Why the moon looks bigger at the horizon

[txd453]

In defense of PhantomJay, there are two aspects to this question: physics and perception, which is clear from the very beginning of the discussion. With all due respect, the back-&-forth with PJ over harvest moons has focused almost solely on the physics (astronomy) side of the question. However, the only way to completely answer his question is to do something like the following: set up a realistic-looking simulator that reproduces the effect and has the ability to vary the size of the (bogus) moon. Get a plot of (simulated) real size (angular diameter) vs. (simulated) apparent size. Locate the (real, un-simulated) lunar size on the plot under various conditions (e.g. apogee & perigee), and compare the ratios of the corresponding apparent sizes. You could even do a statistical study of person-to-person variation.

 

[MagnetDave]

The analogy is "some people think the sky is purple." If everyone answers to the effect of "the sky is blue, here are the physics of the radiation spectrum, and the eye's perception of color, and the net result is 'blue' as defined by common functions" and one person comes in with "I saw a sky a few years ago I thought was really periwinkle," the solution is not to then blue a phantom sky, experimentally slice out the colors and poll every possible person.

There is really no reasonable basis for this to be a nine-page thread apart from sheer stubbornness. If I make a subjective observation of an objective fact, and am presented with evidence that my observation is accounted for by known phenomena, the correct response is NOT to then say, "No, my observation was much huger, and I can't understand why no one else sees this" and then grasp for an explanation, ignoring every reasonable thing that is said for five more pages, and then get upset that people are annoyed with me on the sixth. The apogee/perigee issue was resolved in the first answer at "15% longer," yet he continually ignored that people were taking area into account in the hopes of justifying his "one and half times bigger" observation. We went into explanations about telephoto lens - and zoom lenses got mixed in there.

If *I* make a subjective observation and everyone can explain it, at some point, I have to say, "huh, guess I must have seen it funny," because it was a _subjective_ observation. I don't see why we have to die on this hill. If I observed "the moon occupied x arc-minutes of sky on this day and 1.5x arc-minutes of sky on that day, then yeah, I stand by it. But I also explain my methods, and expose my experiment so we can see if I have an error.

Otherwise, we are just a bunch of guys arguing why a hammer falls faster than a feather with some guy who insists that gravity is different.

 

[PhanthomJay]

https://www.youtube.com/watch?v=

The apogee/perigee issue was resolved in the first answer at "15% longer," yet he continually ignored that people were taking area into account in the hopes of justifying his "one and half times bigger" observation.

I'm glad you enjoyed all 9 pages and then some of the thread! I did not want to bring this topic up again until the special night of March 21, 2011 (mark it now on your calendar), when the moon, near the date of the Vernal equinox, will be nearly full, rise after sunset, and be at near Perigee. But I am forced to rebut your criticism.

The moon at Perigee is about 45,000 km closer to us than when it is at apogee, so it will appear about near 12 % larger in diameter (about 25 % larger in projected surface area) than when at apogee. This is a known fact. But it is difficult to note this increase without a side by side comparison. But also a known fact is the 'Moon Illusion' effect, when the brain makes the moon appear larger when on the horizon. This can be proven by looking through a tube thru one eye, and without a tube through the other eye, and make the side by side comparison. How much larger? I don't know, pick a number, let's just say 'larger'.

Now the combination of the Moon Illusion on the horizon and the fact that the full moon will be at Perigee as it rises on the horizon after sunset on 21 March, will produce an apparent size of the moon that will be very large indeed, I imagine, even without a side by side comparison to a full overhead moon at apogee ...what do you think? I had overlooked this in my earlier postings, until it was definitively pointed out to me that the Harvest Moon was at Apogee, which made the vision a bust. Sorry about that.

Save the date: 21 March, 2011...back to square 1.

 

[JDługosz]

I'm just curious, I've never tried this or heard of it being used, but... is it possible to use a micrometer or a set of calipers (dial calipers for example) held a set distance from the observer's eye, to make size estimates? Would that be a more quantitative way of doing it, rather than using a dime or a pencil eraser?

I think an easy way to do it would be to take a tube like from a roll of paper towels, and put a transparent cover on one end upon which is drawn a scale.

You'll find that if you focus on the moon, the marks on the scale are out of focus, and vice-versa. But it will put some boundary on the measured size. Use a longer tube to get sharper measurements.

Furthermore, if the moon clears the horizon and other objects by enough of a margin, the tube can isolate your view and interfere with the illusion, showing you clearly that it is perception.

 

[raylphscs]

While the atmosphere does have some effect on the Moon near the horizon, it isn't what you think it is. For one, the index of refraction for air is pretty small (1.0003), For the Other the effect it does have is to bend light slightly around the horizon, so that we see objects that, in a straight line, are below the horizon and objects just at the horizon are seen higher in the sky. This bending effect gets stronger as your line of sight nears the horizon. When the Moon is sitting right at the Horizon, the effect is stronger at the bottom of the Moon than it is at the top, so the actual effect is to "flatten" the Moon slightly, not enlarge it.

The moon is so far away, 1.0003 multiply by the distance between the moon and earth, it will make the moon image size substantially bigger.

 

[cjl]

The problem with your line of argument is that it is already conclusively proven wrong (just look at the pictures posted a few posts back).

 

[raylphscs]

Your picture doesn't show your purported atmospheric effect. It cannot for the simple reason that your purported effect doesn't exist. The atmosphere does not act as a lens that magnifies the size of the Moon.

Aside: The word is lens, not lenz. Most browsers have spell checkers and highlight misspelled words. Mine, for example, shows "lenz" with a red underscore. That means that "lenz" is not a word.

Back on topic: Using the well-known equation 1 picture = 1000 words, here is a several thousand word essay on this topic:

Source: http://antwrp.gsfc.nasa.gov/apod/ap020130.html

Source: http://spaceweather.com/submissions/large_image_popup.php?image_name=Chris-Picking-moon_rising_composite_1213915516.jpg
[PLAIN]http://spaceweather.com/submissions/pics/c/Chris-Picking-moon_rising_composite_1213915516_med.jpg

Source: http://greenmanblog.com/archives/P87.html
http://greenmanblog.com/uploads/FullMoons.jpg

Source: http://epod.usra.edu/blog/2007/06/rising-moon-illusion.html
[PLAIN]http://epod.typepad.com/.a/6a0105371bb32c970b011571a50254970b-600wi[/QUOTE]

how was the picture taken? it means everything. where is the scale for time?? what are the different times for each picture?

 

[DaveC426913]

how was the picture taken? it means everything. where is the scale for time?? what are the different times for each picture?

While that would all be interesting information, it wouldn't change anything.

And it can be deduced. Unless you have reason to suspect that the Moon has recently changed how fast it moves across the sky.

 

[D H]

how was the picture taken? it means everything. where is the scale for time?? what are the different times for each picture?

That information regarding the first two photos is right there in the links I provided.

That said, it is rather obvious how those pictures were taken. The first three are time-lapsed multiple exposures while the fourth is a single long exposure. The advantage of the first three is that you can clearly see the horizon and clearly see that the atmosphere does not magnify the size of the Moon. The advantage of the fourth is that you can see nice parallel lines (in other words, the atmosphere does not magnify the Moon).

Surely you have looked elsewhere on the 'net by now and found a vast number of sites that discuss the Moon illusion. If you haven't do so: Google the phrase "moon illusion" and read some of the many links provided.

Wondering how and why our minds are so thoroughly tricked so much is a valid question. Insisting that the phenomenon is real rather than an illusion is not valid. While you are entitled to your own opinions, you are not entitled to your own facts.

 

[Janus]

The moon is so far away, 1.0003 multiply by the distance between the moon and earth, it will make the moon image size substantially bigger.

This argument makes no sense what-so-ever. The distance to the moon doesn't factor into this. The refractive index determines the difference between the angle at which the light enters the atmosphere and it has after it has entered.

If you go back to post #24, I show the effects of no atmosphere to normal atmosphere and thicker than normal atmosphere.
Those images were done with Ray-tracing software. How this works is that you define the positions of a camera, light source(s) and object(s) in a three dimensional space. The software will then calculate the path of light rays leaving the light source, bouncing off of any objects in the scene and determine which would enter the camera. The objects in the scene can be assigned characteristics such as color, finish, transparency etc.
One of the characteristics that can be applied to transparent objects is index of refraction. When tracing a ray, the software will take into account any IOR assigned to an object and deflect the ray accordingly. For example, the attached image was created with the ray-tracer. It shows two "glass" spheres sitting on a checkered plane. The left sphere has had its index of refraction set to 1, while the right one has its set to 1.5

For the images in post #24, I did this:
I created two spheres with their sizes relative to each other being the same as that of the Earth to the Moon.
I placed the smaller sphere at a distance away from the first that corresponded to the
distance of the Moon from the Earth.
I placed the "camera" at a point on the surface of the large sphere so that the smaller sphere appeared at its horizon.
The software rendered the image that the camera would see. This was the view with no atmosphere.

I created a transparent "shell" around the larger sphere with the same relative thickness and index of refraction of our atmosphere.
The software rendered the new image that the camera would see under these conditions.

I increased the index of refraction for the "atmosphere" to increase any effect and had the software render the new image.

The images created were what the software calculated what the camera would see under the different conditions of no atmosphere, normal atmosphere and thick atmosphere. This showed that there is no noticeable difference between atmosphere and no atmosphere, and debunked the idea that atmospheric refraction has any perceptual effect on the apparent size of the Moon.

 

[Buckleymanor]

To your mind, yes, it appears to be larger on the horizon. That is an illusion, however. Here is what a camera sees (time-lapsed photograph):

Is it just me or does the Moon right at the bottom of the picture, the one with the slightly reddish glow and partly obscured look just slightly bigger than the other Moons.

 

[lenfromkits]

1) is the answer I have heard too.

Our perception of the sky - in the absence of visual clues - is that of a low dome, its roof closer to us than its rim. Thus, when we see the Moon above us, we perceive it to be closer than when it is at the horizon. Because we think it's closer yet its disk is the same size, we interpret that as if it is smaller over our heads..

This is in fact the correct answer to the illusion. This is a great topic in psychology. I'm so happy to be able to share some cool psych stuff in a physics forum.

The reason for the moon illusion is very similar to this famous illusion of which line is longer. In the example attached picture, 'A' looks longer than 'B' but they are the same length.

The explanation is the really cool part. Since most of us grew up in a western society where rooms and buildings are all rectangular (ie, not round), we learned very early to recognize these two shapes. 'A' represents the corner of a room when you are looking at it from inside the room. 'B' represents the corner of a building when you are looking at it from outside. Notice that the line in 'A' then is FAR away from you. In fact, the longer the diagonal lines on it, the farther away it would be - if this were a room. If the vertical line in there is the same size for a close corner as for a far away one, the far away one would have to be HUGE (due to perspective).

So the vertical line in 'A' looks like it is far away and large. 'B' shows the opposite. Here the line is the closest point of the building. Being 'big' in this case doesn't mean much.

In the end 'A' looks bigger than 'B' because 'A' looks a lot further away than 'B' does (to the brain). The REALLY interesting point here is that this illusion has no effect on societies in some places of, say, Africa where people did not grow up around rectangle buildings (where they grew up in round houses!)

 

[DaveC426913]

The explanation is the really cool part. Since most of us grew up in a western society where rooms and buildings are all rectangular (ie, not round), we learned very early to recognize these two shapes. 'A' represents the corner of a room when you are looking at it from inside the room. 'B' represents the corner of a building when you are looking at it from outside.

While the "which line is longer" illusion is quite cool, your explanation is nonsense.

Same illusion, cannot be explained by "corners":

 

[JDługosz]

The sky-as-dome explanation is mentioned athttp://www.lhup.edu/~dsimanek/3d/moonillu.htm" , and shown to be wrong.

I thought I had read a detailed explanation on Phil Plait's site, but now I can't find it. Something involving the rotation center of the head being different from the center of the eye, and processing corrections to normal perception can cause certain illusions.

 

[sophiecentaur]

Did you ever notice how slowly Jumbo Jets fly?
Your poor little brain just ca't comprehend how something soooo massive can be up there so it thinks it must be smaller object flying low and slow.

 

[pallidin]

The sky-as-dome explanation is mentioned athttp://www.lhup.edu/~dsimanek/3d/moonillu.htm" , and shown to be wrong.

...Something involving the rotation center of the head being different from the center of the eye, and processing corrections to normal perception can cause certain illusions.

The human eye's are spatially off-set from the center of our head.
Rotation of the human head effects a visual displacement.
OK, understood. Makes sense.
Not sure where you are going with this though.

Wonder if a video cam could further illuminate this displacement phenomenon, and, most importantly, implications.

 

[Mindscrape]

Good lord, folks, the solid angle of the moon (from earth) is
$$\Delta \Omega=A/r$$

which gives
5.74*10^-5 steradians at minimum to 7.71*10^-5 steradians at maximum

Atmospheric lensing? Give me a break... How did this topic go on for 10 pages?

The optical illusions listed by some users are pretty cool though. I wish my brain weren't defect. Oh well, at least everyone else's is defect too!

 

[raylphscs]

Is it just me or does the Moon right at the bottom of the picture, the one with the slightly reddish glow and partly obscured look just slightly bigger than the other Moons.

on this picture, the moon never steered away from being horizontal to being vertical. so the picture means noting for argument. the moon in the picture acturally kept as basically horizontal.

 

[DaveC426913]

Me thinks you have utterly missed the point of the conversation.

Good lord, folks, the solid angle of the moon (from earth) is
$$\Delta \Omega=A/r$$

which gives
5.74*10^-5 steradians at minimum to 7.71*10^-5 steradians at maximum

Great, you showed a formula that describes the Moon's angle. OK, we start with this a premise, you listing the formula adds nothing.

Atmospheric lensing? Give me a break...

Ironclad logic there.

How did this topic go on for 10 pages?

Well, because contributors were doing more than simply skimming to the end and then performing a drive-by judgement.

 

[raylphscs]

Your picture doesn't show your purported atmospheric effect. It cannot for the simple reason that your purported effect doesn't exist. The atmosphere does not act as a lens that magnifies the size of the Moon.

Aside: The word is lens, not lenz. Most browsers have spell checkers and highlight misspelled words. Mine, for example, shows "lenz" with a red underscore. That means that "lenz" is not a word.

Back on topic: Using the well-known equation 1 picture = 1000 words, here is a several thousand word essay on this topic:

Source: http://antwrp.gsfc.nasa.gov/apod/ap020130.html

Source: http://spaceweather.com/submissions/large_image_popup.php?image_name=Chris-Picking-moon_rising_composite_1213915516.jpg
[PLAIN]http://spaceweather.com/submissions/pics/c/Chris-Picking-moon_rising_composite_1213915516_med.jpg

Source: http://greenmanblog.com/archives/P87.html
http://greenmanblog.com/uploads/FullMoons.jpg

Source: http://epod.usra.edu/blog/2007/06/rising-moon-illusion.html
[PLAIN]http://epod.typepad.com/.a/6a0105371bb32c970b011571a50254970b-600wi[/QUOTE]

on this picture, the moon never steered away from being horizontal to being vertical. so the picture means noting for argument. the moon in the picture acturally kept as basically horizontal.

 

[DaveC426913]

on this picture, the moon never steered away from being horizontal to being vertical. so the picture means noting for argument. the moon in the picture acturally kept as basically horizontal.

1] You said this already in post 158.
2] I have no idea which you mean by 'this picture'. Why not just display and reference one picture?
3] I have no idea what you are trying to say about 'horizontal versus vertical'.

 

[raylphscs]

Our perception of the sky - in the absence of visual clues - is that of a low dome, its roof closer to us than its rim. Thus, when we see the Moon above us, we perceive it to be closer than when it is at the horizon. Because we think it's closer yet its disk is the same size, we interpret that as if it is smaller over our heads..

This is in fact the correct answer to the illusion. This is a great topic in psychology. I'm so happy to be able to share some cool psych stuff in a physics forum.

the sky is a low dome is not an illusion, it is the fact. because the visible part of the atomosphere is exactly the low dome in reference to the point we are standing.please see the picture in post#13

please the analogy picture

[

the sky is a low dome is not an illusion, it is the fact. because the visible part of the atomosphere is exactly the low dome in reference to the point we are standing.please see the picture in post#13

 

[DaveC426913]

the sky is a low dome is not an illusion, it is the fact. because the visible part of the atomosphere is exactly the low dome in reference to the point we are standing.please see the picture in post#13

the sky is a low dome is not an illusion, it is the fact. because the visible part of the atomosphere is exactly the low dome in reference to the point we are standing.please see the picture in post#13

raylphscs, you are not helping your cause by repeating yourself. You've just posted the same comment twice in the same post, and that is a repeat of an earlier thread.

While I acknowledge that English is not your first language, this problem has nothing to do with language, this has to do with you organizing your thoughts before posting (a language-independent task), and providing enough detail for the context of what you are trying to say in a single post, instead of continually adding more posts to try to patch up botched posts.

I'm going to request the thread be cleaned up, removing your fractured posts so that you have a chance to make a single clear concise post that gets your point across. This will help you as well as the rest of us.

 

[Redbelly98]

Thread locked pending moderation.

EDIT: Thread will remained locked.

There is really no reasonable basis for this to be a nine-page thread apart from sheer stubbornness.

... we are just a bunch of guys arguing why a hammer falls faster than a feather with some guy who insists that gravity is different.

Good analogy.