Why the moon looks bigger at the horizon

[DaveC426913]

It is not only an optical illusion, it also has to do with the distance of the moon from the earth, amongst othrer things. A full moon at apogee on the horizon appears smaller than a full moon at perigee on the horizon.

By 13%.

It's just like Venus apears larger when it is closer to earth, than when it is fathest.

No, not at all like that. Venus orbits the Sun, not Earth.

I swear I've seen full moons nearly 1.5 times bigger than the one i saw tonight.

Which proves beyond any shadow of a doubt that your ability to judge the size of the Moon is terrible.

 

[D H]

I swear I've seen full moons nearly 1.5 times bigger than the one i saw tonight. And if no one else ever has, you've missed a real great treat.

Nope. At most 12% larger (angular size) or 25% larger in terms of area. A picture is worth a thousand words:

The time between lunar apogee and perigee is approximately 13.8 days. This does not explain the Moon illusion, which is about the the difference in appearance of the size of the Moon in the hour or less it takes for the Moon to rise far enough above the the horizon to make the illusion disappear.

Nor do atmospheric effects explain the illusion. There are indeed atmospheric effects on the angular size of the Moon when it is near the horizon. These effects however make the angular size of the Moon smaller, not larger, when it is near the horizon.

Once again a picture is worth a thousand words.

 

[DaveC426913]

Nor do atmospheric effects explain the illusion.

PJ is not talking about the well-known standard illusion.

PJ is insisting that, over and above this, he personally, is illuded into seeing the Harvest Moon big as a pie plate. He is attempting to confirm this by gonig out and looking at the Harvest Moon and confirming that, indeed, just like he thought, he imagines it to be much larger than it was a month ago.

 

[PhanthomJay]

This is getting silly. If we can't judge size by comparison, when not side by side, how do we know that the moon, when on the horizon, appears bigger than when it is overhead? Whether due to an illusion, or whether the moon over-ate and got fatter, then lost weight as it was overhead, we know it appeared larger when on the horizon. Everyone agrees to that, correct?

 

[D H]

we know it appeared larger when on the horizon. Everyone agrees to that, correct?

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):

 

[PhanthomJay]

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):

Yes, correct. No argument from here. I don't see how the moon could ever get actually larger, without overeating.

I'd like to get back to that nice picture you posted of the moon at apogee and perigee, thanks. Now if you don't mind, i'd like to use that 25% increase in area between the two. I would say, roughly, that a full moon, at apogee, on the horizon, appears to be about 25% greater than its area appears when it is overhead, still at apogee. Thus, a full moon at perigee, on the horizon, already appearing 25% greater in area due to its closeness to the earth, would be, compared to a full moon at apogee, when overhead, about (1.25)^2 or apparently 50% greater in area.
I rest my case.

 

[D H]

I already did the squaring, Jay. The angular size difference between the Moon and perigee and apogee is less than 12%. Square that and you get a 25% difference in area. You are squaring that squared value again.

 

[PhanthomJay]

I already did the squaring, Jay. The angular size difference between the Moon and perigee and apogee is less than 12%. Square that and you get a 25% difference in area. You are squaring that squared value again.

Yes, I am, to account for the moon illusion effect, comparing the apparent size of the moon on the horizon, at perigee, which last occurred in April, 2010 (see link in post #67), with the apparent size of a moon overhead, at apogee,such as is occurring right now as I write. That's a 56% increase, actually...I was trying to err on the low side. 25% increase due to closeness, and 25% increase due to illusion = 56% apparent area size increase amongst these 2 events which occurred 5 months apart.

 

[DaveC426913]

Yes, I am, to account for the moon illusion effect, comparing the apparent size of the moon on the horizon, at perigee, which last occurred in April, 2010 (see link in post #67), with the apparent size of a moon overhead, at apogee,such as is occurring right now as I write. That's a 56% increase, actually...I was trying to err on the low side. 25% increase due to closeness, and 25% increase due to illusion = 56% apparent area size increase amongst these 2 events which occurred 5 months apart.

Well, except for the fact that the Moon is currently at Apogee, meaning it is 25% smaller.

 

[PhanthomJay]

Well, except for the fact that the Moon is currently at Apogee, meaning it is 25% smaller.

Yes, correct. The full moon ,currently at apogee, appears 25% smaller overhead than the full moon would appear in some other month overhead when at perigee, due to the fact that it would be closer to Earth at perigee. Now based on my subjective analysis that any month's full moon appears overhead about 25% smaller than when that same day's moon was on the horizon some 6 hours prior, then the current moon overhead at apogee appears 50% smaller than another month's moon would appear on the horizon when at perigee.

 

[PhanthomJay]

I'm beginning to think that the clouds will never go away over the mighty Atlantic...once again, at moonrise tonight, a thick layer of clouds on the horizon completely obscured its view.
When it finally did rise above the clouds, at say about 10 degrees above the horizon, in a sky now darkened since the sun had already set about 40 minutes prior, its nice orange color was impressive,... but its size was not. It was somewhat bigger than the moon I'm viewing now, but not appreciably so. I'm surmising that since most of the 'illusion' was gone, and since the moon was at apogee, that the hugeness I expected was naught. I've still got a couple of more nights to view it at the horizon if those clouds ever go away, but as of now, I'm convinced that the apparent 'hugeness' of the moon occurs when the full moon is on the horizon and when it is at perigee. That next occurs around the Vernal Equinox, in March, 2011. Six more months of waiting.

At this time, Ladies and Gentlemen, Boys and Girls, I want to thank all who responded in this thread, in particular, but not limited to, DaveC and DH, but also, a very very special thanks to Nucleus, who appeared out of nowhere to advise me that the moon was at Apogee, something I had overlooked.

 

[PhanthomJay]

Tonight, the clouds still hung over the horizon, but it was quite dark at moonrise, and when the moon was about no more than 5 degrees up from the horizon, it made its appearance in and out of the broken cloud layer. Again, its orange color (due to the travel of its light through the thick atmosphere at this angle) was impressive, but again, even at this low angle, it's apparent size was not appreciably large, not impressive at all.

So I was completely wrong about the Harvest Moon being huge on the horizon...the full moon appears 'extra large' on the horizon 'apparently' when it is closer to the earth, at or near perigee, and I will (we will?) have to wait until next year to find out for sure. At this point, after all this discussion, I still don't know exactly why i have seen (and many others have seen), on occasion, an extra large huge moon.

 

[DaveC426913]

I still don't know exactly why i have seen (and many others have seen), on occasion, an extra large huge moon.

Because it is subjective, depending on your circumstances, location and mood?

 

[DaveC426913]

BTW, I took this pic of the Harvest Moon at 6:42PM Wed Sept 22 on the off-chance that it might come in useful in our discussion. It is against a grid on top of a Home Depot.

Not sure if it's any help to you...

 

[PhanthomJay]

BTW, I took this pic of the Harvest Moon at 6:42PM Wed Sept 22 on the off-chance that it might come in useful in our discussion. It is against a grid on top of a Home Depot.

Not sure if it's any help to you...

Thanks very much! Looks bright, but of 'normal' size, as one would expect from a picture...but it looked pretty normal anyway, without the pic, just slightly apparently larger when near the horizon...classically, the past has a bit of uncertainty

 

[DaveC426913]

Looks bright, but of 'normal' size, as one would expect from a picture...

This sentence is absurd.


Tell me. Did I take this picture from 50 feet away, or from 500 feet away and then cropped it so it looks like I'm only 50 feet away? Each would result in a completely different measurement of the Moon's diameter.

You have aboslutely no way of claiming the Moon's size is "normal".

 

[PhanthomJay]

This sentence is absurd.


Tell me. Did I take this picture from 50 feet away, or from 500 feet away and then cropped it so it looks like I'm only 50 feet away? Each would result in a completely different measurement of the Moon's diameter.

You have aboslutely no way of claiming the Moon's size is "normal".

My guess it that you took it about 50 feet away from the building, with no zoom lens...but does it matter?...the moon in that pic looks like the full darn moon I see month after month after month, when not near the horizon...that's what i mean by normal looking...the average run of the mill variety I see with my own eyes while standing on terra firma.. In fact, even when the moon is at perigee, I never noticed it being any bigger, maybe I never looked up, i don't know, or if I did, the increase of 12 % diameter was probably not noticeable anyway...and in fact, when I saw the moon near the horizon last night, I could only notice it's apparent size increase by looking through my curled thumb and forefinger through one eye, and looking normally through the other eye (if i may use that term 'normally)..the moon's apparent size was pathetic...but all i know is this: see that nice pic you took of the moon...thanks...there is a halo of sorts around it...about three times the diameter of the moon...now place a dot on the midpoint between the circumfernce of the moon and the circumference of the halo...do that in all 4 quadrants, and connect the dots to form a circle with a diameter of about 1.5 times the diameter of that moon...now that circle you just drew..THAT's the apparent diameter of the moon I have seen on occasions in the past...over twice the projected surface area of an 'average' sized moon in appearance when overhead...and come by gee or by golly, I'm someday going to find and see that huge moon again...whether due to illusion, perigee, location, subjectivity, mood, or circumstances...or combination thereof... I hope you get a chance to see it someday also...but don't take any pictures...I don't want any tricks...just watch, and marvel at its apparent size: HUGE!

 

[D H]

The Moon near the horizon does indeed appear to be huge, Jay, but that is only in your mind's eye. Atmospheric effects make the angular size of the Moon when it is near the horizon slightly smaller compared to when it is away from the horizon.

Certainly you have googled "Moon illusion" by now and have found that it is an illusion. If not, I suggest you do just that.

 

[PhanthomJay]

The Moon near the horizon does indeed appear to be huge, Jay, but that is only in your mind's eye. Atmospheric effects make the angular size of the Moon when it is near the horizon slightly smaller compared to when it is away from the horizon.

Certainly you have googled "Moon illusion" by now and have found that it is an illusion. If not, I suggest you do just that.

I think the fact that it was an illusion was noted by you much much earlier in this thread, like a few years ago, to whit:

It's an optical illusion. A very, very good one, too.

Again ,I have NO argument with the moon illusion in my mind's eye! But if you happened to look at the moon last night, the moon near the horizon did NOT appear huge...it appeared miniscular larger in my mind's eye, but not HUGE in my mind's eye. I know Dave saw it, took a pic, but never subjectively commented on how it appeared to him with the nak-d eye. However, sometimes it does appear HUGE on the horizon, at least to me, and apparently, to you, per your first sentrence, and apparently to the originator of this post, who first asked the question. My only question is WHY does it appear only slightly larger on the horizon on some full moon's (like this one's did) but hugely larger on the horizon at other times of full moon (like this one did not). I don't know how I can get this question across any better, and no one yet has come up with a satisfactory explanation.

 

[D H]

Well now that you know it is an illusion it never will look quite as big as it did before you knew that. It's a bit like how Christmas changes once you know that Santa isn't real.

 

[DaveC426913]

My guess it that you took it about 50 feet away from the building, with no zoom lens...but does it matter?...the moon in that pic looks like the full darn moon I see month after month after month, when not near the horizon...that's what i mean by normal looking...the average run of the mill variety I see with my own eyes while standing on terra firma.. In fact, even when the moon is at perigee, I never noticed it being any bigger, maybe I never looked up, i don't know, or if I did, the increase of 12 % diameter was probably not noticeable anyway...and in fact, when I saw the moon near the horizon last night, I could only notice it's apparent size increase by looking through my curled thumb and forefinger through one eye, and looking normally through the other eye (if i may use that term 'normally)..the moon's apparent size was pathetic...but all i know is this: see that nice pic you took of the moon...thanks...there is a halo of sorts around it...about three times the diameter of the moon...now place a dot on the midpoint between the circumfernce of the moon and the circumference of the halo...do that in all 4 quadrants, and connect the dots to form a circle with a diameter of about 1.5 times the diameter of that moon...now that circle you just drew..THAT's the apparent diameter of the moon I have seen on occasions in the past...over twice the projected surface area of an 'average' sized moon in appearance when overhead...and come by gee or by golly, I'm someday going to find and see that huge moon again...whether due to illusion, perigee, location, subjectivity, mood, or circumstances...or combination thereof... I hope you get a chance to see it someday also...but don't take any pictures...I don't want any tricks...just watch, and marvel at its apparent size: HUGE!

:facepalm:
PJ, don't go into the sciences or engineering. Bridges don't get built or atoms smashed by "eyeballing" measurements.

 

[PhanthomJay]

Well now that you know it is an illusion it never will look quite as big as it did before you knew that.

I guess you're saying that the moon looked not as big to me last night because I knew it was an illusion, and if I didn't know, it would have appeared larger? Hmm, I never thought of that...I'll find out next year at the vernal equinox.

It's a bit like how Christmas changes once you know that Santa isn't real.

But even the broken bell will toll for those who still Believe (Credit: 'The Polar Express', "...to the North Pole, of course!" (Hanks).)

 

[D H]

Even though I know it is an illusion, the Moon can still at times appear to be quite large near the horizon. I think color has something to do with it. We had some rain as of late, so on the few days when we could see the Moon near the horizon it was only off-white and not all that large-looking. Tonight, after a couple of dry days, the Moon was a bit orangish and appeared to be bigger as well.

That could explain why the harvest Moon looks so big. Late summer / early fall tends to be dry, resulting in increased particulates. Farmers harvesting crops adds even more particulates. These particulates will increase the atmospheric effects that reduce the angular size of objects near the horizon. These are small effects, however, and are overwhelmed by whatever is happening in our minds that makes objects near the horizon appear to be larger -- big red rubber ball type objects in particular.

 

[DaveC426913]

These particulates will increase the atmospheric effects that reduce the angular size of objects near the horizon.

Can you elaborate? Never heard of such a thing.

 

[PhanthomJay]

:facepalm:
PJ, don't go into the sciences or engineering. Bridges don't get built or atoms smashed by "eyeballing" measurements.

Eeeeeeks I've been an engineer for 40 years, and no one has caught on yet! But you seem to be good at eyeballing measurements, as per your quote from post #5 in two-double-ought-seven

use a tube over one eye to eliminate the foreground (close the other eye); note how small the moon suddenly appears. Now open the other eye. The eye with the unobstructed view sees a larger moon, and you have the slightly unnerving experience of seeing two different-sized moons at once.

That's pretty good double eye-balling,there, Dave.

 

[DaveC426913]

Eeeeeeks I've been an engineer for 40 years, and no one has caught on yet!

Don't believe you. No engineer would make such a mistake.

That's pretty good double eye-balling,there, Dave.

Yes. Awesome. I could not have shown you a better example of what you're doing wrong. Thank you.

Notice how I am talking about comparing two things side-by-side and simultaneously? That's a comparison.

 

[PhanthomJay]

Don't believe you. No engineer would make such a mistake.



Yes. Awesome. I could not have shown you a better example of what you're doing wrong. Thank you.

Notice how I am talking about comparing two things side-by-side and simultaneously? That's a comparison.

Well sir, I sure hope you noticed Jupiter to the right of the moon. Neat, huh? And by side by side comparison, I'm willing to bet (although i know you are not willing) that the moon appeared larger than Jupiter to you. Even though we know that Jupiter is bigger, it appeared smaller. Not due to any illusion of course, but simply because it's pretty far away compared to the moon from us.

Now, look at the full moon again some time, when Jupiter is not hanging around simultaneously next to it. Then, a year or 2 later, look at Jupiter when the moon is not around. Then tell me if you think Jupiter still appears smaller than the moon you saw a year or 2 ago. I know I could tell that it still would appear smaller. Most people could. Can you?

 

[DaveC426913]

Well sir, I sure hope you noticed Jupiter to the right of the moon. Neat, huh? And by side by side comparison, I'm willing to bet (although i know you are not willing) that the moon appeared larger than Jupiter to you. Even though we know that Jupiter is bigger, it appeared smaller. Not due to any illusion of course, but simply because it's pretty far away compared to the moon from us.

Now, look at the full moon again some time, when Jupiter is not hanging around simultaneously next to it. Then, a year or 2 later, look at Jupiter when the moon is not around. Then tell me if you think Jupiter still appears smaller than the moon you saw a year or 2 ago. I know I could tell that it still would appear smaller. Most people could. Can you?

That is a silly analogy. You should know this too. This is not helping your cause to convince anyone that you understand the effects involved.

It's been a blast PJ.

 

[PhanthomJay]

That is a silly analogy. You should know this too. This is not helping your cause to convince anyone that you understand the effects involved.

It's been a blast PJ.

I think you are losing your sense of humor, but I can't tell for sure. That's why I don't like April Fool's day.

Anyway, OK, if you don't like the Jupiter analogy, try this :

Have someone place two oranges on a table that is 25 feet in front of you. That someone has measured the orange on the left to be 3.5 inches in diameter, and the one on the right to be 3.0 inches in diameter. You don't know those measurements. Now you are asked, "Which one appears bigger to you"?. I assume, by your Instantaneous Side by Side Comparison Theory, (which I think is a good one, by the way), that you will pick the one on the left . At least I would. I couldn't say how much bigger, but I could only say, by eyeballing it, that the one on the left is 'somewhat bigger'.

Now, a month or so later, again standing 25 feet in front of the table, have that same someone place just one of those oranges on the table (asuming it hasn't wilted). Either the big one or the smaller one, it doesn't matter. Now you are asked, " Is this the big orange you saw last month, or the smaller one?". I don't know what your answer would be, but mine would likely be "I have no idea; without a side by side comparison, since the size difference was small, I really can't tell which one it is!".

Now, a month or so later, the experiment is repeated, with the following difference: one orange is 3.5 inches in diameter, and the other is 2.5 inches in diameter. They are again placed side by side, the 3.5 inch one on the left; and again I would say, 'the one on the left surely appears bigger, that other one is puny looking'. I couldn't say how much bigger, but only say 'noticeably bigger'.

Now, a month after that, again just one of those oranges, say the 3.5 inch one , is placed on the table. And then the question is posed, " Is this the big orange you saw last month, or the small one?". My answer would likely be " Oh my, that's the big one for sure, that other one was puny looking" . In other words, since the apparent size difference was so large to begin with, I think I can make that subjective statement. The greater the difference, the more likely I can make that subjective call (I bet the Earth looks bigger when viewed from the moon, than the moon looks bigger when viewed from earth, and there's no way to make a side by side comparison of the 2 from your location). And it doesn't matter whether it is apparently bigger, or actually bigger, or whether by illusion or too much alcohol that day...it still looks 'bigger'.

I hope this clears up for you my take on this. But I'm ready for you to find yet another flaw in this argument. Unless, by that statement "It's been a blast", you are implying that you will not respond further.

 

[DaveC426913]

I think you are losing your sense of humor, but I can't tell for sure.

I am, yes.

Have someone place two oranges on a table that is 25 feet in front of you.

All your orange examples are not comparable to observing the Moon. There are myriad clues to the size of a known object such as an orange that is sitting on a table, a mere 25 feet away, with your binocular vision.

If I did the same experiment a very long (undetermined) distance away, using discs, which gave no indication of their actual size, you would have absolutely no clue which one I was showing you.

 

[PhanthomJay]

If I did the same experiment a very long (undetermined) distance away, using discs, which gave no indication of their actual size, you would have absolutely no clue which one I was showing you.

I'm sorry about your losing your sense of humor. If it was because of me, I apologize. I would have thought that years of dealing with my kind would have brought you patience.

Let me ask you this: Supposing you were a cave man on Planet X. There were 2 moons orbiting your planet of unknown size and at an unknown distance away. Moon A and Moon B. When orbiting side by side, you notice, by comparison, that one (Moon A) is much much much much larger than the other (Moon B). Now sometime later, Moon B has taken off to parts unknown, and it no longer appears in the sky. Only Moon A remains. Would you know that the remaining moon is Moon A, or would you not know? Assume that no other moon has come into existence, so that the remaining moon must be either moon A or Moon B. Could you tell, without having both moons present to make the call?

 

[DaveC426913]

Supposing you were a cave man on Planet X. There were 2 moons orbiting your planet

Do you think some warning flags should be going up if the scenarios have to get this contrived?

 

[PhanthomJay]

Do you think some warning flags should be going up if the scenarios have to get this contrived?

I did take an extreme example, but since we are at an impasse, let me compromise by saying this:

"Once upon a time, not too long ago, I saw a really really big moon. Or at least I thought I saw it...perhaps, instead, time has rewritten every line. But, whether real or imagined, it makes no difference, for one time, not too long ago, I saw a huge appearing moon, and its memory will stay with me forever."

 

[DaveC426913]

I did take an extreme example, but since we are at an impasse, let me compromise by saying this:

"Once upon a time, not too long ago, I saw a really really big moon. Or at least I thought I saw it...perhaps, instead, time has rewritten every line. But, whether real or imagined, it makes no difference, for one time, not too long ago, I saw a huge appearing moon, and its memory will stay with me forever."

I can live with that.

 

[PhanthomJay]

I can live with that.

Great, thanks, I'm glad this is over! So is my wife...she thinks I spend too much time on this forum, and looking at the moon, and not enough time helping around the house. And I'll have to admit, she's probably right.

Thanks for your valued input.