is this an optical illusion?

"Chris L Peterson" wrote in message
...
: 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.
:
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/

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

On Jun 22, 4:35 am, 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. 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

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/

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

"Guy Macon" http://www.guymacon.com/ wrote in message
...
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.