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Main
Date: 22 Jun 2007 03:35:36
From: brian
Subject: is this an optical illusion?
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Sometimes, when I see the moon lit by earthshine, against a twilit
sky, it looks as if the unlit part of the moon is actually darker than
the background sky - as if the moon is silhouetted against the sky.
This shouldn't be physically possible.
Has anyone here noticed this? Is it an optical illusion, and can
anyone here explain it?
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Date: 25 Jun 2007 15:04:05
From: canopus56
Subject: Re: is this an optical illusion?
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On Jun 25, 3:10 pm, Guy Macon <http://www.guymacon.com/ > wrote:
> While it is true that the twilight sky has significant residual
> brightness that the midnight sky lacks, it is not true that this
> residual brightness occurs in the sky around the moon and not
> in the dark side of the moon. <snip>
> Guy Macon
Good, common sense point, Guy. So the dark side of the Moon should
always have as a minimum brightness, the sky's brightness, as we see
the Moon during its three-quarter daylight passage. - Canopus56
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Date: 25 Jun 2007 10:30:51
From: canopus56
Subject: Re: is this an optical illusion?
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On Jun 22, 4:35 am, brian <brianqsma...@yahoo.com > wrote:
> Sometimes, when I see the moon lit by earthshine, against a twilit
> sky, it looks as if the unlit part of the moon is actually darker than
> the background sky - as if the moon is silhouetted against the sky.
> This shouldn't be physically possible.
> Has anyone here noticed this? Is it an optical illusion, and can
> anyone here explain it?
It is not an optical illusion. Until astronomical twilight about an
hour after sunset, the twilight sky has significant residual
brightness. This residual brightness can be brighter than the dark
side of the Moon. In terms of the photographer's B light scale and an
EV value, the setting sky has brightnesses of:
Sunset, 10 minutes after 120, 9
Sunset, 30 minutes after 0.93, 2
Civil twilight (about 30 min after sunset) 0.46, 1
Night, away from city lights, subject under full moon. 0.3, -3
Night, away from city lights, subject under half moon. __, -4
Night, away from city lights, subject under crescent moon __, -5
Night, away from city lights, subject under starlight only. __, -6
Rural night sky 0.00001, -15
Although I do not have a specific B value for the dark terminator side
of the Moon, during a partially lunar eclipse, the umbra and penumbra
portions of the Moon have a B of about 0.25. This is darker than the
brightness of civil twilight.
After astronomical twilight, the dark side of the Moon is brighter
than the background sky, which can easily be seen even in light
polluted skies using binoculars.
- Canopus56
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Date: 25 Jun 2007 21:10:12
From: Guy Macon
Subject: Re: is this an optical illusion?
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canopus56 wrote:
>
>brian wrote:
>>
>> Sometimes, when I see the moon lit by earthshine, against a twilit
>> sky, it looks as if the unlit part of the moon is actually darker than
>> the background sky - as if the moon is silhouetted against the sky.
>> This shouldn't be physically possible.
>> Has anyone here noticed this? Is it an optical illusion, and can
>> anyone here explain it?
>
>It is not an optical illusion. Until astronomical twilight about an
>hour after sunset, the twilight sky has significant residual
>brightness. This residual brightness can be brighter than the dark
>side of the Moon.
While it is true that the twilight sky has significant residual
brightness that the midnight sky lacks, it is not true that this
residual brightness occurs in the sky around the moon and not
in the dark side of the moon. For that to be true, the residual
brightness would have to be a change in the brightness of the
sky behind the moon. In reality, the residual brightness is
an atmospheric effect that is between the sky and the observer
as well as being between the moon and the observer.
--
Guy Macon
<http://www.guymacon.com/ >
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Date: 28 Jun 2007 22:46:28
From: mc
Subject: Re: is this an optical illusion?
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"Guy Macon" <http://www.guymacon.com/ > wrote in message
news:j86dnaMNS5Srsh3bRVn_vw@giganews.com...
> canopus56 wrote:
>>
>>brian wrote:
>>>
>>> Sometimes, when I see the moon lit by earthshine, against a twilit
>>> sky, it looks as if the unlit part of the moon is actually darker than
>>> the background sky - as if the moon is silhouetted against the sky.
>>> This shouldn't be physically possible.
>>> Has anyone here noticed this? Is it an optical illusion, and can
>>> anyone here explain it?
>>
>>It is not an optical illusion. Until astronomical twilight about an
>>hour after sunset, the twilight sky has significant residual
>>brightness. This residual brightness can be brighter than the dark
>>side of the Moon.
>
> While it is true that the twilight sky has significant residual
> brightness that the midnight sky lacks, it is not true that this
> residual brightness occurs in the sky around the moon and not
> in the dark side of the moon. For that to be true, the residual
> brightness would have to be a change in the brightness of the
> sky behind the moon. In reality, the residual brightness is
> an atmospheric effect that is between the sky and the observer
> as well as being between the moon and the observer.
Bingo. The twilight is in front of the moon, not behind it, so it should
add to the brightness of the dark side of the moon.
The question could certainly be settled by taking a photograph.
A similar effect has been reported with Venus seen through a telescope at
crescent phase. With Venus, silhouetting against the outermost solar corona
has been advanced as a possible explanation. But it might well be an
illusion.
I wonder if color is part of the illusion. The earthlit part of the moon is
gray and has more red in it than the surrounding twilight. Does red light
perhaps desensitize the eye to blue to some extent? Going further -- I seem
to recall that the unlit side of Venus is fairly bright in the infrared.
Does infrared (even at invisible wavelengths) affect the eye's sensitivity
to visible light? I seem to recall that some types of film are partly
desensitized by infrared.
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Date: 22 Jun 2007 12:01:36
From: thad@thadlabs.com
Subject: Re: Is this an optical illusion?
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On Jun 22, 11:45 am, Chris L Peterson <c...@alumni.caltech.edu > wrote:
> On Fri, 22 Jun 2007 18:28:36 +0100, "OG" <o...@gwynnefamily.org.uk>
> wrote:
>
> >This is one of the very best examples I've seen of this
> >http://www.popularscience.co.uk/features/feat16.htm
>
> Very good. Look at these,http://www.nature.com/neuro/journal/v2/n11/full/nn1199_1010.html, for
> similar examples with color.
>
> Basically, the eye/brain is very poor at discriminating absolute
> intensities or colors.
Some others in the same vein:
<http://gpsinformation.info/main/Moving.html >
<http://gpsinformation.info/main/JYimages.html >
<http://gpsinformation.info/main/grayscale.html >
<http://gpsinformation.info/main/humor.htm >
Another one that really surprised me the first time I saw it was
"a" Moon illusion. Not the one when the Moon appears larger
on the horizon than when higher, but when craters look like
mountains. Best I can determine it's caused by a lighting
effect (the position of the Moon with respect to the Sun); it's
quite amazing.
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Date: 22 Jun 2007 13:52:00
From: Sam Wormley
Subject: Re: is this an optical illusion?
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brian wrote:
> Sometimes, when I see the moon lit by earthshine, against a twilit
> sky, it looks as if the unlit part of the moon is actually darker than
> the background sky - as if the moon is silhouetted against the sky.
> This shouldn't be physically possible.
>
The earth shine gives visual detail to the unlit surface of the moon
independent of the darkness of the sky. As long as the sky is not too
bright, the eye can see the unlit surface of the moon.
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Date: 22 Jun 2007 12:19:47
From: Guy Macon
Subject: Is this an optical illusion?
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brian wrote:
>
>Sometimes, when I see the moon lit by earthshine, against a twilit
>sky, it looks as if the unlit part of the moon is actually darker than
>the background sky - as if the moon is silhouetted against the sky.
>This shouldn't be physically possible.
>
>Has anyone here noticed this? Is it an optical illusion, and can
>anyone here explain it?
Next time you see that, grab a camera and take a picture.
If it's not the same as what you see, it's an optical illusion.
If it *is* the same as what you see, then I would really like
someone to explain the effect to me. It seems impossible.
--
Guy Macon
<http://www.guymacon.com/ >
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Date: 22 Jun 2007 07:40:08
From: Louis Boyd
Subject: Re: Is this an optical illusion?
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Guy Macon wrote:
> brian wrote:
>
>>Sometimes, when I see the moon lit by earthshine, against a twilit
>>sky, it looks as if the unlit part of the moon is actually darker than
>>the background sky - as if the moon is silhouetted against the sky.
>>This shouldn't be physically possible.
>>
>>Has anyone here noticed this? Is it an optical illusion, and can
>>anyone here explain it?
>
>
> Next time you see that, grab a camera and take a picture.
> If it's not the same as what you see, it's an optical illusion.
>
> If it *is* the same as what you see, then I would really like
> someone to explain the effect to me. It seems impossible.
I haven't personally noticed that phenomena. It might be possible if
the Moon is positioned in line with one of the brighter parts of the
Milky Way Galaxy (in the general area of Sagittarius) and the area of
reflection of sunlight off of the Earth toward the Moon is from a land
mass less reflective than an ocean. Sometimes "Earth shine" is rather
weak.
It might show up best in late twilight when the sky glow from the Sun
illuminating the upper atmosphere is less bright than the light from the
Milky Way but the angle between the Sun and Moon is small so only a thin
crescent is illuminated on the Moon. Zodiacal light could also add to
the background illumination with the right alignment.
It might be a fun project to try to photograph the phenomena. I'm not
saying it happens, just offering a possible explanation if it does.
Lou Boyd
Director - Fairborn Observatory
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Date: 22 Jun 2007 15:29:50
From: Phil Hobbs
Subject: Re: Is this an optical illusion?
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Louis Boyd wrote:
> Guy Macon wrote:
>> brian wrote:
>>
>>> Sometimes, when I see the moon lit by earthshine, against a twilit
>>> sky, it looks as if the unlit part of the moon is actually darker than
>>> the background sky - as if the moon is silhouetted against the sky.
>>> This shouldn't be physically possible.
>>>
>>> Has anyone here noticed this? Is it an optical illusion, and can
>>> anyone here explain it?
>>
>>
>> Next time you see that, grab a camera and take a picture. If it's not
>> the same as what you see, it's an optical illusion.
>>
>> If it *is* the same as what you see, then I would really like someone
>> to explain the effect to me. It seems impossible.
>
> I haven't personally noticed that phenomena. It might be possible if
> the Moon is positioned in line with one of the brighter parts of the
> Milky Way Galaxy (in the general area of Sagittarius) and the area of
> reflection of sunlight off of the Earth toward the Moon is from a land
> mass less reflective than an ocean. Sometimes "Earth shine" is rather
> weak.
The Milky Way is orders of magnitude dimmer than twilight. It's
invisible even at midnight when you're near a city. Where I am, I have
to go way out into the country to see it at all.
There's a strong optical illusion operating--the one that makes your eye
connect up circles and squares when only portions of them are actually
visible.
If you think about the geometry, the portion of the sunlit Earth seen by
any point on the near side of the Moon is just about equal to the
proportion of the dark part of the Moon as seen from the Earth. At full
moon, the dark hemisphere of the Earth faces the Moon, and at new moon,
it's the bright hemisphere of the Earth. At half moon, the Moon sees a
half-Earth. Except near full moon, then, the brightness of earthshine
isn't very different from night to night. If there's a large area of
dark Moon showing, then the earthshine is bright.
Cheers,
Phil Hobbs
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Date: 25 Jun 2007 12:38:10
From: danek
Subject: Re: Is this an optical illusion?
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Phil Hobbs wrote:
> Louis Boyd wrote:
>> Guy Macon wrote:
>>> brian wrote:
>>>
>>>> Sometimes, when I see the moon lit by earthshine, against a twilit
>>>> sky, it looks as if the unlit part of the moon is actually darker than
>>>> the background sky - as if the moon is silhouetted against the sky.
>>>> This shouldn't be physically possible.
>>>>
>>>> Has anyone here noticed this? Is it an optical illusion, and can
>>>> anyone here explain it?
>>>
>>>
>>> Next time you see that, grab a camera and take a picture. If it's
>>> not the same as what you see, it's an optical illusion.
>>>
>>> If it *is* the same as what you see, then I would really like someone
>>> to explain the effect to me. It seems impossible.
>>
>> I haven't personally noticed that phenomena. It might be possible if
>> the Moon is positioned in line with one of the brighter parts of the
>> Milky Way Galaxy (in the general area of Sagittarius) and the area of
>> reflection of sunlight off of the Earth toward the Moon is from a land
>> mass less reflective than an ocean. Sometimes "Earth shine" is rather
>> weak.
>
> The Milky Way is orders of magnitude dimmer than twilight. It's
> invisible even at midnight when you're near a city. Where I am, I have
> to go way out into the country to see it at all.
>
> There's a strong optical illusion operating--the one that makes your eye
> connect up circles and squares when only portions of them are actually
> visible.
>
> If you think about the geometry, the portion of the sunlit Earth seen by
> any point on the near side of the Moon is just about equal to the
> proportion of the dark part of the Moon as seen from the Earth. At full
> moon, the dark hemisphere of the Earth faces the Moon, and at new moon,
> it's the bright hemisphere of the Earth. At half moon, the Moon sees a
> half-Earth. Except near full moon, then, the brightness of earthshine
> isn't very different from night to night. If there's a large area of
> dark Moon showing, then the earthshine is bright.
>
> Cheers,
>
> Phil Hobbs
To add to the geometry portion of the discussion, the reflectivity of
earth is dependent upon the percentage of cloud cover. More clouds more
light reflected. Not sure if it is enough to see a difference in moon
illumination though.
P. Danek
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Date: 22 Jun 2007 08:15:07
From: Louis Boyd
Subject: Re: Is this an optical illusion?
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Louis Boyd wrote:
> Guy Macon wrote:
>
>> brian wrote:
>>
>>> Sometimes, when I see the moon lit by earthshine, against a twilit
>>> sky, it looks as if the unlit part of the moon is actually darker than
>>> the background sky - as if the moon is silhouetted against the sky.
>>> This shouldn't be physically possible.
>>>
>>> Has anyone here noticed this? Is it an optical illusion, and can
>>> anyone here explain it?
>>
>>
>>
>> Next time you see that, grab a camera and take a picture. If it's not
>> the same as what you see, it's an optical illusion.
>>
>> If it *is* the same as what you see, then I would really like someone
>> to explain the effect to me. It seems impossible.
>
>
> I haven't personally noticed that phenomena. It might be possible if
> the Moon is positioned in line with one of the brighter parts of the
> Milky Way Galaxy (in the general area of Sagittarius) and the area of
> reflection of sunlight off of the Earth toward the Moon is from a land
> mass less reflective than an ocean. Sometimes "Earth shine" is rather
> weak.
>
> It might show up best in late twilight when the sky glow from the Sun
> illuminating the upper atmosphere is less bright than the light from the
> Milky Way but the angle between the Sun and Moon is small so only a thin
> crescent is illuminated on the Moon. Zodiacal light could also add to
> the background illumination with the right alignment.
>
> It might be a fun project to try to photograph the phenomena. I'm not
> saying it happens, just offering a possible explanation if it does.
>
> Lou Boyd
> Director - Fairborn Observatory
On further though the brightness of the Earths atmosphere at twilight
doesn't affect whether the background of the Milky Way with or without
Zodiacal light will be brighter or fainter then the sunlight reflected
off of the Earth to the Moon then back to the Earth. All light from the
atmosphere can do is is obscure the observation by overwhelming it, not
by change which will be brighter. The same is true for the size of the
lunar crescent at the time of the observation.
It should be easy to tell if the phenomena is possible using an ordinary
digital camera with manual controls by taking a photo of the Milky Way
and one of lunar "Earthshine" with the same camera settings and
measuring the relative brightness level of each. If the Milky Way is
significantly brighter then the phenomena will happen occasionally.
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Date: 22 Jun 2007 13:54:43
From: Chris L Peterson
Subject: Re: Is this an optical illusion?
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On Fri, 22 Jun 2007 12:19:47 +0000, Guy Macon <http://www.guymacon.com/ >
wrote:
>Next time you see that, grab a camera and take a picture.
>If it's not the same as what you see, it's an optical illusion.
An optical illusion can be just as strong in an image as in the real
view. But with an image, you can actually measure the intensity in
different spots and verify that the effect is an illusion.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 22 Jun 2007 18:28:36
From: OG
Subject: Re: Is this an optical illusion?
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"Chris L Peterson" <clp@alumni.caltech.edu > wrote in message
news:60ln731t9ddb8973bb054anb06qup85ccn@4ax.com...
> On Fri, 22 Jun 2007 12:19:47 +0000, Guy Macon <http://www.guymacon.com/>
> wrote:
>
>>Next time you see that, grab a camera and take a picture.
>>If it's not the same as what you see, it's an optical illusion.
>
> An optical illusion can be just as strong in an image as in the real
> view. But with an image, you can actually measure the intensity in
> different spots and verify that the effect is an illusion.
>
This is one of the very best examples I've seen of this
http://www.popularscience.co.uk/features/feat16.htm
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Date: 22 Jun 2007 18:45:31
From: Chris L Peterson
Subject: Re: Is this an optical illusion?
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On Fri, 22 Jun 2007 18:28:36 +0100, "OG" <owen@gwynnefamily.org.uk >
wrote:
>This is one of the very best examples I've seen of this
>http://www.popularscience.co.uk/features/feat16.htm
Very good. Look at these,
http://www.nature.com/neuro/journal/v2/n11/full/nn1199_1010.html , for
similar examples with color.
Basically, the eye/brain is very poor at discriminating absolute
intensities or colors.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 23 Jun 2007 20:00:40
From: Androcles
Subject: Re: Is this an optical illusion?
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"Chris L Peterson" <clp@alumni.caltech.edu > wrote in message
news:kj5o739ghgil1a72fedd0e8psbi3ijt2jk@4ax.com...
: On Fri, 22 Jun 2007 18:28:36 +0100, "OG" <owen@gwynnefamily.org.uk >
: wrote:
:
: >This is one of the very best examples I've seen of this
: >http://www.popularscience.co.uk/features/feat16.htm
:
: Very good. Look at these,
: http://www.nature.com/neuro/journal/v2/n11/full/nn1199_1010.html , for
: similar examples with color.
:
: Basically, the eye/brain is very poor at discriminating absolute
: intensities or colors.
:
Yes, that is true.
There are even crackpots who wildly imagine a star can be occulted
by a dark body as large as the star itself and never realize the cause
of the optical illusion is c+v. Such people are too stupid to calculate
how the luminosity regularly varies as a function of a star's relative
velocity.
They are also the same cretins to cry "crackpot" at the first opportunity.
"But the ray moves relatively to the initial point of k, when measured in
the stationary system, with the velocity c-v" -- Einstein
http://www.fourmilab.ch/etexts/einstein/specrel/www/
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Date: 22 Jun 2007 14:20:45
From: Johns Yard
Subject: Re: Is this an optical illusion?
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That is amazing!
"OG" <owen@gwynnefamily.org.uk > wrote in message
news:5e2f65F36ffa8U1@mid.individual.net...
>
> "Chris L Peterson" <clp@alumni.caltech.edu> wrote in message
> news:60ln731t9ddb8973bb054anb06qup85ccn@4ax.com...
>> On Fri, 22 Jun 2007 12:19:47 +0000, Guy Macon <http://www.guymacon.com/>
>> wrote:
>>
>>>Next time you see that, grab a camera and take a picture.
>>>If it's not the same as what you see, it's an optical illusion.
>>
>> An optical illusion can be just as strong in an image as in the real
>> view. But with an image, you can actually measure the intensity in
>> different spots and verify that the effect is an illusion.
>>
>
> This is one of the very best examples I've seen of this
> http://www.popularscience.co.uk/features/feat16.htm
>
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