The Moon Illusion
by Jeff Polston (Oct. 31, 2001)
I think just about everyone is familiar with seeing the moon or the sun near the horizon, seemingly looking larger than normal. The moon can really look kind of eerie. It hangs above the tree tops, usually an orange color. And it looks huge! (though I admit that the image you see here is one I artificially created for effect). When we later see the moon higher in the sky, glowing a silvery white, it seems much smaller. Incidentally, the reason the moon looks kind of orange when low in the sky is because the light is passing through the thicker atmosphere along the horizon so we are seeing the longer wavelengths of light, toward the red end of the spectrum. But this doesn't explain why it looks so much bigger than when it is overhead. Even the constellations of stars seem larger on the horizon.
Ask someone if they know why the moon appears bigger and you'll probably get one of two answers. The first is that they don't know. The second is in line with why the moon appears orange. They will tell you that the thinker atmosphere along the horizon is acting as a lens and magnifying the image. Now, that sounds like a reasonable explanation. When I look through the "thicker" glass of a magnifying lens, it makes objects appear bigger. In fact, as an "informed" astronomer, that's the answer I use to give to people when they asked me why the moon's image was bigger. Unfortunately I was wrong on two fronts.
First of all, according to the laws of physics and the science behind refraction, the image of the moon should not be magnified. I've heard some say that if the thicker atmosphere was really refracting the image of the moon, it would appear smaller! Now of course you could accused me of once again just believing what other people say. I would agree with you if it wasn't for the second reason of why I was wrong. The simple reason is that the moon's image is the same size on the horizon as it is overhead! It looks bigger because of an optical illusion.
The response from most people at this point is disbelief. Their eyes tell them that the image is bigger so that's what they believe. In other words, they say, "hogwash"! There are a couple of simple experiments that you can do to prove that the horizon image is the same size at the overhead image. One is to look at the moon through a paper tube, such as one from a paper towel roll. Look at it on the horizon, and compare its size to the opening of the tube. Then look at it overhead and do the same. The image size will be the same. Another method is to hold a coin, such as a dime or nickel, at arms length, in front of the moon. Use a coin that will approximately appear as the same size as the moon's image. Both of these methods basically give you a solid frame of reference for comparison. The problem is that few people will actually take the time to view the moon on the horizon, then take the time to view the moon when it is overhead, some 5 or 6 hours later. Let's face it, when the common person (i.e. non astronomer) sees the moon on the horizon, it's usually by chance. They just happen to be looking that way when they see the moon, and they take notice of it. I guess I could perform the experiment myself and tell you that my results proved the moon's image was the same size. But would you believe me? I can hear it now....I didn't hold the coin at the same distance, I used different coins, I didn't center my eye properly in the cardboard tube, etc. So, to provide good hard evidence, I figured a picture is worth a thousand words.
With that in mind, I decided to photograph the moon while it was on the horizon, then again when it had reached its high point in the sky. If the image on the horizon was indeed larger, then the picture would show it. The image you see here is the result. The left side is the moon on the horizon and the right side is the moon overhead. And as you can see, the moon's image is the same size on the horizon as it is when it is overheard. I snapped the horizon shot through my 80mm refractor at about 8:05pm, EDT on October 3rd, 2001. The moon was one day past full. The image is orange because of the atmospheric effects on the light wavelengths. It's kind of grainy too since I didn't want to use too long an exposure for fear of blurring the image (the telescope is on an alt-azimuth mount). The overhead image was taken later that night, at around 2:05am. Yes, I set my clock and dragged myself out of bed just to take this image. The horizon shot might seem a little squat, but I think that really is explained by refraction by the atmosphere. And the image is rotated just a bit because when the moon was on the horizon, I was facing east to take the image and when the moon was overhead (on the meridian), I was facing south to take the image. But as you can see, they are basically the same size.
So that settles the case of whether or not the image of a horizon moon is actually larger or not. The pictures I took prove it's an illusion. Now we need to figure out why we have this illusion in the first place. Some have theorized that the reason is because when the moon is on the horizon, we can compare it to houses, trees, mountains and such and that makes us think it looks bigger. On the surface that sounds like a good explanation, but it doesn't resolve the fact that we still see the moon illusion when it is over a flat featureless horizon, such as the ocean. Something is tricking our eyes and brain into thinking the moon image is bigger.
Consider this illustration of railroad tracks going off into the distance. We see a set of "parallel" railroad tracks that converge. Indeed, if you've ever looked at real tracks or even a highway, it does look like the rails or sides of the road eventually converge. Now look at the brown blocks inserted between the tracks. Which one is bigger? The "further" block, or top one, clearly looks larger than the closer (bottom) block. In reality, they are exactly the same size. Measure them if you have doubts. So why does the top block have the illusion of being bigger?
Well, these tracks are not really parallel. This is a two dimensional drawing so they are drawn to appear to look like they are converging. Because of this perspective, our brain perceives the top block as being further away. If the blocks were the same size (in a real 3-D world), the top block would appear smaller, and spaced like the bottom block is between the rails. Since the top block seems to span more distance between the rails, and our brain is telling us that this section of track is further away, we think the block is bigger. We've got two equal size blocks but because of the perspective of the converging rail lines making the top block appear further way, the top block seems significantly bigger than the bottom block.
The same principle is working when we see the moon on the horizon versus seeing it overhead. We perceive the horizon as being further away than the overhead sky. For some reason, the sky overhead is thought of as closer than the sky on the horizon. When see clouds overhead versus clouds on the horizon, the clouds on the horizon are smaller (as they should be since they are actually further away). An airplane overhead is clearly seen while it looks like just a dot when near the horizon. So, when we see the moon on the horizon, we think of it as being further way. Our brain basically says, "Whoa, that moon is suppose to be further away. Since the disk appears to be the same size as overhead, and it's further away, it must be a bigger moon".
As I have shown, the actual image of the moon on the horizon is the same size as the image of the moon when it's overheard. The atmosphere isn't magnifying it at all. It's just an optical illusion. I have heard some people say that if you change your perspective, the illusion will disappear. For example, if you view the moon upside down, they said the illusion will vanish. So next time you see a larger than life moon on the horizon, bend over and look at it between your legs up side down to see if it still appears to be as large. But do be careful. If you do this in public, you may give the illusion that you need a straight jacket!
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