[Laser] Re: Lunar Eclipse #3

[Tim Toast]

Hi John and James

John, you are absolutely right, it is two radii not one. i
was sitting here looking at a little diagram i drew of it
and still didn't realize it till later. 

About the main difference being in amplitudes between
center and limb, i'm not so sure. A ray hitting near the
limb does indeed spread over a wider area than in the
center but somehow the reflection amplitude is almost the
same - to the eye anyway. But this probably only applies to
similar type areas, like rough areas in the center and
rough areas near the limb. Or between two similar smooth
areas. I would expect there would be some difference
between dissimilar areas. 
On several places around the net i've read the limb can
actually have a stronger reflection than the center. But it
should definitely show up in a detailed albedo survey like
James was suggesting. The no limb darkening thing is pretty
much unique to our moon, most other objects have it, even
the sun.
 
James, another experiment i would suggest would be some
sort of setup to test that modulation cancellation effect
of the "multitude of phases" or the "three phases" effect.
Maybe have a small globe model of earth studded with lots
of led's all emitting the same frequency but with different
phases. Laid out in a USA-like shape or other shapes. You
would have a detector some distance away aimed toward the
model to simulate how it would be seen from the moon. Or
have the detector aimed at a small disk of white paper to
simulate the moon (and mix all those phases more
effectively than a large area photodiode might if aimed
right at the earth model). Then, rotate the earth model to
different angles to see what happens to the detected light.

One way to get a lot of separate phases might be to use a
digital shift register IC or a "one of N" decoder - clocked
at N x the mains frequency - so that each output would be
120Hz but at a slightly different phase from the others. 
In fact with this setup, NONE of the led's would have the
same phase, but all would be locked exactly on 120 Hz at a
low duty cycle.





----- Original Message ----- 
"Hi Tim,

Some more thoughts ...
If you have light leaving the earth and illuminating the
whole side of the 
moon facing us, then the path difference between a ray to
the center of that 
face and one to the edge is twice the radius of the moon,
one radius longer 
going and one radius longer coming back.  Pretty much a
pulse would be 
lengthened by the time of flight of two moon radii.  The
main difference 
will be in amplitudes ... a ray illuminating the center
area reflects back 
to us normal to the surface.  A ray going to the edge has
its energy spread 
over a large area and we are way off to the side from
nornal.  Of course, 
objects with surface roughness of even a few mils scatter
light almost 
omni-directionally.  The is not quite like 2 meters where 1
foot roughness 
is "flat".

John Matz KB9II"

----- Original Message ----- 

"I suggest an experiment to measure the optical
reflectivity across the
observable face of the moon.  Take a picture of the full
moon, then compare
the "brightness" of the pixels.  You might need to use
color filters if you
want to get data on different frequencies of light rather
than a composite.
It may be necessary to distinguish between plains and
mountains, which may
provide different results.
...
James
 n5gui"



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