Why the moon looks bigger at the horizon

[DaveC426913]

... but my brain will say otherwise.

Right. Which is why there is no purpose to the experiment at all. You will go out tonight and you will see what you see. Self-fulfilling prohpecies can be quite satisfying.

 

[PhanthomJay]

Right. Which is why there is no purpose to the experiment at all. You will go out tonight and you will see what you see. Self-fulfilling prohpecies can be quite satisfying.

I have a prophecy, but I'm an honest man. If it doesn't pan out, I'll let you know. I don't think it will be tonight...the near full moon rises in my parts at about 6:05 PM EDST or so, and the darn sun doesn't set until 6:40 PM , so it may not be ideal viewing. Friday might pan out better, when the moon rises after the sun sets, I would think the size appearance would be similar though, on that night. Then, next month when the full moon rises, I'll view it, and make a mental comparison between the 2. You say the comparison can't be done? Well, follow these ten simple steps:

1. Go to the post where I have 2 pictures of a full moon.
2. Try not to look at the pictures; cover them with an opaque sheet of paper.
3. Free up all brain storage cells (This step is very important).
4. Forget that one of the pix is a telephoto shot.
5. Lower the paper so that just the top image is exposed.
6. Study it for a minute or so, and gage the appearance of the moon size.
7. Close your eyes, and recall in your mind what you have seen. You might say to yourself, 'yeah, nice healthy looking moon that was, not too big as I recall, but nice looking'.
8. Now go back to the screen and cover the top pic, and look at the lower pic, and ponder it. Unless you didn't free your brain cells, you have got to admit to yourself "that darn moon is huge compared to my mental image of the other one!"
9. Look at both pix together, to confirm your thoughts.
10. Send me your response.

 

[DaveC426913]

I have a prophecy, but I'm an honest man.

I'm not questioning your honesty. I'm questioning whether the observation has any meaning in the first place.

Then, next month when the full moon rises, I'll view it, and make a mental comparison between the 2. You say the comparison can't be done?

I do.

Well, follow these ten simple steps:
1. Go to the post where I have 2 pictures of a full moon.
2. Try not to look at the pictures; cover them with an opaque sheet of paper.
3. Free up all brain storage cells (This step is very important).
4. Forget that one of the pix is a telephoto shot.
5. Lower the paper so that just the top image is exposed.
6. Study it for a minute or so, and gage the appearance of the moon size.
7. Close your eyes, and recall in your mind what you have seen. You might say to yourself, 'yeah, nice healthy looking moon that was, not too big as I recall, but nice looking'.
8. Now go back to the screen and cover the top pic, and look at the lower pic, and ponder it. Unless you didn't free your brain cells, you have got to admit to yourself "that darn moon is huge compared to my mental image of the other one!"

Countless is the number of ways this not at all like what you are proposing to do.

I'll list just a couple of giant ones:
- "a minute or so" as opposed to "a month"
- a context for comparison (such as the boundaries of the pic, a screen of standard size, etc.) as opposed to zero context in the sky
- a 2D pic, compared to a real object in 3D space

Forget "apples to oranges", this is "apples to mushrooms".

 

[PhanthomJay]

Yes, point well taken. Still, I'll bet you mushrooms to apples that the darn moon is going to look HUGE!

 

[DaveC426913]

Yes, point well taken. Still, I'll bet you mushrooms to apples that the darn moon is going to look HUGE!

I will not take that bet. No sane person would.

 

[PhanthomJay]

Date: 22 September, 2010
Time: 6:05PM EDST
Place: Shoreline of Atlantic Ocean, elevation 20' MSL (eyeball height above sea level)
Weather: High clouds on east horizon, haze looking at E.horizon, partly cloudy overhead
Moon angle at 6:05 PM (moonrise) ; 0 degrees above east horizon (invisible)
Sun angle at 6:05 PM: about 7 degrees above west horizon, through thin cloud layer
Air temperature: 83 degrees F (28 degrees C) (unusually warm for this time of year, creating haze layer)
Visibility: 10 miles

Synopsis:

And so, equipped with a dime, pencil eraser, paper towel tube, and an out of shape body, I hastened to the sea wall to view the 'event'.

The weather and sunlight was against me.

I waited patiently until about 6:25 PM. I saw nothing. The clouds and haze and setting sunlight obscured my vision of the thing.

Then , not much later, the Harvest Moon appeared above the haze layer! (but was still partially obscured by the high thin clouds). The sun was almost fully set. The moon at this point was about 7 degrees above the horizon.

The moon was light orange in color, and not very bright.

It was NOT unusually large in appearance.

A dime held at arm's length easily covered it.

The pencil eraser held at arm's length just about covered it exactly.

Looking through the tube with one eye, and without the tube from the other eye, the moon was a bit smaller in appearance looking through the tube , but not appreciably so.

Looking through my legs (hint, kneel down while doing so, it's easier on the back), the moon appeared slightly smaller, like it did when looking through the tube.

A commercial jet passed by the moon, cleared to land on runway 22L. I judged the moon to be about 3 plane lengths across. The plane was about 2 miles from me, at elev 1500 feet or so.

In short, it was a bust. But I haven't given up yet. Friday the moon will rise after sunset, and hopefully the weather will be more cooperative. Give me until the weekend, if you would, before I make my final call.

Dave: the bet's still on if you're insane; I'll send you a bag of fresh apples if I think I'm wrong; you need only send me a half pint of stale mushrooms, if I think I'm right. You don't have anything much to lose, but a lot to gain (Macs or Cortland?). Thus far, you're ahead

 

[nucleus]

The present full moon at Apogee, which is the farthest it will be from earth, so the smallest full moon this year. The Lunar Perigee moon was in winter this year.
http://www.lunarplanner.com/LunarPerigee/index.html
http://www.fourmilab.ch/earthview/moon_ap_per.html

 

[PhanthomJay]

The present full moon at Apogee, which is the farthest it will be from earth, so the smallest full moon this year. The Lunar Perigee moon was in winter this year.

Thank you very much! I didn't realize the harvest moon was at Apogee this year. I wonder if it was at Perigee last year? That explains a lot, thanks. Tonight was another bust, thick clouds obscured the horizon; and when it finally rose above the clouds at about 10-15 degrees above the horizon, it was quite ordinary looking. I took my daughter with me...she clearly remembered some full moon last year that appeared much larger than this one, and so do I. When one sees such a large appearing moon, it sticks in your memory, in spite of the claims that the comparison is impossible. As one who has seen an extra large appearing moon, I can tell you right now that the one i saw tonight paled in size in comaprison to it.

I've not given up yet on this year's harvest moon though...still a few more days to see it if the weather cooperates as it rises over the horizon...bright orange in all its glory. Thanks again for that Apogee/Perigee info..

 

[D H]

Nobody is arguing that the Moon appears to be larger on the horizon that when overhead, Jay. That would be a silly argument to make because to the vast majority of people, it does appear to be larger on the horizon.

The phrase appears to be larger is key here. This appearance is an optical illusion, and a very convincing one at that. It is a function of how our brains work.

 

[PhanthomJay]

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. A full moon overhead at apogee appears smaller than a full moon at perigee overhead, and there is no illusion overhead. It's just like Venus apears larger when it is closer to earth, than when it is fathest. I have never argued about the illusion. I just didn't know why some full moons appears much larger than other moons, when on the horizon. Apogee /Perigee may be one rwson...what is it, an apparent 15% increase in apparent diameter? 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.

 

[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.