“But the illusion persists without foreground objects.”

“The Moon Illusion has nothing to do with foreground objects.”

On Earth we are never without foreground objects.

Nonsense.

Go visit the middle of a flat a featureless plain. Or the middle of a calm sea, out of sight of land. Or a mountaintop above a smooth layer of cloud.

Unless you count the horizon itself as a “foreground object”, there are several ways to view the horizon without foreground objects.

With the exception of looking straight up on a cloudless night everywhere we look there *must* be a foreground in our field of vision. The ground at our feet is visually different than the horizon.

So what?

One cannot observe the ground at one’s feet while gazing at the horizon. It is irrelevant.

We take these as distance cues.

Not really. What we take as distance cues is the way distant objects look fainter and more haze-obscured than nearer ones. In very clear air on high ground, something that is 20 miles away looks much the same as something that is 30 miles away.

The Apollo astronauts found that, on the moon where there is no atmospheric haze to give them distance cues, judging distance was incredibly difficult. The appearance of the ground itself is only a useful distance gauge for a few hundred metres. It is useless at larger distances.

The reason the Moon does not appear larger when it is overhead even when behind powerlines or house eves is because there are no distance cues.

What?

Of course there is a distance cue – a nearby object (the power line or the house). It doesn’t matter whether your distance cue is 2 metres away or 20 kilometres away – it is still a tiny, tiny fraction of the distance to the moon.

So our eyes adjust to a resting focus of about a meter away. When we look on the horizon there are tons of cues for distance so our eyes focus to infinity, which makes an object’s visual angle appear larger.

No.

I’ve seen the moon set behind a tall mountain (well, tall for England, it was about 600 m) while I was in a low valley, so I was looking upward at a steep angle to view the moonset. According to your conjecture, I had the entirety of the slope of the mountain to use as “distance cues”, but the moon still appeared small. You can try this for yourself if you choose not to take my word for it.

Your hypothesis fails.

We then *compare* that with our memory of it overhead and judge the Moon to be both larger and closer.

Not consciously, we don’t. How many people look at the moon when it is high overhead? Probably not much more than 10%. But how many people will comment on how large a full moon appears to be at moonrise or moonset? Nearly anyone who observes it.

The other effects, the Ponzo illusion and the Ebbinghaus illusion, then compete with each other for dominance based on the viewing conditions.

Thus endeth the lesson for today.

Not only does your hypothesis fail on comparison with reality, but you do not address the real explanation of the moon illusion, that it arises mainly through a combination of the brain’s model of the sky with the Ponzo Illusion.

There simply isn’t one explanation on which everyone agrees.

Well, feel free to disagree. That is your prerogative.

However, I think you will find that reality agrees on only one explanation of the illusion.

“A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point.”

This is out of date. Or just simply wrong.

Its perceived angular size is the same whether it is near the horizon or high in the sky (you can measure it’s angular size and find it to be so close to the same that the human eye cannot perceive the difference). You can even take the simple step of viewing the moon through a cardboard tube and observing the distance between your eye and the tube to get the moon’s diameter in your field of view to equal the diameter of the tube in your field of view – you’ll get the same result whether the moon is low or high in the sky.

The only thing that changes is the way the human brain interprets the data it receives from the eye. And, in the ambiguous terminology of your quote, it is the moon’s “physical size” that is being modelled by the brain.

The reason that I objected in the first place was because it seemed to me that Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club.

No. It is simple. His explanation does not match reality.

The relative size hypothesis is an acceptable explanation for the Moon illusion.

Rubbish!

It’s plain wrong. It does not explain the illusion. The “relative size” hypothesis predicts that, if the moon is high in the sky and you view it in close proximity to a nearby foreground object, it will appear larger. The moon does not do this – you can try it for yourself if you don’t want to take my word for it. Conversely, the “relative size” hypothesis also predicts that, when the horizon is flat and featureless (such as is experienced on a calm sea), the moon will appear smaller than if there are other objects (such as buildings, trees, or hills) on or near the horizon. The moon does not do this either.

You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.

And you and he are still wrong.

I have not seen anyone here being a dick about it. We are simply pointing out Tyson’s – and your – error.

“There simply isn’t one explanation on which everyone agrees.”

Fair enough. I think Phil Plait’s explanation is better than Neil Degrasse Tyson’s, but I won’t rehash that. My main point was that they are substantially not just semantically different, and although I’m not sure if you now agree about that, I won’t rehash that either.

“Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club….You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.”

Frankly, this seems to come out of left field. I literally have no idea what you’re talking about. All that Phil Plait did was to say that Neil Degrasse Tyson’s explanation of the Moon illusion was off-base. He even refers to “my pal Neil Tyson” in the post, and links to another post where he praises Mr. Tyson and again refers to him as a friend. No one in the comments attacked him, as far as I can see. Where in the world are you seeing anyone “unfairly ganging up on Mr. Tyson”?

I have no idea what the “internet Village Atheist’s He-Man club” is. I am, I suppose, a member of the “internet Village”, but I certainly don’t consider myself an Atheist or a He-Man, much less a member of any sort of anti-Neil Degrasse Tyson club. Furthermore, I personally don’t think Pluto should be classified as a planet. I just think that Mr. Plait’s explanation of the Moon illusion is better than Mr. Tyson’s – and I freely admit that I could be wrong about that.

What comments, specifically, do you think constitute being “a d!ck”? What comments, specifically, do you think warrant an apology to Mr. Tyson?

For the record, I get the same illusion when I see Orion on the horizon.

Moon illusion
“A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point.”

The reason that I objected in the first place was because it seemed to me that Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club.

The relative size hypothesis is an acceptable explanation for the Moon illusion. You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.

—–
“But the illusion persists without foreground objects.”

“The Moon Illusion has nothing to do with foreground objects.”

On Earth we are never without foreground objects. With the exception of looking straight up on a cloudless night everywhere we look there *must* be a foreground in our field of vision. The ground at our feet is visually different than the horizon. We take these as distance cues. The reason the Moon does not appear larger when it is overhead even when behind powerlines or house eves is because there are no distance cues. So our eyes adjust to a resting focus of about a meter away. When we look on the horizon there are tons of cues for distance so our eyes focus to infinity, which makes an object’s visual angle appear larger. We then *compare* that with our memory of it overhead and judge the Moon to be both larger and closer.

The other effects, the Ponzo illusion and the Ebbinghaus illusion, then compete with each other for dominance based on the viewing conditions.

Thus endeth the lesson for today.

“We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.”

No. “Foreground” and “background” in the “Tyson effect” refer to actual objects we are actually seeing at the same time in the foreground and background of our visual fields. It posits that when we see the moon overhead, there are no foreground objects with which to compare it, so our brains judge its size and distance purely by the space it occupies in our visual field – so it appears small. Near the horizon, the moon is in proximity to foreground objects and sometimes behind them (in the background) which we know to be large but far away, so our brains perceive the moon as being larger but further away – so it appears bigger than when directly overhead.

But the illusion persists without foreground objects. The flat, featureless Kansas plains may not literally be flat, featureless geometric planes, but they’re close enough for human perception (as are, in my personal experience, areas of Kuwait and southern Iraq), and even here the illusion persists. And as Nigel Depledge (#47) points out, “merely having a nearby foreground object (say, a house or a tree or a power cable) close to the moon in your field of view when the moon is high in the sky should make the moon look bigger, and it does not.”

You seem to be using “foreground” and “background” to refer to comparing memories of seeing an object to the current experience of seeing an object – which is NOT how they are used in Neil Degrasse Tyson’s conjecture. And that’s not really what’s going on according to the “Plait effect” either. See my previous posts, or Nigel Depledge at #41, or BA’s original post linked in the current post.

Shorter gdave: “The Moon appears larger because you’re comparing it with (the expected size of) foreground objects.”

We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.

Which was what Neil Tyson said in the first place only without the tiresome pedantry.

Not really.

If this were correct, then merely having a nearby foreground object (say, a house or a tree or a power cable) close to the moon in your field of view when the moon is high in the sky should make the moon look bigger, and it does not.

The Moon Illusion has nothing to do with foreground objects.

And, hey, since when was pedantry tiresome? Oh, that’s right – since it became cool to be wrong.

I guess that serves me right for not re-reading all of Phil’s old post.

I just looked at the Ponzo Illusion bit and thought, no, that’s not the explanation.

I added to the comic for clarity and to touch upon the “shallow bowl” effect. The illusion has nothing to do with objects, it’s about how our brain maps the space around us and interprets objects based on perceived distance, just as it does with the ponzo images.

We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.

Which was what Neil Tyson said in the first place only without the tiresome pedantry.