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Author Topic: Size of hevenly bodies at the horizon!!!  (Read 7129 times)

Offline Prathibha

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Size of hevenly bodies at the horizon!!!
« on: 05/12/2005 00:18:57 »
This is a very common question. We see that the sun, moon and other
heavenly bodies seem too appear larger at the horizon and shrink as
the approch the zenith. Including the stars/constallations seem wide
spread at the horizon.
The answer so far is that "we tend to compare them to the objects on
the earth" like buildings mountains.Is this the right answer? or is it
something to do with "Refraction" of light


 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #1 on: 05/12/2005 01:18:43 »
I remember Patrick Moore talking about this on TV many years ago. He used a coin to demonstrate that the moon is no larger at the horizon than it is when high in the sky. His explanation then was to do with the angle & nothing whatsoever to do with objects on the horizon. I've seen the moon rise when I was on the deck of a ship in the middle of the Indian Ocean, nothing on the horizon whatsoever, & the moon still appeared larger.

P.S. Welcome to the NS forums. If you wish to retain your sanity I would advise you to steer well clear of Neilp, UKMicky & myself as we can seriously damage your cognitive faculties.
The emergency exit is located on the top left of your keyboard! :D
« Last Edit: 05/12/2005 01:51:42 by DoctorBeaver »
 

Offline neilep

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Re: Size of hevenly bodies at the horizon!!!
« Reply #2 on: 05/12/2005 03:26:18 »
Hi Prathibha.

With cunning you can use this phenomenon to your advantage.

At certain times, I always make sure that I am between the horizon and my wife
:D

You may wish to use this tactic when wooing horizontally !:D

Errhmm !...please don't leave yet :) there is in fact plenty of real science here...welcome ;)

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Offline ukmicky

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Re: Size of hevenly bodies at the horizon!!!
« Reply #3 on: 05/12/2005 04:14:42 »
HI Prathibha and welcome

I think the effect is called atmospheric lensing (maybe there's a better name)and is due to refraction of light in the air. Basically the atmosphere acts like a lens on a camera enlarging your view of the sun or moon.
I believe its the same effect that can be seen when (silly people)walk through a desert and see a mirage of a town or oasis,it looks like its right in front of them but in reality can be many miles away. the effect works better when in the vicinity of water:)



Michael                                      
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #4 on: 05/12/2005 11:30:27 »
Michael - I don't think it's that. If it were then the image of the moon would be larger, which it isn't. I can't remember how Patrick Moore explained it, it was probably 20 years ago now.
 

Offline ukmicky

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Re: Size of hevenly bodies at the horizon!!!
« Reply #5 on: 06/12/2005 00:38:34 »
TO Doctorbeaver

Hi Eth

I was looking forward to knocking down your dam.:D however after doing some internet searches it seems your right.

(I HOPE YOU DONT SUPPORT LIVERPOOL):)

Its is a weird one though,with many contrary opinions on the net.

The scientists however believe its a visual trick called the Ponzo effect.

http://www.griffithobs.org/IPSMoonIllus.html

Michael                                      
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #6 on: 06/12/2005 00:58:53 »
quote:
I was looking forward to knocking down your dam


That's not very nice [:(!]

Interesting link. I'm pretty sure that's not what Patrick Moore said, but our knowledge of science has moved on quite a lot in the past 20-or-so years.
« Last Edit: 06/12/2005 01:09:17 by DoctorBeaver »
 

Offline ukmicky

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Re: Size of hevenly bodies at the horizon!!!
« Reply #7 on: 06/12/2005 01:15:19 »
Sorry doc, just couldn't resist.:DAnyway there bad for the environment aren't they.

Interestingly though, did you know that they are in the process of releasing beavers back into the uk. I can't really see it working though as the uk hasn't got the type of  uninhabited land like say in America where they will be allowed to get on with there dam building.


Michael                                      
« Last Edit: 06/12/2005 01:24:56 by ukmicky »
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #8 on: 06/12/2005 01:45:34 »
Beaver dams are not bad for the environment.They create pools for aquatic & semi-aquatic life, & their channels help to irrigate the surrounding area. The detritus they cause breaks down & fertilises the ground encouraging further growth.

Yes, I know of the campaign to reintroduce Castor Fiber into the UK.  I became aware of it a couple of years ago.
This site will tell you all about it

http://www.scotsbeavers.org/aboutus.html
 

Offline ukmicky

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Re: Size of hevenly bodies at the horizon!!!
« Reply #9 on: 06/12/2005 02:17:05 »
I didn't realize they have been releasing them in Scotland. The case i read about was in England where a wealthy landowner had released some on his property. Hopefully they won't cause to many problems and will be left alone because I suppose beavers don't have any protection in the uk as they've been missing for so many years?. Its all good though,and hopefully there reintroduction will be as successful as the red kite and the osprey.

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Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #10 on: 06/12/2005 02:42:56 »
I don't think any have actually been released yet in Scotland.
I know there's a landowner in Scotland who is also seriously considering reintroducing wolves & bears on his land. THe problem would be keeping them there, though. Beavers would not cause the same problem.

(I wonder if beavers look larger on the horizon)
 

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Re: Size of hevenly bodies at the horizon!!!
« Reply #11 on: 06/12/2005 02:47:33 »
On the subject of size, did you know that European beavers (Castor Fiber) are considerably larger than their North American cousins (Castor Canadensis)? A female Canadensis will average 50-55lbs, a female Fiber averages 70-75lbs. Castor Fiber is the world's 2nd largest rodent (2nd only to the capybara of Sth America).
 

Offline Soul Surfer

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Re: Size of hevenly bodies at the horizon!!!
« Reply #12 on: 06/12/2005 08:59:02 »
What we "see" is a mental model of our surroundings created by our brain and has very little to do with the optics of our eyeballs or anything else. Our visual field is a bit like the map of our physical sensations where the amount of space and detail of our touch sensations depends on their importance in our daily life.  When we are standing up. most of the important things are around the horizon, slightly less important things are down at our feet and the least important things are up high so the horizon looks biggest things at our feet look normal and things up in the sky look small.  try looking at the moon very high up in the sky while lying down so that your body is positioned to put the moon in a horizobntal plane with respect to your own body you will find it tlook large.  Another effect of this illusion is that uphill slopes look less steep than downhill slopes.

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Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #13 on: 06/12/2005 10:59:10 »
quote:
try looking at the moon very high up in the sky while lying down so that your body is positioned to put the moon in a horizobntal plane with respect to your own body you will find it tlook large


I've slept under the stars on numerous occasions & I can't say I've ever noticed that.

 
quote:
What we "see" is a mental model of our surroundings created by our brain and has very little to do with the optics of our eyeballs or anything else


Although your statement is essentially correct, I think, maybe, you're confusing sensation/perception with perception/pre-conception. In general, we see what we expect to see - or, at least, that is how we interpret it. This gives rise to the "I could see something wasn't right but I didn't know what it was" syndrome. Our eyes are registering something that should look different from how it actually looks, but our brain wants to interpret what we see as being how we expect it to look. This causes a conflict and, hence, confusion.
Various theories have been put forward for this, but I believe the most convincing is the mental schema. We automatically try to fit what we percieve into a pre-existing schema & will distort reality, if necessary, to do so.
This theory of schemas applies to many areas of psychology, including prejudices, so it is not merely a sensory phenomenon.
As for the slope seeming steeper from the top, that can also be explained in psychological terms. An inexperienced fell-walker will see a slope as being steeper than would a veteran mountain-climber. It has been postulated that it is the brain's way of indicating a possible danger. An experienced mountain-climber will realise that the slope is not dangerous & hence not take much notice of it. But to someone less-experienced, the threat may be a lot more real.
This same mechanism can explain why slopes look steeper from the top than from the bottom - no-one gets injured walking uphill.
There is also a possible physical explanation for this. Standing at the top of a sheer cliff & looking down, your eyes are more likely to be in line with the cliff face than if you are at the bottom looking up. Being at the bottom will add a slight angle to how you view the face. Our vision is very sensitive to angles, which is how we see in 3D, so this could well be more of a factor than you may at first think.
« Last Edit: 06/12/2005 11:02:17 by DoctorBeaver »
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #14 on: 06/12/2005 11:03:46 »
quote:
(I HOPE YOU DONT SUPPORT LIVERPOOL)


Good Lord no. Your hubcaps are safe with me! [:o)]
 

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Re: Size of hevenly bodies at the horizon!!!
« Reply #15 on: 07/12/2005 13:18:54 »
Quote
Originally posted by ukmicky

HI Prathibha and welcome

I think the effect is called atmospheric lensing (maybe there's a better name)and is due to refraction of light in the air. Basically the atmosphere acts like a lens on a camera enlarging your view of the sun or moon.

Hi All,
there is a real optical effect,(not psychological), where heavenly bodies appear squashed (oblate) when they are near the horizon.
Here is an image of the transit of Venus which I took about an hour after sunrise, (through cloud).
 Venus, the black blob, is noticably oblate, not circular.

As UKMicky said this is due to refraction by Earth's atmosphere.
« Last Edit: 07/12/2005 13:20:48 by ROBERT »
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #16 on: 07/12/2005 13:30:45 »
quote:
I think the effect is called atmospheric lensing (maybe there's a better name)and is due to refraction of light in the air. Basically the atmosphere acts like a lens on a camera enlarging your view of the sun or moon.


That's not the case. The image of the moon at the horizon isn't actually larger than when it's high in the sky, it only appears to be so. Lensing would cause an actual increase in its size.

I wasn't intending to imply that this effect is psychological. My post on that was in reply to the point raised about slopes looking steeper from the top.

By the way, that's a great photo. It's as good as I've anything I've seen. Do you do much astronomical photography?
« Last Edit: 07/12/2005 13:33:37 by DoctorBeaver »
 

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Re: Size of hevenly bodies at the horizon!!!
« Reply #17 on: 07/12/2005 15:04:13 »
I'm not denying that there is an illusion, I believe that there is a psychological effect which makes astronomical bodies APPEAR larger when they are near the horizon.
I was merely saying that there is an optical effect which distorts the light from heavenly bodies when they are near the horizon: the Earth's atmosphere acts like an anamorphic lens. This is a lensing effect due to refraction as UKMickey said.  

In reality (optically) the size (area) of heavenly bodies near the horizon is smaller than at zenith,
 because their vertical dimension is compressed when they are oblate.

Note: lenses do not necessarily magnify, they can make objects appear smaller.
« Last Edit: 09/12/2005 14:04:22 by ROBERT »
 

Offline DoctorBeaver

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Re: Size of hevenly bodies at the horizon!!!
« Reply #18 on: 07/12/2005 15:56:13 »
But the image of the moon at the horizon is identical to when it's high in the sky. Therefore nothing is actually happening to the light.

 
quote:
Note: lenses do not necessarily magnify, they can make objects appear smaller.


Indeed. I stand corrected
 

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Re: Size of hevenly bodies at the horizon!!!
« Reply #19 on: 07/12/2005 16:08:37 »
 

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Re: Size of hevenly bodies at the horizon!!!
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