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Shadows in the Earthshine



 
 
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Old April 3rd 06, 07:25 PM posted to sci.astro.amateur
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Default Shadows in the Earthshine

I covered occultation observations in a class just a couple of weeks
ago, so last weekend's Pleiades occultation was too good a demonstration
to pass up. We did about 1-1/4 hours of nonstop video as star after star
popped out of view (a few leaving a brief glimpse of a companion). The
video was done with a 125-mm guide scope, so we could do parallel CCD
imaging through the main 0.4m instrument. Someday I'll pretty up that
3x3-frame montage just as the Moon was surrounded by a garland of the
bright Pleiads, but that will take a while with all the regions wiped
out by charge bleeding on long exposures and scattered light crossing
the dark limb.

But now to something really interesting. We all make a point that
Earthshine always exactly matches full moon, since we're on the light
source. However, looking at many of our images of the Earthlit limb, it
sure looks as if there are shaded sides to elevations right next to
the limb (i.e. it looks more like some hours short of full Moon than
right at it). An example near Grimaldi may be viewed at
http://www.astr.ua.edu/keel/earthlight.jpg
which is from a 5-second I-band CCD exposure with a couple of stars
visible. Now that I think of it, this makes sense. We were viewing
the event down to about 20 degrees off the horizon, meaning that, from
the lunar point of view, we were nearly a degree to the side of the
centroid of the light source. If not true shadows, we might well be
in a position to see a shading.

Anyone else out there noticed something like this?

Bill Keel
 




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