Re: blow your alphorns

Marcus L. Rowland <forgottenfutures@xxxxxxxxxxxx> wrote:
> Keith F. Lynch <kfl@xxxxxxxxxxxxxx> wrote:
>> James Nicoll <jdnicoll@xxxxxxxxx> wrote:
>>> http://zebu.uoregon.edu/~probs/therm/sunstrok/sunstrok.html

>> That problem makes the same mistake as Clarke himself made: It
>> disregards the fact that sunlight is not all parallel. Sunlight
>> reflected off a mirror will spread out, and most of it will miss
>> the target.

> It's so close to parallel that energy losses in this context are
> negligible. Unless I'm missing something ...

You're missing something. The fact that the sun's distance isn't
infinite is indeed irrelevant in this context. But the fact that the
sun is a disk, not a point, is very relevant. The sun's disk is about
half a degree across. If a spectator armed with a mirror is 200 meters
from the target, the spot of light will be at least 1.7 meters across.

Another way to look at it is that no reflection of the sun can ever
be brighter, per angular area, than the sun itself. To fry something
with sunlight, you need to fill a substantial portion of the target's
"visual field" with mirrors.

In the original story, if I recall correctly, it wasn't mirrors, but
shiny programs distributed to the attendees. I won't even get into
the fact that the programs aren't 100% reflective, probably aren't
particularly close to flat, and, with hundreds of spots of light on
the playing field at once, are all but impossible to aim.

I once briefly worked with what was then the world's largest solar
furnace, at Kirtland Air Force Base, New Mexico. There I met the only
man ever to suffer from moonburn. One night with a full moon, they
had him sit at the focal point with a walkie-talkie all night, to help
align the mirrors. Ten hours with ten thousand reflections of the
full moon is equivalent to about one hour on a sunny day -- more than
sufficient for an unprotected person to get burned. Next time maybe
he'll wear moonscreen.
--
Keith F. Lynch - http://keithlynch.net/
Please see http://keithlynch.net/email.html before emailing me.
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