Worms, crows, maple syrup and the moon illusion

Tonight – and tomorrow night – the Full Worm moon will rise in your neighborhood. Be sure to catch it. Photo: Bob King

The March full moon goes by a variety of names, all appropriate in their own way in this month of seasonal transition. Full Worm Moon. Crow Moon. Sap Moon. Even Crust Moon to describe the icy crust that forms on snow overnight after a day of above freezing temperatures. These were the names given by Indian peoples across the country to tonight’s full moon.

Your calendar may show tomorrow the 27th as the date of full, but since the moon is “fullest” around 4 a.m. Wednesday Central time, it will appear slightly fuller tonight than tomorrow, at least for sky watchers in the western hemisphere.

Most of us won’t be able to tell the difference anyway with our eyes; such subtleties are reserved for binocular and telescope users.

I’ll bet you’ve witnessed the famous moon illusion sometime in your life. When near the horizon, the rising or setting moon looks HUGE compared to when it’s higher up in the sky. Our logical self knows the moon can’t really be bigger. Matter of fact, it’s actually very slightly smaller, since we have to look around the curve of the Earth to see it at moonrise.

How big does the moon look to you when near the horizon? This illustration shows the impression shared by many – the rising moon appears bigger. Illustration: Bob King

If you photograph the moon near moonrise and then again when it’s high up and measure its size, you’ll find there’s no difference in diameter.

Generations of philosophers and scientists at least as far back as Aristotle, who believed it was magnified by the thick air near the horizon, have tried to get to the heart of why we see it so. Our best current explanations to explain this persistent illusion are the “relative size” and “oculomotor micropsia/macropsia” theories.

Relative size is easy to understand and based on the perceived size of an object relative to objects near it. Trees and buildings in the nearby distance and sharing the horizon with the moon make it appear larger in our brains.

When the moon ascends higher and is surrounded by a large expanse of empty sky, it appears smaller. Psychologists call it the Ebbinghaus Illusion and it’s all about context.

An illustration of the Ebbinghaus illusion. The two orange circles are exactly the same size, yet the one on the right appears larger. The left side demonstrates seeing a full moon in large areas of open sky; the other simulates seeing it in the company of many smaller foreground objects. Credit: Wikipedia

Take a look at the illustration above and you’ll see what I mean by context. Assuming the relative size explanation is correct, one could perform an interesting test by comparing the moon’s apparent size in a busy setting with lots of visual cues to one viewed across an empty field or open lake. Would the illusion disappear in the latter?

Now let’s get acquainted with oculomotor micropsia/macropsia. This lovely bit of terminology provides a physical explanation for the illusion and boils down to this:

* When you look at the moon and then momentarily converge your eyes to focus on closer objects in the landscape, the moon will appear smaller (micropsia) than it was before. If you then glance back at the distant moon, your eyeballs straighten out and the moon’s apparent size becomes larger (macropsia).

We’ve all seen the afterimage of a camera flash suspended in front of us after getting our picture taken. If you recall, the afterimage looks large against a wall on the other side of the room, but if you were to hold your hand in front of your face, the ghostly flash would appear small.You can simulate a camera flash by staring at a bright light for a minute in a semi-darkened room and then looking away.

The human eye is our portal to both the real world and one of illusion. Photo: Steve Jurvetson

Consider trying an experiment described by Don McCreedy, professor emeritus of psychology at the University of Wisconsin-Whitewater, when you’re watching the full moon tonight. Cross your eyes by looking at the bridge of your nose while at the same time paying attention to the moon in front of you. While the moon will be blurred, it should look smaller. Then uncross your eyes and stare at the moon – it will immediately look larger.

Scientists are still working on the ultimate explanation for the moon illusion. For now, it appears to be a combination of both the relative size effect and micropia/macropsia. If you really want to dig into the details, you’ll find much to learn at McCreedy’s Moon Illusion Explained site. To find times when the moon rises for you town tonight, click HERE.

Have fun and don’t get your eyes stuck :)

17 Responses

  1. Gary Morris

    Hi Bob,

    Great article Bob thanks and I’m always learning when I read your blog.
    Looks like spring is here this week anyways.

    Gary Morris
    Thunder Bay, ON

    1. Profile photo of astrobob

      Thanks Gary. Yes, I agree that there’s finally some spring in the air. It’s gotten a little warmer and the snow’s starting to clear off the roof on sunny days.

  2. SebastienP

    Hi Bob, in your illustration with a high and a low moon, shouldn’t you have taken the same moon? I see with a picture editor’s measurement tool that the ‘high’ moon is less wide than the lower one. In reality, both moon woul;d have the same width, isn’t it?

    1. Profile photo of astrobob

      Hi Sebasien,
      I enlarged the same moon deliberately for purposes of illustration otherwise the two moons – horizon and higher up – are identical sizes. I wanted to show the impression one gets when looking at the horizon moon. Make sense?

  3. sm

    Wow Astro Bob, I stumbled upon your dramatic pix and diagrams
    cos of the supermoon report in Yahoo!News. Your blog is a
    great mix of science fact and punchy writing style that I’ll
    bookmark for future reading!

    I recall once looking at the moon on the horizon with a pair of
    binoculars that had lines in the field of view, sorta like a range-
    finder. A few hours later I looked again with the moon overhead
    and the size of the moon was definitely smaller.

    Andy’s post (pls see link below) suggests that his “astronomer
    friend” may have had an experience not unlike mine. While
    Antony, Mr H and Jegbert referred to practical experiments
    to check out this ongoing debate I was unable to fiind
    BrustyBnail’s link.

    Hey Astro Bob, you just made my day.



  4. Giorgio Rizzarelli

    I read this nice article again now since you linked it. I remember also the very interesting article “Moon Illusion Explained” you linked here. It’s very interesting the 2006 addendum, about that experiment which substantially showed that, when an object is perceived with bigger angular size than usual, the image is really bigger on the visual cortex, although it’s not on the retina. I like to trace an analogy with digital zoom, which magnifies the object on LCD although the object is not magnified optically on the CCD. So it seems that we have a “digital zoom” which enters in function when we watch landscape (I think the main explanation of the moon illusion is simply the Ebbinghaus illusion you quote). This makes lot of sense if we think that our sight had its evolution while catching preys or watching to avoid predators.

      1. Giorgio Rizzarelli

        Region of Interest, which is basically the same (crop) may be an even better analogy. A few days ago I did a little “experiment”: watching cars and persons at distance, from the window at an upper floor, paying attention to how narrow is the area I was concentrating on, ignoring the rest. It was of order of 1 degree. That’s about 1/100 of our field. It’s probably no coincidence that the fovea centralis, where we have only cones and can focalize objects, covers just around 1 degree. By the way that is exactly the order of magnitude of the Moon angular size. Not bad having a 100x digital zoom built-in! :)

          1. Giorgio Rizzarelli

            Haha yes.. I was just giving orders of magnitude since I had vague info. So I did a bit of research about the numbers….

            Fovea (the sharp-vision region of retina) has a field of view (FOV) of around 6° (and feeds no less than 50% of optical nerve and visual cortex info!). Foveola or fovea’s center (the top hires zone, since it has tightly packed cones) has a FOV of 1.5°. So, assuming for the retina (eye) FOV (subjective and vaguely defined quantity) an average of 150°, human vision’s Region Of Interest is between 6°/150°=25x and 1.5°/150°=100x.

            So the Full moon image, with its 0.5° size, occupies a third of foveola, so we can see it all focused in a single sight without moving eye.
            Interestingly, foveola has just about 20×20 cones or “pixels”, so we see the Moon naked eye with a size of about 7 pixels: no wonder Galileo needed telescope to see craters!

            Of course returning to the moon illusion effect we’re speaking of a different phenomenon, that is, relating the Moon to the surrounding landscape. This gives just a 1.5x-2x (or possibly more) for our subjective perception (respect the high Moon), which (as from that MRI experiment) corresponds to an actual bigger image size on the primary visual cortex.

            So human vision digital zoom for an object in landscape context is 2x, while the ROI for a prey is 25-100x. And all this with a camera weight of just 7 grams per eye. Better than a Sony videocamera! 😀

          2. Profile photo of astrobob

            Enjoyed your analysis. Are you sure you can make a one to one connection of cones to pixels? It would seem the moon’s image is far more detailed on the retina. For instance, at full moon I can make out many lunar features with the naked eye including at least three craters.

          3. Giorgio Rizzarelli

            You’re right Bob, the info I had was incorrect. It’s difficult finding clear quantitative info about eye optics. In addition one should consider that 3 cones make a pixel with color information (or better 2 cones, because foveola misses blue cones). The resolution of the focusing zone of eye is about 10000 pixels. They are hexagonally packed, but if they were packed as on a CCD this would be equivalent to 100×100. Again, take this as an order of magnitude. On Moon this would mean about 30×30 pixels.

          4. Giorgio Rizzarelli

            Yesterday I watched carefully the Moon naked eye and I too can see a few craters like Copernico as white spots (of course before Gailieo’s telescope people didn’t know these were craters).
            I also realized there’s a simple way to measure the Moon “pixel size” at naked eye: since naked eye resolution is about 1 arcmin (really 50arcsec) and Moon is about 30arcmin, we get 30-40 pixels, confirming what I wrote in previous comment. Likewise for foveola, which is 1.2° or 72arcmin, we get about 80 pixel, i.e. 80×80=6000 pixel.

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