History of the moon

I am wondering...
When I look at the moon and it is not full, I can kind of see an "outline"
of a "full" moon.
However, the outline appears smaller than a circle would be from the lit
side.
I don't know why the "apparent" circle looks smaller.
I am wondering what the ancient people thought about this.

John wrote:

When I look at the moon and it is not full, I can kind of see an "outline"
of a "full" moon.
However, the outline appears smaller than a circle would be from the lit
side.
I don't know why the "apparent" circle looks smaller.
I am wondering what the ancient people thought about this.

You're seeing "earthshine"; from the moon's limb comparatively little
light gets reflected back to us because of the shallow angle of
incidence, making the 'second-hand' illumination look as if it came
from a 'soft' spotlight aimed at the centre of the disc. I haven't
encountered an ancient reference to the phenomenon (I'm not well read
in such materials, so one might well exist), but to a thoughtful
observer it might have served as an additional clue that the moon is
actually a sphere rather than a disc.

--
Odysseus

John wrote:

When I look at the moon and it is not full, I can kind of see an "outline"
of a "full" moon.
However, the outline appears smaller than a circle would be from the lit
side.
I don't know why the "apparent" circle looks smaller.
I am wondering what the ancient people thought about this.

You're seeing "earthshine"; from the moon's limb comparatively little
light gets reflected back to us because of the shallow angle of
incidence, making the 'second-hand' illumination look as if it came
from a 'soft' spotlight aimed at the centre of the disc. I haven't
encountered an ancient reference to the phenomenon (I'm not well read
in such materials, so one might well exist), but to a thoughtful
observer it might have served as an additional clue that the moon is
actually a sphere rather than a disc.

--
Odysseus

"John" wrote in
:

I am wondering...
When I look at the moon and it is not full, I can kind of see an
"outline" of a "full" moon.
However, the outline appears smaller than a circle would be from the
lit side.
I don't know why the "apparent" circle looks smaller.
I am wondering what the ancient people thought about this.


There is a combination of physiological and optical effects at work.

Take two cirles exactly the same diameter. One is white a disk on a black
background the other a black disc on a white backgroung. The white-on-black
disc will seem disctinctly larger. The same effect is operating when
looking at the bright portion of the moon against a black sky making the
bright portion look larger than it realy is.

It may also be compounded by residual abberations in the eye scattering
some light beyond the edge of the bright portion of the disk and
irradiation in the retina adding to the illusion that it is a bit larger.

This also happens in photography. It is seen in images of an over-exposed
crescent moon. In this case the light of the bright portion scatters in the
film emulsion beyond the edge of the moon due to irradiation making the
bright portion larger than the earthshine portion.

An example of bloaded bright crescent portion due to irradiation can be
seen here
http://antwrp.gsfc.nasa.gov/apod/ap020419.html

Martin Lewicki

"John" wrote in
:

I am wondering...
When I look at the moon and it is not full, I can kind of see an
"outline" of a "full" moon.
However, the outline appears smaller than a circle would be from the
lit side.
I don't know why the "apparent" circle looks smaller.
I am wondering what the ancient people thought about this.


There is a combination of physiological and optical effects at work.

Take two cirles exactly the same diameter. One is white a disk on a black
background the other a black disc on a white backgroung. The white-on-black
disc will seem disctinctly larger. The same effect is operating when
looking at the bright portion of the moon against a black sky making the
bright portion look larger than it realy is.

It may also be compounded by residual abberations in the eye scattering
some light beyond the edge of the bright portion of the disk and
irradiation in the retina adding to the illusion that it is a bit larger.

This also happens in photography. It is seen in images of an over-exposed
crescent moon. In this case the light of the bright portion scatters in the
film emulsion beyond the edge of the moon due to irradiation making the
bright portion larger than the earthshine portion.

An example of bloaded bright crescent portion due to irradiation can be
seen here
http://antwrp.gsfc.nasa.gov/apod/ap020419.html

Martin Lewicki