> 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}> "Off parallel" rays will focus on different parts of
> eye retina making the picture in a human brain. Unless,
> of course, there is very bright object in the sight, Sun
> for example, lightning other parts of eye retina because
> of imperfection of eye (aberration etc.) Just observe the
> sky standing in a shadow.
> In fact, I think, observing the sky from a tall chimney
> would bring worse results, because eyeball diaphragm would
> accommodate to dark and exposed part of a retina would be
> overexposed.
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}> Mike.
>
> > > Russell McMahon wrote:
> > > > > Why does this stuff always have to happen at night?
> > > > > What's with that?
> > > >
> > > > It also happens in the daytime, but you need a lonnnnnnng
> > > > pipe with a black painted interior pointed at the sky to
> > > > see them. (Or at least, I assume that MIGHT work).
> > >
> > > You're right Russell, but partly, I think. Actually, black
> > > painted interior is not needed. And you forgot to specify
> > > precisely the length of the pipe. I'd suggest, it should reach
> > > upper layers of atmosphere, but not higher then meteor's paths.
> >
> > It was actually a serious suggestion.
> > The pipe needs only be a few tens of metres long - maybe less.
> > The black interior helps minimise reflections from rays that
> > enter off parallel.
> > As someone else noted - you can see on some occasions stars
> > up a tall chimney in daylight. The long narrow path provides
> > only parallel rays coming "straight down". The blackened
> > interior helps exclude rays which enter non vertically at
> > the top.
>
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