is this an optical illusion?

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?

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/

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

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

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.

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

"Chris L Peterson" wrote in message
...
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

That is amazing!

"OG" wrote in message
...

"Chris L Peterson" wrote in message
...
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

On Fri, 22 Jun 2007 18:28:36 +0100, "OG"
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/...1199_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

On Jun 22, 11:45 am, Chris L Peterson wrote:
On Fri, 22 Jun 2007 18:28:36 +0100, "OG"
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/...1199_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.