The Moon illusion

Link to column: Why does the moon appear bigger near the horizon?

Cecil got this one wrong – he says that the Moon appears bigger on the horizon because of visual cues of surrounding objects. Trees, houses, etc. appear small so the Moon appears large by comparison. He then says that even waves at sea appear smaller. It’s clear that The Master has never spent much time at sea. Phil Plait, aka The Bad Astronomer, aka former Doper BA debunks this explanation as well. Out at sea (where wave sizes cannot be accurately judged) or in the desert (are we comparing the Moon to grains of sand?) the illusion still holds.

As further proof of this, try looking at the Moon upside down, through your legs. Even with visual cues such as houses and trees, the Moon appears normal size. In fact, no one is quite sure why the illusion occurs, but the best explanation so far is that the sky itself is the visual cue. Since the sky appears as a flattened bowl (Cecil got this part right), the sky appears farther away at the horizon, giving the illusion that the Moon is bigger by comparison.

Unless you’re one of the characters in Waterworld, “visual cues” and “flattened sky” are inextricably tied together, as Cecil rightly points out.

Don’t forget, there have been a lot of technological advances made in the field of moon-measuring since this column was first printed.

But that’s just the point – this phenomenon is observed in water world all the time. The point being that trees and houses have nothing to do with it.

I am just amazed at the extent to which our eyes trick us, day in and day out.

Seeing really shouldn’t be believing.

Very interesting and informative link. Ever notice how much larger the moon appears in photos where it is the main backdrop for the subject? Ala the Coyote howling or the romantic moon always behind the couple. They are always huge. This intentional distortion has also contributed to our desire for a moon that is bigger than it actually is.

Just an observation that I have never seen the Big Moon in Ireland or the UK. The first time I saw it was in Tennessee, and it happened very frequently in Hong Kong, This might be due to the relative closeness of the horizon in our hilly northern countries, but that doesn’t explain why I saw it near the Himalayas in Nepal. Is there any chance that latitude could have an effect?

jjimm, that’s just weird. I’ve never heard of that before.

Good observation. I would also add that we tend to conflate bright objects with big objects. How many people have experienced the almost point-like Venus as a 2-dimensional space craft?

Question for y’all, and no cheating now: If you were to go outside during a full moon, and you held up a US quarter at arm’s length, could you block out the entire moon with it? Could you block it out with any other coins?

I know you could block it out easily with a quarter. I suspect you could block it out with a dime.

In fact, you could block it out with an aspirin, with room to spare. Until I had this proved to me, I wouldn’t have thought it to be true.

Ah. So you were born on a boat and have almost never visited dry land, then? Formed all your childhood impressions at sea, did you?


One does not to be born on board a boat to board a boat. Are you saying that the only way to observe the phenomenon at sea is to be born on a boat? Or are you saying…

What are you saying?

Your perceptions of the Moon were formed on dry land, where the psychological clues that Cecil mentions predominate. When you go to sea, these psychological habits go with you, and you still see the Moon the same way.

IOW, this perception of the night sky as a flattened bowl continues at sea. All due respect to Phil, but the existance of the phenomenon at sea does not invalidate the psychological hypothesis.

Oh, I see. I disagree, but I see the point of the argument.

It doesn’t invalidate the psychological hypothesis, the psychological hypothesis does not necessarily have anything to do with houses and trees. It may just be the sky alone. Also, the hypothesis that we are conditioned on land to see it a certain way at sea is an interesting one, but not falsifiable. Unless we can find someone who has never seen dry land and ask him, we’re left with guessing. As the “flattened bowl” holds at sea, I’m going with “sky alone”, but of course that’s just my guess.

This thread has me wondering why we see the sky as a flattened bowl. I don’t think it is the trees, etc. Not that I couldn’t be wrong. You wouldn’t need a water world inhabitant to test this. Just someone from an island small enough that land is never on the horizon.

My guess would be that it there is a greater apparant star density away from the horizon, where the dust and atmosphere are thicker. Fewer apparant stars at the horizon leading you to assume the horizon is further away than zenith.

I don’t believe the issue of star density at night has anything to do with it. I - and apparently Minnaert as well - see the “vault of the sky” as flattened even on a clear blue day. Quite why this is, I don’t know.

In the Bad Astronomy book, Plait insists that although the psychological / flattened bowl explanation that tdn cites makes the most sense, in Plait’s words “no one know, exactly”, what causes the moon size illusion. It’s still somewhat mysterious.

That shouldn’t surprise anyone too much. Visual psychology is still a young science.

But it is clear that Cecil was wrong in his explanation, though he was very close-- if you exclude his comments about the comparative size of objects on the horizon, which as has been shown here are quite incorrect.

The One, True Master needs to update his answer…

I have an answer to the flattened bowl theory. If it’s correct, I don’t know.

When you look at something one of the first questions you ask is “how far away is that?” Even if it’s something as nebulous as sky. If you see what looks to be a huge tree, you assume it’s close by. If you see a tiny little tree, all thing being equal, it’s far away.

The horizon is obviously further away than the tiniest speck of a tree.

If you look up, you can’t normally judge distance on the same scale. The sky/moon is obviously above the top of the tallest tree, but as far away as the horizon? It’s a big jump for a lot of people to make.

The same arguement for the sun. When it’s over head, it’s hot and bright. When it sets, it’s cooler and dimmer. If you’re close to a regular fire, it’s hot and bright. A distant fire is cold and dim. Ergo, the setting sun is further away than the midday sun.

Could we just try reading Cecil’s column while paying attention to what it actually says? Which is that the visual-cues explanation for the moon illusion does not work, but visual cues remain a possible explanation for a habitual flattened-bowl view of the sky.

How about clouds? They’re the object we most commonly see in the sky. And cloud cover does form a flattened bowl shape. So if one supposes that the clouds are in the sky, and the clouds are a flattened bowl, why, then, the sky must be a flattened bowl. Notably, this would work just fine in Montana, Kansas, or Gilligan’s Island.