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BigDon
2010-May-15, 03:39 PM
After spending nearly two years at sea all told, I noticed that during the exquisite viewing conditions that occur in the middle of the ocean at night a full moon, as long as it was at about two or less closed fists above the horizon, looked like it was sitting on top of a column of "blackness" the apparent width of the full moon itself.

Is there a name for that?

Messier Tidy Upper
2010-May-21, 11:53 AM
Not that I know of. Don't know if that's helpful or not but I guess its one answer for you.

Tog
2010-May-21, 12:14 PM
I don't know a name for it, but I would suspect it's an optical illusion. The moon would also (probably) be causing a reflection on the surface of the water that was directly "under" it and the same width. That would leave a bright circle, a dark bit, and a long bright bit, like a huge, well lit, lower-case 'i' (sans font).

My guess is that your brain expects that void to have edges, and fills them in for you.

JohnD
2010-May-21, 05:22 PM
I think you're right, Tog.
I googled for images of "moon over sea" and the first page of hits showed what looked exactly as Don describes.
When viewed full size, the effect is not as clear, but I think it's still there, yet if you cover the moon with your hand, the sky is uniform.
Same if you cover the sea.
http://www.digitaljournal.com/blog/267
It's similar to that illusion where a chequerboard has a shadow across it. A white square in the shadow is the same shade as a grey square in full light!
http://visualfunhouse.com/color_adapting/chequer-shadow-illusion.html

John

astromark
2010-May-21, 07:54 PM
Yes. I also spent some years at sea and yes, As if the moons brightness blackens the sky around it. While its within 15 deg of the ocean that reflection does wash out the sky's features. : generally, there's no light pollution at sea. Two hundred km south of New Zealand on a clear winter night... Wow. I have seen that sky under the moon also.

BigDon
2010-May-21, 08:50 PM
I don't know a name for it, but I would suspect it's an optical illusion

Tog, I really can't decide what to say to that...

Mark, thanks for the support. I've seen the Milky Way so bright it cast shadows. (middle of the Indian Ocean along the equator.)

mugaliens
2010-May-21, 09:38 PM
After spending nearly two years at sea all told, I noticed that during the exquisite viewing conditions that occur in the middle of the ocean at night a full moon, as long as it was at about two or less closed fists above the horizon, looked like it was sitting on top of a column of "blackness" the apparent width of the full moon itself.

Is there a name for that?

Yes: Spatial Encoding (http://en.wikipedia.org/wiki/Retina#Spatial_Encoding). It's not an optical illusion per se'. Rather, it's simply the way our retina works so as not to overload our optic nerves and the visual portion of our brains while still transmitting most of the information, without transmitting 100% of the data.

Rather ingenious! Mildly analogous to the difference between jpg and bmp image files.

This reminds me of the "three-letter code" whereby all commonly-used words, numbers, and special characters could be represented by just three of them. That's 26 lowercase, 26 uppercase, 10 numbers, and 26 special characters, for a total of 88 possibilities, which, in various combinations of two provide 7744 variations, but in combinations of three, provide 681,472 combinations. Limiting that to a much smaller subset of words provides for multiply redundant means of encoding any of the commonly used words, numbers, and special characters. It hogs the bandwidth, but in the last 30 years, such has not a problem, and far larger dictionaries have been used for similar purpose.

Even so, PKI using sufficiently large keys has rendered the dictionary approach all but obsolete.