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The Supermoon Illusion

Wow, the moon's bigger than that tree! Credit starryskies.com

You’ve probably all seen it before, a huge Full Moon sitting on the horizon and you wonder why it looks much bigger than at other times? It isn’t, really; it’s an illusion.

And now, if you have heard about the close approach of the moon, or so called “Supermoon” on March 19th and are concerned about the disasters and mayem it may cause, there is no need to worry. And surely, when this so-called “Supermoon” occurs on March 19th — at its closest approach to Earth in two decades — people will indeed report that the Moon looks much bigger than normal. But it won’t really be much bigger in the sky at all. It’s all an illusion, a trick of the eye.

The moon does have an effect on the Earth with its gravity affecting ocean tides and even land to a lesser extent, but the moon on the 19th won’t interact with our planet any differently than any other time it’s been at its closest (also known as perigee).

If anything we may get slightly stronger tides, but nothing out of the ordinary.

The Moon orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the Moon ever gets to Earth (called perigee) is 364,000km, and the furthest is ever gets (Apogee) is around 406,000km (these figures vary, and in fact this Full Moon on March 19, 2011 will see a slightly closer approach of 357,000km).

So the percentage difference in distance between the average perigee and the average apogee is ~10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee.

This is quite a significant difference, and so it is worth pointing out that the Moon does appear to be different sizes at different times throughout the year.

Moon at Perigee and Apogee Credit NASA

But that’s NOT what causes the Moon to look huge on the horizon. Such a measly 10% difference in size cannot account for the fact that people describe the Moon as “huge” when they see it low on the horizon.

What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, so well known that it has its own name: the Moon Illusion.

If you measure the angular size of the Full Moon in the sky it varies between 36 arc minutes (0.6 degrees) at perigee, and 30 arc minutes (0.5 degrees) at apogee, but this difference will occur within a number of lunar orbits (months), not over the course of the night as the Moon rises. In fact if you measure the angular size of the Full Moon just after it rises, when it’s near the horizon, and then again hours later once it’s high in the sky, these two numbers are identical: it doesn’t change size at all.

So why does your brain think it has? There’s no clear consensus on this, but the two most reasonable explanations are as follows:

1. When the Moon is low on the horizon there are lots of objects (hills, houses, trees etc) against which you can compare its size. When it’s high in the sky it’s there in isolation. This might create something akin to the Ebbinghaus Illusion, where identically sized objects appear to be different sizes when placed in different surroundings.

Ebbinghaus Illusion – the two orange circles are exactly the same size

2. When seen against nearer foreground objects which we know to be far away from us, our brain thinks something like this: “wow, that Moon is even further than those trees, and they’re really far away. And despite how far away it is, it still looks pretty big. That must mean the Moon is huge!”.

These two factors combine to fool our brains into “seeing” a larger Moon when its near the horizon compared with when it’s overhead, even when our eyes – and our instruments – see it as exactly the same size.

Source: “Moon Illusion” on Dark Sky Diary Special thanks to Steve Owens

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Comments on this entry are closed.

  • marlon March 16, 2011, 3:17 PM

    moon illusion? yes. anyway, the earthquake and tsunami in japan are also an illusion. water in mars- illusion, water on the moon- illusion, earthlike planets- illusion, alien life- illusion, brown dwarfs- illusion, sunlike stars- illusion, binary stars- illusion…. universe today- illusion???

    • Jhanu March 17, 2011, 2:22 AM

      Hey…. Do you really think that something is going to happen????????

    • Hon. Salacious B. Crumb March 17, 2011, 8:12 AM

      Perhaps I’m an illusion too, and my words have just popped out of the aether or the quantum vacuum. In truth, only the present is real, but it has already moved on! Believe in frozen moments, therefore, is the only true illusion of this world.
      Dream on, baby!

    • Astrofiend March 17, 2011, 4:25 PM

      Off the bongs Marlon. They’re doing you no good.

  • GalileoX March 16, 2011, 5:37 PM

    Marlon, What are you talking about?

    • Hon. Salacious B. Crumb March 17, 2011, 8:14 AM

      What she says is probably also an illusion (or is it, instead, a delusion, here)?

  • Jeb15663 March 16, 2011, 10:20 PM

    I want some of whatever Marlon is smoking. Or maybe I don’t, ha!

    • Feenixx March 17, 2011, 12:32 AM

      Just relax, and don’t feed the Troll, leave him/her howling at the moon… ;)

  • retepaskab March 16, 2011, 10:30 PM

    Constellations are usually not visible near the horizon, so it is very interesting how huge the big dipper can seem, viewed next to a mountain peak.

  • joed293 March 17, 2011, 4:37 AM

    marlons comments – illusion???
    Nice article, cheers

  • Duncan Ivry March 17, 2011, 5:23 AM

    An additional source of the super moon illusion could be, that people have some pictures in their memory, where photographers or cinematographers artificially brought together things or persons in the foreground with a moon in the background, e.g. by using a telephoto lens or a computer program. I remember the boy with the ETI on the bicycle, and several cases of a cowboy on a horse on a hill or a tree on a hill, all in front of a faked big moon. I think, some people “see” those pictures instead of what really is — or better: would be — in front of their eyes. As far as I’m concerned the moon’s size is always roughly equal to the size of my thumbnail at the end of my stretched arm.

  • xfiles March 17, 2011, 6:33 AM

    as soon as you guys understand the meaning of illusion you would understand marlon statement. we all are just an illusion to the higher MAN. we are dispensible in all sense.we have created 99% of all things that are in turn doing damage to us…..we will eventually destory ourselves.

    • Hon. Salacious B. Crumb March 17, 2011, 8:07 AM

      The only higher MAN I know in this world was leant from the philosophies of Frederic Nitschke. I.e By definition. “Faith: not wanting to know what is true.
      Or as Joseph Roux also said; “Experience comprises illusions lost, rather than wisdom gained.”
      Clear from your comment here, you are superficially and rather naive.

    • Astrofiend March 17, 2011, 4:30 PM

      “as soon as you guys understand the meaning of illusion you would understand marlon statement. ”

      I suspect that to understand the meaning of ‘illusion’ in the sense that you’re talking about, I would have to ingest an enormous amount of hallucinogenic drugs over a lengthy period. But you’re correct – with the sheer density of people at zero risk of developing the ability for critical reasoning and a sense of proportion of the world, the human race quite possibly will destroy itself.

  • alcyone March 17, 2011, 7:02 AM

    @xfiles

    Actually I wish you would “destory” your interest in posting comments on Universe Today!

  • Hon. Salacious B. Crumb March 17, 2011, 7:52 AM

    Of course, one of the biggest differences not mentioned in this article, is the size of the moon during solar eclipses. This is between annular and total solar eclipses, whose times can be about 13 minute, to zero, then out to about 7.5 minutes for a total eclipse. This 20.5 odd-minutes is caused by the variations between perigee and apogee.
    As for the moon illusion, the trick is also caused by the horizon, where the brain via the eye interprets the relative size as bigger. Without any comparative references when the moon is high up in the sky, the moon appears smaller. A similar effect is observed with the bight constellations, which also appear larger than when high up.
    A simple experiment, however, shows this is not absolutely true, meaning there are other effects going on. The best way to test it is to look at the moon in a telescope where the eyepiece field is just slightly smaller than the moon. Again looking at the moon when it is higher up, you will find from the fixed diameter of the eyepiece field, there is some difference in the apparent size! Why?
    Of course, one of the other aspects not mentioned here is the refraction of light through the atmosphere, where the apparent positions deviate somewhat from the true position. Atmospheric refraction means at sea level during moonrise or moonset, that the moon is actually below the horizon, even though you can visually see the entire moon. [Also, carful observers will not that the moon rise or moon set (as with sunrise and sunset) that the movement of travel from the horizon is not a straight line, but actually appears curved, as the object travels through the varying atmosphere densities.] The moon or sun appears larger near the horizon because the rays of light have to bend and pass through denser air layers. This larger apparent size is due to the continuos bending of light from the object. [As temperature is important for atmospheric refraction, the apparent size is also slightly different between summer and winter. Variations also affects the degree of atmospheric seeing or twinkling. In winter, the stars twinkle less near the horizon than in summer, as the variance in air density with height is more stable and has lesser turbulence than on average ]
    Some also think it is partially also the same kind of mental illusion is the perceived physical size of our limbs and body parts, where the brain, for example, focusses on our working hands making them larger than they really are.

    The Moon Illusion is far actually more complicated than this article portrays.

    Adrian West might have to comment more on this, I think!

  • Guigo March 17, 2011, 2:30 PM

    Well, we must not forget, that 7000 km in terms of local space is a pretty big number :) It’s half of our planets diameter, and if Moon comes closer to us for that distance, i would not be surprised it has immediate interaction with our planet.

    We should always remember, that things in our Universe are so closely tied together :)

  • marlon March 17, 2011, 5:30 PM

    I believe in God because I feel Him. I believe in aliens because I feel them! How about you? Do you belive in God?

    • Hon. Salacious B. Crumb March 17, 2011, 6:22 PM

      As our beloved friend Friedrich Nietzsche says;

      “Judgments, value judgments concerning life, for or against, can in the last resort never be true: they possess value only as symptoms, they come into consideration only as symptoms – in themselves such judgments are stupidities.”
      or
      “Is man one of God’s blunders? Or is God one of man’s blunders?

    • Hon. Salacious B. Crumb March 17, 2011, 6:45 PM

      God lousily created our Moon for only main reasons.
      To curse all women with the lunar cycle and to smite all astronomers with its brightness so they cannot photograph or see faint celestial objects.

      • Hon. Salacious B. Crumb March 17, 2011, 6:47 PM

        Correction: God lousily created our Moon for only two main reasons.

      • DrFlimmer March 18, 2011, 2:24 AM

        However, if God wouldn’t have created the moon, the earth’s axis would be far more unstable and earth would be rotating like hell. ;)
        I guess, this would have made tracking even more difficult for astronomers than it is today.

    • Aqua March 18, 2011, 9:54 AM

      So you are saying that if you met an alien from a far advanced civilization that would be God? I agree… for Thou art that! All beings share the infinite SPIRIT! There are sparks and fires and quantum flames everywhere because the entire universe is alive!

      • Hon. Salacious B. Crumb March 18, 2011, 11:19 AM

        Only one god has one chance to create the universe. All the rest of the being in the joint are all just bit players in the movie that is the evolution of the Universe.
        Yet even if he did create it, he had very little choice in setting the cosmological parameters for it to behaviour properly so life might exist.
        Regardless if god created the universe or not, it certainly wasn’t created by magic. It is only Man genuine inherited stupidity is behaving in a herd mentality that is organised religion.
        Only science holds the truth to unlocking the secrets of the universe, and the undeniable truth, that it can either prove or disprove the existence of god or gods; because it only works on the observational methods and proofs to explain the physical universe. God does not exist as testable subject in the physical sense. Like wise, the discovery of the nature of the universe, if god does exist, is for human curiosity to find the truth about the universe — if only to give us something to do with our time.

        • Hon. Salacious B. Crumb March 18, 2011, 11:21 AM

          Correction : All the rest of the beings in the joint are all just bit players in the movie that is the evolution of the Universe.

  • shzad1 March 18, 2011, 10:06 AM

    Even so, there is no clear evidence that any of these phenomena influenced the Japan earthquake and tsunami the truth about biggest Full Moon after 18 Years Supermoon 19th March 2011

  • Aqua March 18, 2011, 10:21 AM

    La Selena es supremal!

    The closest full Moon eh? Electro magnetically levitated dust storms on the Lunar terminator anybody? Are there any high energy CME’s incoming to up the ante? What does a solar wind disconnect event in the Earth’s magneto tail do to our overall magnetic field during a full moon? Does that cause the Earth’s and Moon’s multiple resonant nodes to vibrate at a slightly higher frequency?

    • Hon. Salacious B. Crumb March 18, 2011, 11:23 AM

      “The closest full Moon eh?”

      Just remember to wear your heaviest boots, in cause you are completely sucked of the Earth!

      • Hon. Salacious B. Crumb March 18, 2011, 11:25 AM

        Correction: Just remember to wear your heaviest boots, in case you are completely sucked of the Earth!

        Sorry. It is a bit late in the morning for me!!

    • Aqua March 18, 2011, 2:32 PM

      I note here the 617 earthquakes last week… up from the usual 200-300 by far~

  • justa man March 18, 2011, 3:01 PM

    Marlon & Hon. Salacious B. Crumb?!??? You kidding me?
    The both of you need to change ur underwear, take a shower, brush your teeth, get dressed and get jobs so that you can stop living in your Mother’s house, being a burden to her.

    • Hon. Salacious B. Crumb March 19, 2011, 1:59 AM

      Eh? How I spend my time is none of your business.
      Either contribute to the comments and discussion or just shut up.

      • shadowmajestic March 22, 2011, 12:49 AM

        dont feed the troll

  • ad March 19, 2011, 3:17 PM

    I have been intrigued by the craters and other features on the surface of moon.See the compilation at Moon photo blog: Photos of moon from different parts of world: http://arundevarajlife.blogspot.com/2011/03/moon-photo-blog-moon-as-seen-from.html

  • Olaf March 19, 2011, 10:49 AM

    So whats up? Another failed end of the world prediction?
    In a few hours the Super moon is over and I am bored.

    • Hon. Salacious B. Crumb March 19, 2011, 11:00 AM

      Me too.
      Pity these jackasses have never heard of results plus errors…

      • Olaf March 19, 2011, 7:05 PM

        Well I enjoyed the nice moon view, but except for that, still, no end of the world. LOL

  • ad March 19, 2011, 7:58 PM

    Today evening supermoon indeed looked splendid!! with its size and yellow colour. I was lucky enough to get a bunch of photos of it today from Denton, Texas, USA. Find the pics of supermoon at http://arundevarajlife.blogspot.com/2011/03/supermoon-photo-from-denton-tx-usa.html

  • xSiiLvErII March 20, 2011, 5:51 PM

    why are people asking if the moon’s close proximity to earth caused the japan earthquake? earthquakes are caused by the movement of tectonic plates due to currents of molten rock underneath them.. the moons gravity is way too weak to move a tectonic plate.. do people learn nothing in school these days…

    • shadowmajestic March 22, 2011, 12:54 AM

      but the moon drags around the earth. influencing both the sea’s, and to lesser extent the landmasses and the mantles. As posted above, the amount of earthquakes doubled last week compared to average amount. As i dont really keep track of them, im not sure whether these are normal spikes, or seem to happen when the Moon comes in a closer orbit.
      But considering the size and magnetic pull from the moon on the earth.. its unlikely this has no effect on the landmass and mantles.

      • Olaf March 23, 2011, 1:49 PM

        Of course it doubles. You are referring to ons of aftershocks after the big Japanese Earth quake. These are expected and would have occurred without the moon.

        Magnetic pull of the Moon? The Moon does not pull magnetically!

        Look it is very simple. Earth rotates, if Earth quake does not happen within a 12 hour period of the so called Supermoon then you can’t call it supermoon that caused it. You should rename it to Earth’s rotation that causes Earth quakes.

        Earth basically rotated 3 times around the nearest point and 6 ups and down of the sea.

        • wjwbudro March 23, 2011, 2:12 PM

          Excuse my naivety and Astrology aside but, I would suspect that the influence of the moon’s gravity is continually having an effect on the earth’s geology and how would we determine whether or not it’s influence, at any particular position, caused a geological phenomenon that led to a major quake or perhaps even a volcanic event?
          I would further postulate that, based on Newtons law of gravity, the closer the moon, the more likely a “stronger” geological perturbation might occur.

  • xSiiLvErII March 20, 2011, 5:52 PM

    as for god and science and the creation of the moon. Stop arguing, science is based on theory. Religion is based on theory. Stop trying to prove either one of you is the truth and fact when you both have no justification for it

    • Hon. Salacious B. Crumb March 20, 2011, 6:36 PM

      Science cannot prove or disprove god. Science is based on theory. Religion is based on faith. So what are you talking about?

      • shadowmajestic March 22, 2011, 12:58 AM

        Science can disprove god. But if God excists and someone proofs he excists, he will stop to excist. I’m not sure who said this and what the exact saying and conclusion behind this was, but sounded pretty valid.
        Science is based on facts. Theory’s are ways to find out those facts. A theory alone is nothing more then faith. As you _believe_ a theory is true untill its either proven to be a fact or false. The bases of religion and science are the same. But the advantage of science is, anyone can attempt to proof or disprove someone’s theory. In religion the theory’s are set. Unable to be proven or disproven. I saw a very nice documentary on this a year or 2 ago, if i can figure out which one it was, its a very good watch.

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