a much larger time spread between Full and the moment of perigee. And finally, when its perigee near the Full Moon, its apogee in two weeks during New, when the May 20th annular eclipse of the Sun occurs… this means that this “distant New Moon” will be too tiny to cover up the visible disk of the Sun; more on that in May as it approaches! In the meantime, be sure and take that special someone on a moonlight stroll and wonder at all that is Luna at its finest!



  1. [...] wondered whether they may in fact actually be Ophiuchians. Then there’s the yearly round of the “Supermoon,” a Proxigean Full Moon that always seems to fail to wreak havoc for yet another year, all to great [...]

  2. [...] tanto de la variedad profesional y patio trasero, no les gusta el tĂ©rmino “informal supermoon ”. SurgiĂł en los cĂ­rculos de la astrologĂ­a en los Ăşltimos decenios, y al igual que el [...]

  3. [...] tanto de la variedad profesional y patio trasero, no les gusta el término “informal supermoon ”. Surgió en los círculos de la astrología en los últimos decenios, y al igual que el [...]

  4. [...] tanto de la variedad profesional y patio trasero, no les gusta el término “informal supermoon ”. Surgió en los círculos de la astrología en los últimos decenios, y al igual que el [...]

  5. [...] just exactly what a perigee Full Moon isn’t capable of. Yes, we still prefer the quixotic term “Proxigean Moon,” but there you [...]

  6. [...] Most astronomers, both of the professional and backyard variety, dislike the informal term “supermoon”. It arose in astrology circles over the past few decades, and like the term “Blue Moon” [...]

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