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American Moon Illusion

Dear Alice
Recently a friend of mine told me that her friends from India insist that the moon in Seattle is bigger than the moon they see back home. Could this be true?
I’ve often wondered why the moon appears larger near the horizon and why I can’t capture that huge moon in a photograph but it has never occurred to me that the moon might appear larger or smaller from different places on the planet. What’s with that?
Thank you for any insight you might have.

-Terry

Terry,

In short, the Moon is always pretty much the same size, no matter where you’re looking from, and no matter where it is in the sky. All those differences are an illusion. I’ve been poking around though, and I can’t find anything about the “American Moon” vs “Indian Moon” illusion. I’ve got a number of resources on the Moon Illusion in general (why the Moon looks so big near the horizon) but only guesses about why it would happen more here than in India.

Moon Illusion

There are many books and papers written on the subject, but in short, no single theory really explains the Moon illusion. The popular explanations are these:

  1. There are reference points for our brains near the horizon, but there are not reference points straight up. Hence we do a better job of judging the distance and size of the Moon when it is low – near mountains and trees.
  2. Our brains flatten the sky. If you ask someone to judge how high something is in the sky they will always overestimate. This is why people often think the Sun is directly overhead, and if you’re up here near the same latitude as Seattle, the highest it will get is about 66 degrees. Therefore we think things above us are closer, and therefore they must be smaller. (I’ll bet this is related to our estimation of slopes – you know how you were telling your friend about that mountain you skied down that was at a 45 degree angle? Well, it wasn’t, it was much less of a slope than that.)
  3. Relative size: this seems similar to #1, but it’s a little different. Because there is stuff next to the Moon when it is near the horizon it looks larger. When the Moon is high there is no stuff next to the Moon and we assume it is small.
  4. There are also some ideas about how the eye perceives things.

None of these theories actually accounts for the illusion. The only for sure part is that the illusion is definitely all in your head – if you measure or photograph the Moon when it looks big and when it looks small you’ll see that the two measurements and pictures are pretty much identical.

Also, I’ve heard it told that if you see one of these large Moons, and you turn your head so you’re looking at it upside down, the illusion disappears. (Actually, what I heard is if you turn around, bend over, and look back through your legs the Moon will look normal, no illusion, because you’re looking upside down).

Indian Moon vs American Moon

I am absolutely positive that your friends are telling you exactly what they see. I also checked in with a couple of my husband’s coworkers who are originally from India, and visit home regularly. They agree that the Moon seems larger here, though they’ve never heard it talked about.

Here are my two thoughts:

Pollution

I don’t have good numbers to back this up, but of the over a billion people in India, tens of millions live in the big cities. Also, and here I’m really speculating, I would believe that of the people who visit or move to the US, a high percentage are from the cities.

The cities in India are notorious for their air pollution. That’s no good for looking at the sky. Also, if you’ve ever seen the Sun during a dust storm, or, more likely here in Seattle, through a haze of clouds (NEVER LOOK DIRECTLY AT THE SUN) you know you can see the disk of the Sun, and it looks smaller than you expected. The particles, be they dust, smoke, or water droplets, in the sky dim it down and take away the glamor of the Sun. It’s not as glowy, so it looks smaller.

How High the Moon

The latitude of India is between 5 degrees North and 30 degrees North. The latitude of the US is between 25 degrees North and 50 degrees north (yes, I’m ignoring Alaska, because that would just strengthen the argument). I give you those numbers to support for the idea that in India the Moon spends more time higher above the horizon than it does in the US. This would mean that we get the regular Moon Illusion for more of the Moon’s path through the sky (not just at Moonrise and Moonset) here in the US than you would in India. (Obviously the Moon can always be much lower, because it has to rise from the horizon, which is why I’m only looking at the highest part of the Moon’s path)

The red is the highest possible altitudes for the Sun in the US, the blue is the highest possible altitude for the Sun in India. [1]

The red is the highest possible altitudes for the Sun in the US, the blue is the highest possible altitude for the Sun in India.

Here are some numbers to support that argument. If you don’t like numbers, skip this paragraph. Those latitudes means the highest the Moon gets in the US (in Florida) is two degrees past the zenith, and the lowest high (in Washington State) is about 15 degrees above the horizon. In India the lowest the Moon can get (when it is at it’s highest point in its path across the sky) is 30 degrees above the South horizon, and the highest it can be is all the way over to 72 degrees above the North horizon.

P.S. Moonwatch Week

Hey! I just [2] noticed [3] that it is Moonwatch week [4] over in the UK. Heck, it’s the same Moon, so it might as well be Moonwatch week over here. So – this post is belatedly in honor of Moonwatching. Go out and look!

Alice Enevoldsen
~ A l i c e !

Where’d I Get My Info

NASA’s Moon Illusion Page [5]

The Moon Illusion [6]

Griffith Observer [7] I read a good article in a recent issue, but I can’t find the details right now.

Time and Date [8]