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'[OT]: Meteors from Halley's comet'
2003\05\06@042334 by Russell McMahon

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>    Russell, what are you trying to state? Meteors aren't
> visible due to some mysterious "off parallel" light
> rather then just for the air brightness on a line towards
> meteors?

The [OT]: tag had vanished so I restored it.

I obviously didn't make myself clear enough - I was TRYING to say almost
exactly the opposite :-)

I was suggesting that they are essentially INVISIBLE in daytime due to some
entirely NON-MYSTERIOUS off parallel light. ie the light from the sun plus
it's reflections and refraction by particles in the air provides a "haze" of
light which tends to obscure dim distant objects. You can't do much about
the
light in the direct path, but the chimney is a tool for attempting to deal
with all the rest.

There seems to be a terraserver embedded somewhere in my brain which, when
it chooses and in a somewhat uncontrolled fashion,  produces "useful"
tidbits on many occasions - a (variable) majority prove more or less
accurate and or useful and or interesting. This particular gem appears to be
more in the latter category :-)

Here's a good discussion on this exact subject.

       "Can stars be seen in the daytime from the bottom of a
         tall chimney, a deep well, or deep mine shaft?

           http://www.faqs.org/faqs/astronomy/faq/part2/section-15.html

His conclusion is something like "yes, sort of, in theory, but ..." but he
then goes and ruins it in the last sentence (effectively saying that you
can) by effectively simulating such a situation using a technological
simulation (aka a telescope). Interesting to read.

{Quote hidden}

Most of the above "off axis" sources are excluded by the chimney.
The star light at the eye then only has to "compete" with direct path  light
from the patch of sky surrounding it. The taller the chimney the smaller the
patch. Star never gets very occluded at any sensible chimney height so star
gets relatively brighter with height.


       Russell McMahon

Starters for 10 points:

   The dark adapted eye can resolve a single photon.

   The Romans used to make ice in the Sahara Desert area by
   using entirely natural cooling resources.



       :-)



{Quote hidden}

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2003\05\06@043440 by Jake Anderson

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----- Original Message -----
From: "Russell McMahon" <apptechspamKILLspamPARADISE.NET.NZ>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Tuesday, May 06, 2003 6:21 PM
Subject: Re: [OT]: Meteors from Halley's comet


<mass snippage>
>
>         Russell McMahon
>
> Starters for 10 points:
>
>     The dark adapted eye can resolve a single photon.

no, the minimum is about 200 of some paticular frequency around green
(thats why a 1mw green laser pointer is so much "brighter" than a red one of
the same power level
(yes there would be some increase in power with green having a shorter
wavelength but it is insignifigant really))

>     The Romans used to make ice in the Sahara Desert area by
>     using entirely natural cooling resources.

dunno but i'd guess knowing you. yes
no idea how (though verry hot places also generally get verry cold)
hmmm
google time
well thereya go
minimum temperature is below freezing

partial credit with use of google? ;->

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2003\05\06@070145 by Russell McMahon

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> > Starters for 10 points:
> >
> >     The dark adapted eye can resolve a single photon.
>
> no, the minimum is about 200 of some paticular frequency around green
> (thats why a 1mw green laser pointer is so much "brighter" than a red one
of
> the same power level
> (yes there would be some increase in power with green having a shorter
> wavelength but it is insignifigant really))

That was there to be contentious :-)
Here's a reference that largely agrees with you.


omega.cohums.ohio-state.edu:8080/hyper-lists/classics-l/00-07-01/0629
.html

But, I have read it argued before that while about 80 photons need to be
received on average to get 1 to a receptor, that 1 is able to be perceived.
Which disagrees with the above and your version. Staring into a single
photon source would settle it.


               Russell McMahon

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2003\05\06@070153 by Russell McMahon

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> >     The Romans used to make ice in the Sahara Desert area by
> >     using entirely natural cooling resources.

> dunno but i'd guess knowing you.

why thankyou :-)

> yes
> no idea how (though verry hot places also generally get verry cold)
> hmmm
> google time
> well thereya go
> minimum temperature is below freezing
>
> partial credit with use of google? ;->

Indeed.

Make pit in ground.
Make pit waterproof.
Partially fill with suitable quantity of water.
Expose to sky at nights.
Cover with best available insulating layer in daytime.
In time you get ice.

I've idly wondered about placing a parabolic mirror in an insulated longish
tube (see blue sky and stars story :-) ) and seeing if you got noticeable
focused coolth from deep space. The intervening atmosphere seems to mess
this up rather badly. Unlike high temperature photons from eg the sun coming
in, the outward physics works against you. A basic experiment with a
polystyrene insulated tube was inconclusive.

You can certainly cool something by radiation cooling if you "point" it  at
a cold area. Try sitting across a warm  room from an uncurtained window on a
cold day. Quite apart from convection.


       Russell McMahon

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2003\05\06@073944 by Jake Anderson

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I think the one I read was the number of photons per second or some such to
be accuratly detected by a person
{Original Message removed}

2003\05\06@085735 by Bob Ammerman

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> Starters for 10 points:
>
>     The dark adapted eye can resolve a single photon.

Perhaps a much higher energy photon than normal visible light?

>     The Romans used to make ice in the Sahara Desert area by
>     using entirely natural cooling resources.

It does get pretty cold at night out there.

Bob Ammerman
RAm Systems

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2003\05\07@064433 by Peter L. Peres

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>>     The dark adapted eye can resolve a single photon.
>
>Perhaps a much higher energy photon than normal visible light?

One that would really light you up inside ?

>>     The Romans used to make ice in the Sahara Desert area by
>>     using entirely natural cooling resources.
>
>It does get pretty cold at night out there.

It's not the cold that does it, the earth barely goes below zero. It's the
clear dark sky at the the radiation temperature of space (4K) and
evaporation from the large water surface. You put cold water in a (already
cold) blackened tray and put it on a blanket or other insulating material
raised above ground so it can only 'see' the sky. By early morning it is
iced over. It only works when the sky is really clear and far away from
structures that keep the heat and reradiate it a long time after the sun
sets.

Peter

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2003\05\07@153447 by Russell McMahon

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> >>     The Romans used to make ice in the Sahara Desert area by
> >>     using entirely natural cooling resources.
> >
> >It does get pretty cold at night out there.
>
> It's not the cold that does it, the earth barely goes below zero. It's the
> clear dark sky at the the radiation temperature of space (4K)


That's the trick.
The thermal input of the atmosphere has an opposing affect of course (or
we'd all die). Arguably this would work in daytime as well with a larger
than star-viewing black internal painted and now well insulated pipe.



       RM

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2003\05\08@041539 by Alan B. Pearce

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>> It's not the cold that does it, the earth barely goes below zero. It's
the
>> clear dark sky at the the radiation temperature of space (4K)
>
>
>That's the trick.
>The thermal input of the atmosphere has an opposing affect of course (or
>we'd all die). Arguably this would work in daytime as well with a larger
>than star-viewing black internal painted and now well insulated pipe.

Been watching this discussion with interest, as it relates to a project that
was being done at the lab here recently. See
http://www.startiger.org/news.htm for the project info.

The machine scanned the hand looking for radiation, and the guy running it
reckoned that outside the sky was a noise source of about 20K, but inside
they used a piece of polystyrene with a flood of liquid nitrogen behind the
hand as the "low noise" background.

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