Moon & Sun: Newsletter Challenge (05/23/06)

Prismatic effect of the atmosphere at low angles of incidence provide a distortion of the image of the moon/sun which appears larger than when viewed straight up.

The atmosphere also filters out the shorter wavelength of light at those angles and makes the moon and sun look redder than normal.

Surely a prismatic effect would just make the image elliptical?

Correct. Watch a sun set or sun rise and the disk is definitely flattened. It is best seen when setting or rising over a large body of water.

Another effect is the aparant angular speed of the sun or moon appears to speed up at the horizon. Watching a sunrise or set on the ocean is really amazing.

The atmosphere will bend the light so you will actually see the sun rise before it would normally happen if there was no atmosphere (i.e., on the moon). In the case of a sun set you see the sun after it has technically pased below the horizon.

Absolutely correct. At low angles from the horizon the atmosphere produces a marked lens effect magnifying celestial objects.

That is a better way to describe it. The atmosphere actually acts as a lens. Good catch!

Ah, but you've not carried your...old time conjecture to its logical conclusion....Why do not the other celestial objects--planets and stars-- also appear to be magnified? And, consider this ramification of the "air lens" proposition. When viewing celestial objects it is universally agreed, that it's preferable to view them through a perspective in which the atmosphere is least thick. But this flies in the face of the air lens phenomenon--if it were true. One would want to make observations of objects near the horizon--so as to exploit the air lens phenomenon--again, if it were true.

It appears, at least from this angle, that the difference in size is really psychological after all!

It is, as Stupott described. Good work!

The refraction of the atmosphere will distort the shape when low on the horizon, giving it a slightly flattened look. However, the refraction from the atmosphere actually makes the moon or sun's size slightly smaller by about 1.7%, not larger!

From what I just read it appears that some people perceive this effect more so than others.

Looks like the whole thing has been blown out of proportion.

The seeming size increase is merely an illusion. It's a psychological effect that comes from our perception that objects visible at, or near, the horizon are far away and therefore must be large if we're able to see them at all. The illusion also results from the juxtaposition of the sun (or moon for that matter) near familiar terrestrial landmarks such as distant trees or houses, which gives them dimensionality. When high in the heavens, the tiny half-degree Sun and Moon are dwarfed by the immensity of the 180° hemisphere of the sky. That it's really always the same size can be proven by viewing the Moon (probably best not to try this with the sun!!!)through a cardboard tube when it's low and again when it's high. You'll see that it appears the same size in both cases.

Don't forget those telephoto lenses they use in commercials to make the sun or moon look huge next to their new car. ;-)

Had'nt thought of that..........there you go, then ;-)

I recall this illusion from my childhood. I was told to look at the moon when it's near the horizon and compare it to the size of my fingernail, held at arm's length. Then look at it again when it's overhead. Pretty much the same. There may be subtle differences due to refractive effects, but the overwhelming difference is psychological; having a point of reference (or photographic trickery).

No no no...

The sun and moon are actually BIGGER for other people. They shrink when they are on your side of the earth. This is due to the natural law pertaining to fences and the greenness of grass.
(sorry...)

He's right: all the foregoing refraction stuff is off the mark. The moon appears large near the horizon because it's viewed through a biological eye connected to a biological brain--a brain that doesn't like to believe that it can be fooled.

Here's how all you in the "Refraction Camp" can convince yourselves of the fallacy in the Refraction theory. Place yourselves in a darkened location while watching the mood climb from the horizon. Place your hands together, with thumbs outstretched but fingers closed, in front of your faces. Note that you can expand and diminish the size of the triangular opening defined by your thumbs and forefingers by sliding your hands medially and laterally with respect to the midline plane of your body--the midline of your noses. Orient your hands, and move them to adjust the size of the triangular opening such that (1) all incidental light is blocked from your view; and (2) you see only the moon through the opening (let's call it the pinhole) between your hands through which you are viewing the moon's light and only the moon's light. You will quickly realize that, having removed from your field of view all light sources but the moon, including the horizon--including any other relatively near-distance reference points--that the moon has shrunk to the same size it would appear if viewed overhead. Repeat this as the moon rises to convince yourselves that this...mis-perception phenomenom always holds true; that the refractivity thing is not the answer to the challenge.

Oh yes, please don't try the pinhole test while viewing the sun. You'll just have to take that on faith.

CowAnon wrote:

"Place yourselves in a darkened location while watching the mood climb from the horizon. Place your hands together, with thumbs outstretched but fingers closed, in front of your faces. Note that you can expand and diminish the size of the triangular opening defined by your thumbs and forefingers by sliding your hands medially and laterally with respect to the midline plane of your body--the midline of your noses. Orient your hands, and move them to adjust the size of the triangular opening such that (1) all incidental light is blocked from your view; and (2) you see only the moon through the opening (let's call it the pinhole) between your hands through which you are viewing the moon's light and only the moon's light."

I see YOUR HEAD between my thumb and forefingers and say:

"I'm squeezing your head, I'm squeezing your head!"

BTW, my mood never climbs from the horizon! ;-)

I think it is:

"I'm crishing you! I'm crushing you" Then he gets beat up and undergoes massive rehab.

Kids in the Hall.

Okay,OKAAY! I knew right after I posted it was "crushing", not "squeezing", but it definitely was "your head", not just "you", because he would put his fingers around the guy's head and that was all you saw through his fingers, then he closed them, visually "crushing your head!"

Now for 15 extra bonus points, which one of the "Kids in the Hall" was it who uttered those immortal words?

No it's "I squash your head"

STL Engineer (646) wrote:

"I see YOUR HEAD between my thumb and forefingers and say:

"'I'm squeezing your head, I'm squeezing your head!'

"BTW, my mood never climbs from the horizon! ;-)"

LOL... Oooh! But, please. Wait until I've taken my med's--and eaten my 30 tacos. I've got Risperidone and Valproic sodium. Want some? For that mood thing? (-;

Public television had a program about this a few years ago. A scientist came to the same conclusion as to why the moon appears larger to us when on the horizon. I believe he did testing with viewing distant objects from someplace on Long Island (viewing across water).

It is easy to prove to yourself that the moon is no larger on the horizon than when high in the sky. If I hold my thumb up at arms length, the diameter of the moon is about 1/2 the width of my thumb whether it is on the horizon or high in the sky.

You get a similar effect when taking pictures of mountains. In photos, mountains always appear much smaller than they seemed than when you took the picture.

In my pictures the people are smaller, too. I have some of basketball players close to 7 feet tall and they fit on a 3"x5" print.

If you are going to use somebody's material, you should give them credit for it. In this case it would be Picasso.

When you view object high in the sky you have nothing to which to compare them to gage their size. At the horizon they "appear" to be larger but it is in essence an optical illusion.

Agreed, but nobody seemed to notice this fact: the moon's angular size when directly overhead is 1.7% larger than when the moon is near the horizon! Directly overhead, the distance to the moon is shorter by one Earth radius compared to when viewed near the horizon.

Good point. However, if you had to gage two objects that were in different directions, one 100 inches long and the other 101.7 inches long, I would not expect anyone's choice of which were longer to be much better than pure chance. Some things are below the threshold of our natural metric ability. This is why we use instruments to extend the accuracy of our perceptions which fall prey to many demons.

Breathe deep the gathering gloom,
Watch lights fade from every room.
Bedsitter people look back and lament,
Another day's useless energy spent.
Impassioned lovers wrestle as one,
Lonely man cries for love and has none.
New mother picks up and suckles her son,
Senior citizens wish they were young.

'Cold hearted orb that rules the night
Removes the colors from our sight.
Red is grey and yellow white,
But we decide which is right.
And which is an illusion?

- Graeme Edge

If the atmosphere was a lens effect it would make the object look oval shaped (like some sunset pictures)due to the vertical gradient of the atmospere. Side to side there's little change.

I've seen this debated as a psychological issue, physiological issue (eye angle!), optical illusion and the ACTUAL refractive explanation. If it were other than refractive it would ALWAYS look large when near the horizon, but the moon on certain nights is remarkable as the "harvest moon", and appears huge as it rises. I believe the effect is enhanced by the relative humidity which adds to the magnifying effect. That the enlargement is due to this natural phenomenon is easily proven by photographing the moon near the horizon and when overhead (it helps to use maximum zoom on both photos to enhance the size difference). The sunsets in the tropics also exhibit this effect (high humidity over the tropical horizon).