The Not Very Supermoon

The media are abuzz over this summer’s trio of “supermoons.” The second of them, coming up on August 10th, will be the largest and brightest full moon of the year. Consequently, some have dubbed it a “super-duper moon.”

But before you get overly excited, let’s examine how super it really is.

Astrologer Richard Nolle first coined the term supermoon in a 1979 issue of Dell Horoscope magazine. A “supermoon,” by his definition, occurs when either new or full Moon falls close to perigee, the Moon’s closest point to Earth in its slightly elliptical monthly orbit. The astronomical term for this event has long been a “perigean” full or new Moon, but that wasn’t as catchy. Myths have sprung up that supermoons trigger earthquakes and other Earthly misfortunes, but these claims have not withstood scientific scrutiny.

Nor does a supermoon look noticeably bigger or brighter than average, unless you measure it pretty carefully. The Moon’s orbit is only a little bit elliptical.

Supermoon Specs

Laurent Laveder

A comparison of the full moon at perigee (left) and apogee (right). Laurent Laveder

There are five supermoons in 2014: two when the Moon was new on January 1st and 30th, and three with the Moon full on July 12th, August 10th, and September 9th. The middle of these three is indeed somewhat special: perigee will occur at 17:44 UT (1:44 p.m. Eastern Daylight Time), just 26 minutes before the Moon is precisely full. Moreover, August 10th marks the moon’s proxigee, or closest perigee of the year, 356,896 km (221,765 mi) from Earth.

But how special is special? The August supermoon is less than 1,500 km closer to Earth than its July and August counterparts, and only 27 km closer than the one in January.

Go out on the evening of the 10th, and if you didn’t know in advance, you’d be hard pressed to tell whether this was a supermoon or the ordinary meh kind. It will appear just under 8% larger than the average full Moon, and about 15% larger than the “minimoon” at apogee. The difference is discernable in a side-by-side comparison like the one at right, but a glance up at the night sky isn’t going to bowl anyone over.

The Moon Illusion

Media outlets advise their audiences to admire the supermoon when it’s just above the horizon, because that’s when it will appear the most augmented. In reality, you’re actually 4,000 miles (Earth’s radius) farther from the Moon when it’s on your horizon than when it’s overhead, an extra distance of about 1.6%. Nevertheless, the Moon looks bigger when it’s low due to the famous Moon Illusion. The leading theory for why this happens is that we perceive the sky near the horizon as being farther away than the sky overhead, where we have no distant scenery to serve as a comparison. The same illusion happens to constellations when they’re low.


Senior Editor Alan MacRobert also contributed to the reporting on this story.

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20 thoughts on “The Not Very Supermoon

  1. Robert-VictorRobert-Victor

    The “Supermoon” visible at dawn and dusk on August 10 is not the brightest Moon of 2014, because of the phase effect, making Moon’s brightness decrease sharply a few degrees away from exactly 180 degrees from Sun. You can duplicate this effect some night by standing between a stationary bright light source and a highly reflective road sign, so that the shadow of your head is cast upon the sign. Note how the sign really lights up around your head’s shadow, giving the shadow a brilliant “halo”. Take a step or two to left or right, and notice how quickly the road sign fades away.

    August’s Full Moon passes 4 or 5 degrees north of Earth’s shadow center, causing the Moon to be less bright. Further, the timing of Full Moon in daytime for North America places Moon even farther from the shadow center, NW of it at sunrise and NE of it at sunset.

    I haven’t done the calculations, but I suspect September’s Full Moon will appear brighter even though more distant, because it will pass closer to Earth’s shadow without being in it, and the Full Moon will occur during nighttime evening hours for eastern U.S., on Sept. 8. October’s Full Moon, though even more distant, should be a contender for brightest of year, just at the start of penumbral lunar eclipse, on the morning of Oct. 8. It certainly won’t be brightest at the middle of the total eclipse, when it is closest to being 180 degrees from the Sun!

    1. lecomptechristolecomptechristo

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  2. rocksnstarsrocksnstars

    One half degree is not very big. Instead of showing the difference in apogee and perigee moon sizes as in this article, they should be shown so that they subtend 30 minutes of arc (average). On my laptop screen they measure 1.75 and 1.5 inches in diameter. If I look at them from a distance of 12 inches, they subtend angles of approximately 8.3 and 7.1 degrees, which means they are nearly 16X.

    A circle only 0.1 inches in diameter viewed from 12 inches subtends an arc of 30 minutes. With a ruler 12 inches away from your eye, hold it up to the moon. You will see a dia of 1/10th inch. The difference between an average moon and a super moon is 7%. 7% of 1/10th inch is LESS THAN 1/100th of an inch! Neither your eye nor the ruler can see 1/100th of an inch. Therefore, every full moon, from the smallest to the largest, will measure 1/10th of an inch. And that is the bottom line of why the super moon is super silly.

    But just like Ancient Aliens and Big Foot, there are people who want to believe it and the general news media of course is more interested in hype than truth, so it will never go away. That is an opinion. The trigonometry is fact.

      1. Frank-ReedNavigation.com

        Mike, you wrote:
        “Anything that gets the public outside and looking up can’t be bad.”

        The Full Moon is the worst possible day in any month to look at the night sky. Respect your students. Respect your audience. Don’t defraud them with fantasies about things they can’t actually see. Get them outside when there is actually something worth seeing it.

        1. marty0307

          I say get them out anytime there is a a moon planet or what ever and get them to see it in a way they haven’t seen it before. Doesn’t matter what is up. You don’t have to wait for prime time events. Their fascination is as much induced by how you explain things as what they see through the telescope.

  3. tapcitytapcity

    The moon looks larger on the horizon because we see it through a longer atmospheric viewing tangent than when we look up. The atmosphere acts as a magnifying lens.

    1. Frank-ReedNavigation.com

      Tapcity, you wrote:
      “The moon looks larger on the horizon because we see it through a longer atmospheric viewing tangent than when we look up. The atmosphere acts as a magnifying lens.”

      No, that isn’t true at all. The Moon is actually slightly REDUCED in size by atmospheric refraction when it is low in the sky. The increase in the Moon’s subjective size near the horizon is an amazing perceptual illusion –it’s entirely in our heads. It’s not magnified by refraction. This fact has been known for centuries, and “natural philosphers” (later scientists) have been discussing the cause of this illusion for centuries, too. You can easily test this and prove it to yourself with a common index card or a piece of ruled paper (see my comment below).

  4. Frank-ReedNavigation.com

    The article mentions that an astrologer coined the expression “supermoon” back in 1979. That information is probably taken from the Wikipedia article. But the real story is more interesting. The expression had zero traction before the 21st century. It was entirely a private expression used by that astrologer, and It was almost unheard of until the year 2011. While the astrologer repeatedly peddled his idea that supermoons caused earthquakes, and it gained a very small falling around 2000 in the “usenet newsgroups”, it took a coincidence to make this term a really common expression. The astrologer finally got “lucky” in March of 2011. If you yell “wolf” often enough, eventually a wolf will appear on cue. Less than two weeks after space.com had published an idiotic article describing the astrologer’s grossly incompetent theory claiming that an upcoming “supermoon” might cause eqrthquakes, the terrifying and tragic earthquake/tsunami/nuclear disaster devastated Japan (NOT on or even near the date of the Full Moon). At that point the “viral Internet” took over and the term “supermoon” entered the English language (and note this last point: this concept is almost non-existent outside the English-speaking world). Don’t believe it’s only three years old? Then visit here. Since the viral explosion in 2011, this term has been peddled by anyone who can see a buck in it, including astronomy interest groups. It’s all pretty shameless. Telling people to go out and look at the sky on the very worst possible night to see the stars is not a “teaching moment”…

  5. Frank-ReedNavigation.com

    I have come to the conclusion that part of the problem with the expression “Moon Illusion” is that people understand it’s an illusion –they know that the Moon isn’t actually bigger in physical diameter. But unfortunately they still think it’s an “optical illusion” frequently believing that the Moon is somehow magnified in apparent size by refraction …or something like that. When teaching about this “Moon illusion” we need to emphasize that it is a “perceptual illusion” and that it is “all in our heads”. Suggest to anyone interested that they should grab a common index card (or a piece of ruled paper). The spacing between lines on (US) index cards is usually a quarter of an inch. Held at arm’s length, that quarter of an inch spacing subtends an angle of just a bit more than half a degree. Suggest to any skeptics that they should hold the index card out and compare the Moon’s size to the index card spacing just as it’s rising, and then do it again when it is higher in the sky. It’s always right around half a degree in diameter. You can also encourage them to try this on other nights which are not proclaimed to be “supermoon” nights. The Moon’s size does change very slightly as it travels from perigee to apogee and back, but the change is very small and not noticeable, and it’s not connected in any way with the PERCEPTUAL Moon illusion.

    1. Lazor1one

      What about the distance from where you are standing on the earth to the edge of the earth when the moon is first coming up. That makes the moon farther away. But when its straight up above you that distance is eliminated those shortening the distance. And everybody knows when something is farther away it will appear smaller.But here its opposite I think that you see things like trees and mountains when its low and they are not very big those giving the moon the appearance of this massive size. But that is just my opinion and you know what they say about them.

  6. marty0307

    Unfortunately you didn’t test the trigonometry fact against experiment! (ie employ the scientific method). The peri-apo sizes on my screen are 4cm and 3.5 cm. Trigonometry says I should stand 4.3 meters away to match the apparent size of lunar disks to about 0.5 deg as they really appear in the sky.

    The difference in size between peri and apo is blatantly obvious. Not to just me but to several people who stood in the same spot. This confirms my own and others experience that super moons are indeed objectively bigger to the eye than when at average distance or at apogee.

    1. Frank-ReedNavigation.com

      Marty, you wrote:
      “The difference in size between peri and apo is blatantly obvious. Not to just me but to several people who stood in the same spot. This confirms my own and others experience that super moons are indeed objectively bigger to the eye than when at average distance or at apogee.”

      Sigh… Marty, OF COURSE, you can see the difference when you are looking at a paired comparison like this, side-by-side. But there’s a big problem with that logic: you NEVER see the real perigee Moon side-by-side with the apogee Moon, nor do you get to see a “blink” comparison (alternating one with the other). To test this using the “scientific method”, as you have proposed, you need to show your test subjects the images in some random order with a significant interval of time between observations and ask which image was bigger. This size difference is very close to the limit of the resolution of human vision. As shown in the accompanying graphic, it’s only four minutes of arc difference in diameter from apogee to perigee ( and only TWO minutes of arc from “average” full moon to so-called “supermoon”) while the resolution of human night vision is one minute of arc. In digital lingo, on average the Moon has a diameter of 32 “pixels” on the human retina, while at perigee its diameter is 34 pixels. That’s a mighty small difference. Again, side-by-side, this is a measurable difference, but it is undetectable in normal viewing when you see a perigee full moon one month and an apogee full moon a few months later. You have been convinced by social media mania that you are seeing something that you cannot actually see.

      1. Dieter Kreuer

        Did you read the linked article in my post above (#3)? This guy is a science writer and amateur astronomer with many decades of experience. He has contributed articles to S&T and he has published books. He has Emily Lakdawalla as follower of his Twitter account. And he claims, he can distinguish perigee and apogee Moons. He certainly knows, what he is talking about.

        1. Frank-ReedNavigation.com

          Hello Dieter. Of Daniel Fischer, you wrote:
          “He has Emily Lakdawalla as follower of his Twitter account.”

          That’s really neat. Fischer is probably awesome and even “super-awesome” (when at perigee) all on his own. And me, too: my name has turned up now and again in S&T and a few other places… Heck, I was on the CBS Evening News last week, very briefly, with sextant in hand (after measuring the Moon’s diameter the previous evening!!). But these are not credentials for either of us. They’re just “neat” little claims to ephemeral fame by association…

          And you added:
          “And he claims, he can distinguish perigee and apogee Moons.”

          Maybe he can. Maybe he can’t… Produce some evidence. I read this article, and all he really says is “I think I can”. So what?! All I can see is that he had been persuaded by fore-knowledge, by what he already knows about the Moon’s perigee cycle. Could he see the small difference in diameter even if that knowledge could somehow be expunged?? Maybe not. Or maybe yes… with a lot of training. Yet, as we all should know, “you must not fool yourself – and you are the easiest person to fool.” Observations tainted by prior knowledge, ESPECIALLY when they conform with our pet theories, are not observations at all. But let’s get back to the broad question: can MOST people detect the difference in size?? No! It’s a very small difference. There is no evidence that they can. For thousands of years, no culture, anywhere on Earth, spoke of this “supermoon” as an interesting visual phenomenon. Even Daniel Fischer makes that point and (in a preposterous defense of his claimed observations) suggests that maybe they did, and those observations somehow haven’t been noticed historically. That’s absurd. Yet since March 2011 (AND ONLY SINCE THEN), the Internet has gone crazy for the “supermoon”, and dutiful “followers” have assured us that they can see a phenomenon that endless centuries of observers did not see. … Somewhere in the crowd, a child is crying out, “the emperor has no clothes!” And that’s the key. This is a social media “thing”, not a real “thing”. The supermoon “has no clothes”… If Fischer had written about this some long years before 2011, it would be interesting. But for now, he’s just another follower admiring the emperor’s new suit.

          1. Dieter Kreuer

            But these are not credentials for either of us.

            While it’s certainly true that an argument by authority is not a real argument, at least it adds to his credibility. He’s not a tabloid writer, he knows the scientific method.

            Maybe he can. Maybe he can’t… Produce some evidence.

            He explains that he had done blind tests, without prior checking the actual distance of the Moon. This reminds of Richard Feynman’s “Surely you’re joking”, where he claims he could find out by the smell if somebody had touched a cola can beforehand, something common wisdom only attributes to the sensitive noses of dogs and other animals. Maybe one just has to experiment with one’s abilities. Fischer has. Have you?

            But let’s get back to the broad question: can MOST people detect the difference in size?? No!

            No argument about this.

  7. marty0307

    Most people may well not notice a difference casually. But I think Daniel Fischer is right. Once one knows there is a difference then one becomes aware of it. Just think of how experienced observers train their eye to see faint detail in deep sky objects or in planetary features that the novice can’t. It’s worth keeping in mind that the difference in the full moon’s illumination between perigee and apogee is 30%! And that’s pretty obvious. And that is because the moon’s apparent area in the sky is 30% larger or smaller Hard to miss that if you are aware of it. I think the angular diameter argument leads to an incorrect conclusion. It could even convince you that you are not supposed to see any difference!

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