At 06:28 PM 02/22/2001 -0800, hal@finney.org wrote:
>Ian Goddard wrote:
> > So (phew... finally to the question), anyone know more about this?
> > Would this effect itself constitute evidence of being on the moon
> > (as opposed to only being an effect that produces an "anomaly")?
>
>It seems like I've heard about this being noted by astronomers. The
>moon is brighter at full moon than what you would predict if you had
>uniform reflectance in all directions.
>
>According to http://image.gsfc.nasa.gov/poetry/ask/a11387.html, "The Full
>Moon is about a factor of 10 times brighter than the Half Moon phase."
>
>According to http://www.hockey.net/~wb0bwl/tom/paper/old/magnitud.htm,
>after several pages of calculus assuming "the surface brightness appears
>the same for any direction relative to the surface normal", we find,
>"One interesting thing I found that comes out of this is that the phase
>factor is 1 for full phase and 1/pi for quarter phase which means the
>planet is over 3 times brighter when it is full compared to half full."
>
>So according to this calculation (which I didn't try to verify),
>uniform reflectance predicts that a full moon will be pi times as bright
>as a half moon, while observations show a much larger factor of 10.
>This would suggest that the moon's surface does preferentially reflect
>light back in the direction it shines from, since during a full moon we
>are looking at the moon from the same direction as the sun.
IAN: Yes, and the odd effect where the ground gets very
luminous is when the sun is behind the camera, and thus
the camera is looking from about the same direction as the
sun. The result is that there is a halo around the caste
shadow of an astronaut's head, as seen in these images:
http://cass.jsc.nasa.gov/expmoon/Apollo11/A11_WestViewFS.gif
http://www.hq.nasa.gov/office/pao/History/alsj/a11/a11.1103147_bm.jpg
http://images.jsc.nasa.gov/images/pao/AS17/10075956.jpg
Halo around head shadow not bright, but it's there:
http://images.jsc.nasa.gov/images/pao/AS14/10075609.jpg
http://images.jsc.nasa.gov/images/pao/AS14/10075620.jpg
Halo around shadow of astronaut head seen in his visor:
http://images.jsc.nasa.gov/images/pao/AS11/10075267.jpg
In this case the "halo effect" is around the top of the
caste shadow of the lunar modle, probably near where
the head shadow of the astronaut on the module would be:
http://images.jsc.nasa.gov/images/pao/AS11/10075276.jpg
I got this reply on the matter from an astronomy list:
From: "Les Cowley" <ast1@sundog.clara.co.uk>
To: <astro-l@uwwvax.uww.edu>
Subject: Re: Moon Shine
Date: Thu, 22 Feb 2001 10:50:21 -0000
> One thing I notice as I review Apollo images is that the
> lunar surface appears to have a specular-reflection effect
> that causes some areas of lunar surface to reflect brighter
> than other areas relative to sun & viewer positions
There is a definite effect on the Moon in which the soil directly
opposite the sun appears much brighter. If you have 'Full Moon'
by Michael Light, take a look at plate 44 of the Lunar Module
(Apollo 12) and plate 45, the astronaut's shadow imaged during
the Apollo 17 mission. In the latter shot the ground in the
antisolar direction is very bright making the astronaut appear
to be surrounded by a bright halo.
This effect is caused by the lack of shadows near to the antisolar
direction rather than specular reflection. The lunar soil structure
is such that there are deep shadows in the interstices between
the particles. These shadows are not visible when looking directly
away from the sun and so the soil then looks brighter there. The
effect called the 'opposition effect' contributes to the brightness of
the Moon around full as seen from Earth. It also makes Mars
brighter around opposition - hence the name.
The opposition effect can be seen weakly on Earth - when
flying over winter forests for example. I have seen it once (weakly)
in dry beach sand. I did not have a camera and have been back to
that the beach many times since without seeing it again.
On Earth there is a related effect, the 'heiligenschein' (holy light).
This is caused by quite the opposite conditions from those on the
Moon. Droplets of dew on the ground scatter sunlight sharply
backwards producing a bright light around the shadow of the head
of the observer. Stand on a sunlit dew covered lawn early in the
morning to see it. Take a photograph ad the holy light surrounds
the camera instead.
The opposition effect is not exclusive, there are no doubt other
optical effects produced by the lunar soil. Take a look at the Apollo
images in Michael Light's book. The lunar soil looks like.. ..nothing
on Earth!
Les Cowley
Halos: www.sundog.clara.co.uk/halo/halosim.htm
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