Why the moon looks bigger at the horizon

[john.phillip]

He is claiming it's a phenomenon of atmosphere and viewing angle, which will be specific to one's location.

I reviewed PhanthomJay posts, and I think you are right.

 

[PhanthomJay]

No one is arguing that you looked at the Moon and saw it was huge. In fact, that is exactly what we're saying.

But without any objective measurement, who are you to claim that what you saw was not simply an illusion?

I am NOT claiming it wasn't an illusion. I am claiming that the Harvest Moon ,in particular, on the horizon , appeared , last year, to me (and my grandkids), subjectively, to be bigger than other full moons I've seen on the horizon, and I don't know why. Let me put it this way: When I viewed the full moon over the horizon , about 3 weeks ago I believe, it appeared, as one would expect, due to the moon illusion effect, larger than it appeared when directly overhead. I placed a dime at arms length in front of me (I wanted to use a pencil eraser, but I can never find one when I need it most), but anyway, the dime more than covered the moon. When I removed the dime and viewed it, it appeared big again, oh I don't know, maybe the size of a 10 inch cheese (or pepperoni) pizza. I think we all agree as to why. Now next week, when I view the rising Harvest Moon over the Horizon at sunset, I fully expect that holding a dime at arms length in front of me, (or perhaps even a pencil eraser that i'll bring with me), will also more than cover the moon. But when i remove that dime, the moon will appear bigger than a 10 inch pizza, oh, say, maybe the size of a 16 inch pizza fully loaded with all the toppings. I don't know why the illusion on this special night will be so big. It won't appear to be as big as say the frontal area of a Mack Truck, but it will be big for sure, and until i view it, I can't yet describe how big. Again, you should view it yourself, and give me your 'trustworthy' subjective take on how big you think is its apparent size. Mind you, I'm not skilled in astronomy and talking as a science advisor, but rather, as a layman who has seen an extra HUGE Harvest Moon. And I hope I don't strain my back when viewing it through my legs. I am really looking forward to this event. It'll probably be cloudy, but if not, I'll give you my honest take on it.

 

[DaveC426913]

I don't know why the illusion on this special night will be so big. It won't appear to be as big as say the frontal area of a Mack Truck, but it will be big for sure.

Oh I know why. The harvest moon is a full moon and a particularly bright moon because that particular full moon, of all the 13 in the year, it is directly opposite the sun AND right near the horizon. That is the whole thing about the harvest moon and why it is so spectacular.

But its disc does not subtend any larger angle of the sky.

 

[PhanthomJay]

Oh I know why. The harvest moon is a full moon and a particularly bright moon because that particular full moon, of all the 13 in the year, it is directly opposite the sun AND right near the horizon. That is the whole thing about the harvest moon and why it is so spectacular.

OK, I'll buy it.

But its disc does not subtend any larger angle of the sky.

Agreed

 

[JDługosz]

Oh I know why. The harvest moon is a full moon and a particularly bright moon because that particular full moon, of all the 13 in the year, it is directly opposite the sun AND right near the horizon. That is the whole thing about the harvest moon and why it is so spectacular.

Hmm, how is that different than any other full moon? That's what it means for it to be full. And it will be near the horizon at some point during the day.

 

[Buckleymanor]

OK, I'll buy it.Agreed

Oh I know why. The harvest moon is a full moon and a particularly bright moon because that particular full moon, of all the 13 in the year, it is directly opposite the sun AND right near the horizon. That is the whole thing about the harvest moon and why it is so spectacular.

But its disc does not subtend any larger angle of the sky.

Then there is the real reason

 

[PhanthomJay]

Then there is the real reason

Grazie molte!

 

[seto6]

doesnt it have to do with light bending? near Earth surface?

 

[DaveC426913]

doesnt it have to do with light bending? near Earth surface?

No. In fact, refraction near the horizon works to make the Moon look smaller.

Read the thread.

 

[wortzman]

I would think that the light bends through the same angle whether the moon is above or in the horizon. The difference is only that it travels much further, getting to your eye, when it is on the horizon.

 

[DaveC426913]

I would think that the light bends through the same angle whether the moon is above or in the horizon. The difference is only that it travels much further, getting to your eye, when it is on the horizon.

Refraction is a well-known phenomenon. If you look directly down into a swimming pool (like the Moon high overhead), you see no bending of light rays, but if you look at through it at an oblique angle (like the Moon near the horizon), you see a significant bending.

So, the phenom is real enough, it just doesn't have a significant effect here.

 

[PhanthomJay]

2 more days till the Harvest Moonrise over the horizon at the Equinox... I hope it looks like this:

 

[DaveC426913]

2 more days till the Harvest Moonrise over the horizon at the Equinox... I hope it looks like this:

Please tell me you're kidding around with that pic .

 

[PhanthomJay]

Please tell me you're kidding around with that pic .

Hey, you're the humorist around here...I'm dead serious.
By the way, I looked at the moon last night a bit after moonrise...the sun had not set yet, and the moon was about 5 degrees or so above the horizon, rather dim with the sunlight still glowing...it looked quite ordinary...and white...nothing special about it...but come this Thursday, or maybe Friday, when the moon rises a bit after sunset...well, we'll see, we'll see...

 

[DaveC426913]

Hey, you're the humorist around here...I'm dead serious.

Well, the reason I'm asking is you don't really think that the Moon literally appears that large...


The pic is clearly taken without proper context - it is taken with a telephoto lens. Telephoto lenses can dramatically enlarge foreground objects in relation to background objects.

 

[DaveC426913]

Hey, you're the humorist around here...I'm dead serious.
By the way, I looked at the moon last night a bit after moonrise...the sun had not set yet, and the moon was about 5 degrees or so above the horizon, rather dim with the sunlight still glowing...it looked quite ordinary...and white...nothing special about it...but come this Thursday, or maybe Friday, when the moon rises a bit after sunset...well, we'll see, we'll see...

What you need to do is take a picture tonight, with some skyline object as refernce, then take the same pic again tomorrow night.

 

[PhanthomJay]

Big enough for you? I know its a telephoto, but As I remember the harvest moon last year, it almost did appear that big...in a day or 2, I'll know for sure...

 

[cjl]

Telephotos make it appear bigger than it actually is. What you really need to do is take a picture of it in a day or two with a known lens and camera from a known position. Then, a couple of weeks (or a month) later, take another picture with the same lens and camera. That'll show that there's effectively no difference.

 

[PhanthomJay]

Telephotos make it appear bigger than it actually is. What you really need to do is take a picture of it in a day or two with a known lens and camera from a known position. Then, a couple of weeks (or a month) later, take another picture with the same lens and camera. That'll show that there's effectively no difference.

That is very likely true...but nonetheless, it's appearance without the photo will be larger than usual for a full moon rising over the horizon...I think...I'm a bit concerned that the near full moon is presently rising about an hour before sunset, dimming its view... I saw it tonight, but arriving home late from work, it was about 20 minutes after moonrise and about 5 degrees or more above the east horizon, and the sun was also about 5 to 10 degrees above the west horizon, so the view of the moonrise was a non-event. I might have to wait until a couple of days after the equinox , when the moon rises just after sunset, to confirm my judgement of its apparent size. Time will tell.

 

[DaveC426913]

That is very likely true...but nonetheless, it's appearance without the photo will be larger than usual for a full moon rising over the horizon...I think...I'm a bit concerned that the near full moon is presently rising about an hour before sunset, dimming its view... I saw it tonight, but arriving home late from work, it was about 20 minutes after moonrise and about 5 degrees or more above the east horizon, and the sun was also about 5 to 10 degrees above the west horizon, so the view of the moonrise was a non-event. I might have to wait until a couple of days after the equinox , when the moon rises just after sunset, to confirm my judgement of its apparent size. Time will tell.

What I have yet to understand is how you will verify it.

Will you just look up at it and say "Yep, that's bigger"?

 

[PhanthomJay]

What I have yet to understand is how you will verify it.

Will you just look up at it and say "Yep, that's bigger"?

If it looks as big as the "Mothership" in that movie "Close Encounters of the Third Kind' (if you remember it), that's my verification. I would certainly encourage you to view it as well, to see what I think I mean. Certainly, I can judge a 10 inch (25 cm) pizza pie from a 16 inch (40 cm) pizza pie. The moon tonight near the horizon looked like the 10 inch/25 cm version. In a couple of nights, weather permitting, I'll judge its apparent size accordingly. And if I'm right, i'll say, "Yep, that's bigger"!

 

[DaveC426913]

If it looks as big as the "Mothership" in that movie "Close Encounters of the Third Kind' (if you remember it), that's my verification. I would certainly encourage you to view it as well, to see what I think I mean. Certainly, I can judge a 10 inch (25 cm) pizza pie from a 16 inch (40 cm) pizza pie. The moon tonight near the horizon looked like the 10 inch/25 cm version. In a couple of nights, weather permitting, I'll judge its apparent size accordingly. And if I'm right, i'll say, "Yep, that's bigger"!

Thank you. There were a couple of brain storage cells I was setting aside, giving you the benefit of the doubt, content to wait and see how this played out.

But since, as I've feared all along, your method for verification is utterly subjective and ultimately self-fulfilling (you'll see the Moon as big as you think you see it, no way for you to determine otherwise, let alone anyone else) - you've freed up those storage cells for me.


So I'll just provide my answer to tomorrow's post right now:

"Gee, there's a surprise."

 

[PhanthomJay]

But since, as I've feared all along, your method for verification is utterly subjective and ultimately self-fulfilling (you'll see the Moon as big as you think you see it, no way for you to determine otherwise, let alone anyone else) - you've freed up those storage cells for me.

See the 2 photos below for comparison. Now I know one is a telephoto, but can't you see , subjectively, that one is bigger than the other, without having to take a scale and measure it across the computer screen? Gee...

 

[cjl]

The fact that one is a telephoto completely ruins the comparison.

The thing is, in a telephoto lens, things look bigger. To have a legitimate comparison, take a picture with a camera and then take another picture in a few days with exactly the same lens settings.For example, I think you can plainly see that one of these looks bigger than the other:

http://blog.lib.umn.edu/draeg001/regionalpartnerships/snow moon.jpg
http://muneebastronomy.files.wordpress.com/2007/07/moon.jpg

That doesn't mean the moon actually changed size though - just that the image composition and camera settings are different.

 

[PhanthomJay]

The fact that one is a telephoto completely ruins the comparison.

The thing is, in a telephoto lens, things look bigger. To have a legitimate comparison, take a picture with a camera and then take another picture in a few days with exactly the same lens settings.


For example, I think you can plainly see that one of these looks bigger than the other:

http://blog.lib.umn.edu/draeg001/regionalpartnerships/snow moon.jpg
http://muneebastronomy.files.wordpress.com/2007/07/moon.jpg

That doesn't mean the moon actually changed size though - just that the image composition and camera settings are different.

You are missing the point...the full moon, when approximately the same distance from the earth, is always the same size, but due to the 'moon illusion', our brain makes it appear bigger, visually, forget the darn camera, when it's close to the horizon as compared to when its overhead. That's a given fact. What I am saying is that the Harvest Moon, when on the horizon at the Autumn Equinox (day of equal day and equal night at the Equator, September 22,23, this year) , appears bigger than other full moons when on the horizon. It may be due to its brightness and the fact that it is directly opposite the setting sun at the Equinox. I know the camera will prove otherwise, but my brain will say otherwise.

 

[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.