# Moon Illusion Revisited...and a Train

I know it’s been batted around enough - the question of why the moon looks so big on the horizon. One explanation is that our eye is comparing the moon against the objects on the horizon, making the moon seem so large…

Well, here’s an interesting twist. Eveyday, I cross a mile wide river. A freight train line parallels the highway by about 75 feet, max, I WAG. An amazing thing happens when I catch sight of the train approaching the bridge. The train appears normal-sized, of course. But, on the bridge with only the wide river for the eye to compare it to…the train looks like a model HO train…maybe even N gauge! I assume this is the same phenomenon, and it gives credence to this theory about the moon.

• Jinx

Link to the Column in question: Why does the moon appear bigger near the horizon?

I’ve noticed a similar effect, oddly enough also involving a train.

My wife has a studio on the second floor of an old industrial building. Her studio is in the front. There’s a raised train track running at an angle just across the street; it’s probably 100-150 feet away and positioned so that passing freight trains are at roughly the same height as the second floor studio.

Well, like I said, my wife’s studio is in the front of the building. In fact, the interior hallway running the length of the building terminates exactly at her door. If you open her studio door and raise the curtain on the window, you can walk all the way to the other end of the building and look straight down the hallway, through her studio, and out the window.

The fun this is that, when there’s a train on the tracks outside, you lose all sense of scale. From the studio, standing next to the window, you can easily see that the train is ~100 feet away. However, when you walk down the hall and turn around, everything is lined up just right so that the train fills the window – it honestly looks like the train is five feet outsice the building instead of across the boulevard. Or, conversely, that the train is simply huge.

Very interesting, but tell me: Since the train is running at an angle, is it still ~100 from that window, or closer? In my case, the train is parallel and about the same elevation as the cars. You know, even Galileo had trouble convincing people that his telescope wasn’t pulling some kind of trick on the eye. Makes ya wonder…ho do we know that seeing is always believing?

I wonder if others have experiened this, say in the mountains, perhaps? Also, I really wonder how moonrise looks from an airplane - esp during take-off! Does the huge moon suddenly shrink as the apparent “scale” of the landscape changes as the plane climbs??? - Jinx

I just checked yesterday: the angle is pretty shallow (perhaps 15 degrees), so the train stays a reasonably constant distance away from the building. Also, when you’re looking down the hall and through the door, you only see a chunk of train that’s probably about five feet wide and eight feet high. That particular chunk is ~100 feet away.

If you work or visit some building that has a long corridor with a window at the end, you might see the same thing. Take a look out the window, then retreat down the hallway and check the scale again. Normally, I think, people tend not to notice these types of illusions because they don’t think to look out the window from a long ways away.

My brother-in-law was the first person to point this effect out to me – he worked on the third floor of a building with a window at the end of the hall. Normally just blue sky showed through the window, but then a building went in next door, and for a time the construction crane was positioned directly out the window. It was pretty cool: The farther away from the crane you went (down the hall, in other words), the closer it appeared to be.

I would like to point out that I have tried this whole thing with the moon at the horizon, and reading these things about the trains, and I really believe there is more at stake here than the mere presence of houses on the horizon, or thye window. The brain is also using the increasing depth of focus extending all the way from the eye all the way to the horizon, or down the hall to the window. Without this distance there is no effect – look up through a tree at the moon straight overhead and the branches do nothing, because they are too close.

I would like to point out that I have tried this whole thing with the moon at the horizon, and reading these things about the trains, and I really believe there is more at stake here than the mere presence of houses on the horizon, or thye window. The brain is also using the increasing depth of focus extending all the way from the eye all the way to the horizon, or down the hall to the window. Without this distance there is no effect – look up through a tree at the moon straight overhead and the branches do nothing, because they are too close.

AS in my other post:
I would like to point out that I have tried this whole thing with the moon at the horizon, and reading these things about the trains, and I really believe there is more at stake here than the mere presence of houses on the horizon, or the window. The brain is also using the increasing depth of focus extending from the eye all the way to the horizon, or down the hall to the window. Without this distance there is no effect – look up through a tree at the moon straight overhead and the branches do nothing, because they are too close.

But in this case of your “HO-scale train” the depth is BEHIND the train, there is nothing near it or between it and your eye, so the effect is the opposite

It’s more than that. When the moon is low on the horizon, you are looking through more of the atmosphere. The atmosphere diffarcts and refracts. The same thing happens at sunrise/sunset. The sun looks bigger. Same effect.

Actually, the atmospheric effects on the size and shape of the Moon or Sun are negligible. You get more variation in the Moon’s apparent size from the changing distance between the Earth and the Moon, and nobody ever notices that.

Of course, the atmosphere does have a significant effect on the color, which is why both the Sun and the Moon look reddish when they’re near the horizon.

Are you saying the illusion is due to the eye having to focus differently for something 100 feet away versus something at the horizon or beyond? Your aperture is not that big - anything beyond 20 or 30 feet from you is effectively at infinity. There’s the effect of having two eyes, but even that is pretty much a non-issue beyond 100 feet.

Just checking in: I have experienced a few more instances where distance subtly changes the apparent scale of things. First, the Wal-Mart by us put up a banner to announce they are moving into their (larger) new store just down the road. My wife said I bet that banner looked huge on the ground before they put it up on the building, but now I’d bet they say it looks so small against the wall of the building.

Another case is at Niagara Falls where the people like so tiny against the mammouth size of the falls. Perception is a very strange thing indeed!

• Jinx

Your analogy between the eye and a camera lens gives me a thought. Maybe when you are looking at the moon up in the night sky your eyes are in “wide angle lens” mode, but low on the horizon your eyes switch to “telephoto” mode? I know that in taking a photo of even a very large, very faraway object behind another (say a mountain in the distance behind a barn) the mountain can be made to seem much closer and larger by using a longer focal length lens, even when backing off to make the barn appear the same size. Is it possible that the moon illusion is similar to telephoto compression, and our eyes have some ability to “change focal length”, or are we pretty much stuck with a 50mm?