05-November-2001, 02:29 PM
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Established Member
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Join Date: Oct 2001
Location: USA
Posts: 311
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On 2001-11-05 08:44, GrapesOfWrath wrote:
Quote:
Actually, I was startled by your claim that the moon would be as bright as the sun, but now that I think about it, it makes perfect sense. If there were a giant mirror orbiting the earth, reflecting the sun, we would see an image of the sun in that mirror which would be approximately the size of the sun, since the sun is so far away compared to the distance to the mirror. If the mirror was smaller than that, we'd see less of the image of the sun, and have less light, obviously--but since the moon is about the size of the sun in the sky, such a reflective moon would be able to reflect the whole image of the sun. And it would be about as bright as the sun. That's fifteen magnitudes brighter than what it is. So, it doesn't reflect very much light.
Sure is bright in the eyepiece, though.
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As an interesting part of the side issue, a polished aluminum mirror put in orbit to perform the function you are describing would run very hot, the a/e values for that material are listed as: absorptance = .35 and emittance = .04 for and a/e of 8.75
A 'mirror' made of black paint would run much cooler: absorptance = .97 emittance = .91
If you really want it to run cold, make your mirror out of Optical Solar Reflectors ( silvered quartz mirrors with Teflon): absorptance = .08 emittance = .81
It's amazing how intutition fails totally when dealing with thermal optical properties.
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