Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 14/12/2006 04:35:37
-
Dear All
I received this email (below) recently, and I'd be grateful for an evidence-based argument for why the lensing effect of the atmosphere is wrong.
Chris
"Love your show!
And I love your mind, Chris.
But your answer to the question of why does the moon appear larger on
the horizon (you said it was an optical illusion) is surely wrong.
While the context of trees might exaggerate the phenomenon, clearly my 8th
grade science teacher was right when he told me that the greater amount
of atmostphere that we must look through to see the moon on the horizon
is magnifying it's image. The higher the moon rises, the less
atmosphere distorts its image.
No? If atmosphere isn't causing the enlarged image, than what?
Because surely the huge ruddy lunar image we're seeing is being distorted by
something prior to our perception of it.
Thanks for all the great shows!!!
Eric"
-
I think it has been well established that apart from a slight quashing effect there is no real change in the measured size of the moon when it low on the horizon, it is easy to check this by comparing its size to that of a finger held at arms length or more precisely by taking photographs or using a suitably equipped telescope
-
Thanks. I'm actually looking for some sums and numbers that would disprove this, but my optics / physics isn't good enough! Can anyone help? Where's Dave when you need him?
Chris
-
It's an optical illusion:
our brain thinks the sky is flat. Imagine to look at a circular bright spot on the ceiling of a very long room. If the spot is far from the viewer, it appears shorter, because of the different visual angle. What would happen if, instead, it would appear of the same dimension? Our brain would conclude that the spot becomes as bigger as it's far. I hope my english has been good enough to explain the concept.
-
I remember Patrick Moore addressing this in an episode of The Sky At Night. He used an old halfpenny to prove that the moon is indeed the same size no matter where in the sky it is.
-
As a firm believer in empirical study I just conducted an experiment with an orange and a grape !!
Result ?...as the grape gradually moved towards the horizon I saw it breathe in a few breaths as if to impress a girlie on a beach !!..
Conclusion...the Moon likes to impress us as it moves towards the southern hemisphere. This is confirmed by the big ' sigh ' as it finally breathes out when out of view !
QED.
You're welcome.
-
I think that it's all to do with the space allocated to images in our brain things around the horizon are important to us and get more attention than things right over our heads and so the brain allocates the more space in our model of the world about us and so things near the horizon look bigger.
-
If you look at the moon through a long tube when it is near the horizon it will suddenly look small again as you loose the sense of scale. Have you seen the optical illusions where a room is set up with exagerated paralax, so things at the back are tiny, and things at the front are huge, if someone walks though it they appear to change size, because you are judging the distance by the size of objects you know about.
-
I think that it's all to do with the space allocated to images in our brain things around the horizon are important to us and get more attention than things right over our heads and so the brain allocates the more space in our model of the world about us and so things near the horizon look bigger.
I would consider a boulder falling from above my head to be more important than 1 on the horizon
-
Dear All
I received this email (below) recently, and I'd be grateful for an evidence-based argument for why the lensing effect of the atmosphere is wrong.
Chris
Musings on the Sun
Concerning the Apparent Difference Between the Size of the Sun at Sunset and Noon
By Capt. Raleigh C. Willems, USAF Training Analysis and Development Division, Mather Air Force Base
Blast From the Past
The following article originally appeared in the ION Journal, NAVIGATION in June 1952 (Vol. 3, No 4).
It appears that some navigators do not understand why the sun appears larger at sunset and sunrise than it does at noon. One of the more popular misconceptions is that it is caused by refraction. Actually, refraction makes the sun appear smaller at rising or setting, because there is more refraction acting on the lower limb of the sun (because of its lower altitude) than on the upper limb. You can check this. Measure the horizontal and vertical diameter of the sun at noon with a transit. You will find it approximately 32 minutes of arc each way. Repeat this measure- ment at sunset. The horizontal diameter will still be 32 minutes of arc. The vertical diameter will be approximately 27 minutes.
Our eyes give our brains a false impression of apparent size for the following reason: To the observer on the surface of the earth, the sky looks like a large, flat dome instead of a hemisphere. Points on this flat dome which are at the observer’s zenith appear to be nearer to him than points on the rim of the dome (the observer’s horizon). Haze near the horizon aids the illusion of greater distance. The human eye automatically makes allowance for distant objects appearing smaller than closer objects of the same size. The image of the sun received by the eye is the same size (discounting the refraction effect) at both the horizon and the zenith. However, because of the illusion of greater distance at the horizon, the eye makes an adjustment in the message transmitted to the brain and the sun appears larger.
You can check this illusion in an effective, if undignified, manner. Face away from the setting sun, bend down, and look at it from between your legs. The sun will appear smaller than it did at noon.
http://www.ion.org/newsletter/v12n1.html#feature2
The "looking from between legs" experiment proves that the apparent greater size of the sun is psychological not atmospheric.
-
When you are gazing at the Sun from between your legs take Care that that your dark glasses do not slip to your forehead resulting in burnt retinas.
-
You can also use the "looking between your legs" method to spot several planets. Uranus is particularly visible.
Chris
-
You can also use the "looking between your legs" method to spot several planets. Uranus is particularly visible.
Chris
Chris !!..LOL
This man is the best !!
-
You can also use the "looking between your legs" method to spot several planets. Uranus is particularly visible.
Chris
tut!
-
Ye Gods!
Have you forgotten refractive index, which depends on density?
The sun/moon have sufficient apparent diameter that the amount of deflection of the light beam is significantly different from the top edge (lower density atmosphere = small deflection)to the bottom edge (more dense atmosphere = large deflection).
sum = objects appear larger, red shift in apparent frequency.
The apparent size of the sun and moon vary constantly depending on the angle above the horizon.
Graham
-
This is true, Graham, but the difference it makes is tiny - actually about 2%, and it actually makes the moon smaller, not larger!
However, I've subsequently come across this excellent graphical representation of the phenomenon, which is from a webpage addressing the issue by Donald E. Simanek (http://www.lhup.edu/~dsimanek/3d/moonillu.htm).
[ Invalid Attachment ]
The thick black horizontal line in each case is the same length, confirmed by measurement. But they certainly don't look that way!
Chris