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Date: 29 Jun 2007 18:09:48
From: Matthew Ota
Subject: The three dimensional eclipsed Moon effect
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I have a question about a known but seldom discussed lunar eclipse
phenomenon that I personally experienced a couple of years ago in
Fullerton, California.
As you know, the full moon appears as a flat disk to the human eye.
It has absolutely no limb darkening like Jupiter or other planets. The
reason why was not answered until the Apollo moon landings.
The surface of the moon is covered with very fine particles of
moondust that was formed by the pulverizing effect of eons of
meteoroid bombardment. This is an effect that was coined as
"gardening" by lunar geologists. The moondust is as fine as talcum
powder, but is not smooth in texture. The particles are all angular
with faceted surfaces. So the particles reflect light directly back to
the viewer.
As a result no matter what the angle of incidence of the curved
surface of the moon, light is reflected straight back to your eye in a
linear fashion, so equal amounts of photons strike your retina from
not only the center of the moon's face, but also the edges of the
moons surface at the limb. So the moon appears to be as flat as a
dinner plate pasted into the sky.
However, this effect is nullified during a total lunar eclipse. When
the eclipse occurs, all of the sunlight illuminating the moon is from
a scattered source as the sunlight is refracted through the earth's
atmosphere. So the light striking the moon is coming from more than
one angle.
The light reflecting back to your eyes from the moon is not only red
colored, but the light level falls off as you look towards the limb of
the moon's disk. This is the only time that you can see a limb
darkening effect.
With this limb darkening in view, and especially when the moon's
face is framed by clouds or terrestrial objects like trees or
buildings, the eclipsed moon magically and spookily appears to you as
a three-dimensional sphere hanging in the sky. The first time I saw
this I was absolutely stunned.
Now my question is, I learned about this effect by reading an
article about it somewhere in a book, magazine or online. I cannot
remember who wrote it and where I read it.
Can somebody point me to the article or references to this effect?
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Date: 30 Jun 2007 08:56:28
From: Matthew Ota
Subject: Re: The three dimensional eclipsed Moon effect
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On Jun 30, 7:03 am, Thomas Smid <thomas.s...@gmail.com > wrote:
> Hi Matthew,
>
> It seems you have misinterpreted something you read: the fact that the
> moon has no limb darkening is due to it having no atmosphere, not to
> the peculiarities of its surface. All bodies with a rough (i.e. non
> mirror-like) surface will scatter light into all directions and thus
> will appear to have the same surface brightness everywhere. The point
> is that if a body has additionally an atmosphere, light will be
> absorbed in it, and as line of sights closer to the limb have a logner
> path trough the atmosphere, the absorption is stronger, hence limb
> darkening.
>
> What you saw during the lunar eclipse was most likely the variation in
> the darkness of the earth's shadow. Also, you have to take into
> account that different regions on the moon have different
> reflectivity.
>
> Thomas
Thank you for the correction, and I really need to find the article so
I can get the story straight.
I am getting ready for the next eclipse and will need to be able to
explain all of this
stuff to the lay public.
Matthew Ota
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Date: 30 Jun 2007 07:03:24
From: Thomas Smid
Subject: Re: The three dimensional eclipsed Moon effect
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Hi Matthew,
It seems you have misinterpreted something you read: the fact that the
moon has no limb darkening is due to it having no atmosphere, not to
the peculiarities of its surface. All bodies with a rough (i.e. non
mirror-like) surface will scatter light into all directions and thus
will appear to have the same surface brightness everywhere. The point
is that if a body has additionally an atmosphere, light will be
absorbed in it, and as line of sights closer to the limb have a logner
path trough the atmosphere, the absorption is stronger, hence limb
darkening.
What you saw during the lunar eclipse was most likely the variation in
the darkness of the earth's shadow. Also, you have to take into
account that different regions on the moon have different
reflectivity.
Thomas
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Date: 01 Jul 2007 13:45:14
From: Peter Webb
Subject: Re: The three dimensional eclipsed Moon effect
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"Thomas Smid" <thomas.smid@gmail.com > wrote in message
news:1183212204.836932.129320@n2g2000hse.googlegroups.com...
> Hi Matthew,
>
> It seems you have misinterpreted something you read: the fact that the
> moon has no limb darkening is due to it having no atmosphere, not to
> the peculiarities of its surface. All bodies with a rough (i.e. non
> mirror-like) surface will scatter light into all directions and thus
> will appear to have the same surface brightness everywhere. The point
> is that if a body has additionally an atmosphere, light will be
> absorbed in it, and as line of sights closer to the limb have a logner
> path trough the atmosphere, the absorption is stronger, hence limb
> darkening.
>
> What you saw during the lunar eclipse was most likely the variation in
> the darkness of the earth's shadow. Also, you have to take into
> account that different regions on the moon have different
> reflectivity.
>
> Thomas
>
Sounds very plausible.
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Date: 29 Jun 2007 23:18:27
From: Ben
Subject: Re: The three dimensional eclipsed Moon effect
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On Jun 29, 9:48 pm, mov...@webtv.net (Marty) wrote:
> >Now my question is, I learned about this
> > effect by reading an article about it
> > somewhere in a book, magazine or
> > online. I cannot remember who wrote it
> > and where I read it. Can somebody
> > point me to the article or references to
> > this effect?
>
> Wish I could help Matthew, but I can't. I did enjoy your post though! I
> first noticed the 3D appearance of the totally eclipsed moon when I was
> a kid... I remember standing in the back yard, looking up at it, and
> thinking it looked like a red balloon hanging there. However, I've
> never heard the phenomenon discussed, much less explained. Thank you!
> Marty
I've never noticed the effect, but we are having another total in
August and
I will work it over.
Thanks,
Ben
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Date: 29 Jun 2007 21:48:06
From: Marty
Subject: Re: The three dimensional eclipsed Moon effect
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>Now my question is, I learned about this
> effect by reading an article about it
> somewhere in a book, magazine or
> online. I cannot remember who wrote it
> and where I read it. Can somebody
> point me to the article or references to
> this effect?
Wish I could help Matthew, but I can't. I did enjoy your post though! I
first noticed the 3D appearance of the totally eclipsed moon when I was
a kid... I remember standing in the back yard, looking up at it, and
thinking it looked like a red balloon hanging there. However, I've
never heard the phenomenon discussed, much less explained. Thank you!
Marty
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