How dark is dark ?

In ,
Chris L Peterson typed:

If you couldn't see your hand in front of your face, you weren't very
well dark adapted, or there was some haze. Skyglow, zodiacal light,
gegenschein, and other natural sources of light are significant in a
clear, dark site. Even in the darkest of areas, you can see well
enough to get around easily after an hour outside. Certainly, the sky
background doesn't look at all black. (It makes quite a difference how
active the Sun is, too.)

I live in Powys in mid Wales, which has dark skies. Even so, on a moonless
night and away from any artificial lights there is enough light to see well
enough to get around...but it does take 30-40 minutes for the eyes to dark
adapt. The night skies here are spectacular and are the main reason I have a
telescope...it would be a waste of good sky not to have one :-)

Jo

"canopus56" wrote:

It's no myth. I personally saw it during the 2004 Venus transit.
Venus was bright enough that a fuzzy shadow of the scope and my body
could be seen with the naked eye at a dark sky in the Intermountain
west. - Canopus56

LOL - the shadows were indeed magnificent. In fact I had to wear those
eclipse glasses to see Venus without going blind!

Cheers

Martin

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

Jo,

How many clear nights a year do you get ?

dan


Jo wrote:
In ,
Chris L Peterson typed:

If you couldn't see your hand in front of your face, you weren't very
well dark adapted, or there was some haze. Skyglow, zodiacal light,
gegenschein, and other natural sources of light are significant in a
clear, dark site. Even in the darkest of areas, you can see well
enough to get around easily after an hour outside. Certainly, the sky
background doesn't look at all black. (It makes quite a difference how
active the Sun is, too.)


I live in Powys in mid Wales, which has dark skies. Even so, on a moonless
night and away from any artificial lights there is enough light to see well
enough to get around...but it does take 30-40 minutes for the eyes to dark
adapt. The night skies here are spectacular and are the main reason I have a
telescope...it would be a waste of good sky not to have one :-)

Jo

On Sat, 23 Apr 2005 23:14:36 +0000 (UTC), "Jo"
wrote:

I live in Powys in mid Wales, which has dark skies. Even so, on a moonless
night and away from any artificial lights there is enough light to see well
enough to get around...but it does take 30-40 minutes for the eyes to dark
adapt. The night skies here are spectacular and are the main reason I have a
telescope...it would be a waste of good sky not to have one :-)

Same for me. My sky brightness is 21 mag/arcsec^2 - less than a
magnitude from the darkest possible skies. When I walk outside, it is
truly like being struck blind. But after an hour, you think it must
still be twilight, it seems so bright. Where I really notice this effect
the most is at star parties, where I'm outside with no artificial
illumination for hours. Skyglow and gegenschein are really _bright_
under those conditions.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Andrew:
Years ago I had the pleasure of living in Tonga, a small island state
in the Pacific ocean. At that time I had no interest in astronomy, but
I do remember the night sky looked like someone had splashed paint
across it, such was the density of stars. At times I, literally, could
not see my hand in front of my face. Anyone here have an idea what
sort of mag skys they may have been, and has anyone been anywhere
darker ?

It probably doesn't get much darker than you experienced. I have
observed from the Australian Outback under skies that showed so many
stars that I could not immediately distinguish well-known
constellations. Also from remote, high-elevation (5000 ft) parts of
the former Yemen Arab Republic where there were few villages and those
that existed had no electricity. Airborne dust was a problem, however.

Davoud

--
usenet *at* davidillig dawt com

Gaz wrote:
I love readng Patrick Moore, he writes like he talks, you can hear his
voice as you read.

did that make sense?

Gaz


Right. And if you're in the US or The Galapagos Islands and don't know
what Gaz is talking about, have a look:

http://www.bbc.co.uk/science/space/s...proginfo.shtml

There are currently 41 episodes of Moore's monthly astronomy television
show *The Sky At Night* archived on the above page. RealPlayer or a
similar streaming video player is necessary to view them.

Clear Skies *at Night*!

Uncle Bob
(A Sir Patrick Moore fan)

Davoud wrote:

It probably doesn't get much darker than you experienced. I have
observed from the Australian Outback under skies that showed so many
stars that I could not immediately distinguish well-known
constellations...

I've observed from rural locations in Canada, Australia,
the U.S.A. and Costa Rica, and can certainly attest to the
way you start to lose the constellations when the sky is
really dark.

Tropical locations have the advantage of brief twilight and
invariant sunrise and sunset times. I have family who live
in northern British Columbia and who enjoy magnitude 6.5 (at
least) skies in the winter. In the summer, however, it's
twilight all night... :-(

Laura Halliday VE7LDH "Yeah but no but
Grid: CN89mg yeah but no but..."
ICBM: 49 16.05 N 122 56.92 W - V. Pollard

Chef!:



Chef! wrote:

[clip]

Darker skies probably do exist. The full story is at:
http://www.astropix.com/HTML/L_STORY/SKYBRITE.HTM
should be required reading for all visual observers, IMNSHO

Regards
Chef!



From the article you quoted,
http://members.ozemail.com.au/~loomberah/agw1.htm :

Then banded appearance of the sky may be due to the airflow rotations that
change the temperature and dew point of the moisture in the air. Refractive
and dispersive effects of photon paths may cause the banded lighter and
darker appearance.

Several years ago I was in Western Colorado. I saw the weather report that
said that the jet stream was going to be over the Grand Mesa near Grand
Junction, CO, and that the stream was going to be lower in altitude than
usual. I happened to be on the Grand Mesa at 10.000 ft. altitude at sunset.

The jet stream was not a single stream like a river of air. Rather, it was
made of 7 or 8 counter-rotating tubes of air located side by side. The jet
stream seemed to be at 12,00 to 13,000 ft. of altitude. The color of the
clear sunset sky over the Gunnison River Valley in summer is
characteristically clear pink to fuscia, and the visibility to mountain
tops that were at the same altitude or greater was more than 100 miles in
the clear air. Visibility was only ten miles at the lower elevation of
5,000 ft. in the valley below. The experience of the spatial depth of the
sky on these evenings is awe inspiring. The condensation, smoke or dust
made the tops of the tubes visible and the rotating effect could be seen.
My guess is that the air flow rotated at possibly 1 rpm., and the tubes
were possibly 1,000 ft. in diameter.

Another place to view air tubes is Hurricanes. The high altitude outflows
from the eye of the hurricane do not flow in a spiral path. They flow
radially in a disk. The air in the hundreds of counter-rotating tubes flows
from the center at high altitudes. They can be seen prior to the arrival
of a hurricane, and the usual counter-clockwise spiral inflows to the eye
that occur at lower altitudes. The high altitude radial outflows can be
clearly seen in some stop-motion motion pictures taken from satellites of
hurricanes. All the storms have that effect, and it is visible when clouds
don't obscure the effect.

Hypothesis: Could the banded appearance of the sky be due to the high
altitude counter-rotating tubes of air flows due to the jet stream or a
hurricane?

Ralph Hertle

The darkest dark I can remember was in a mile-long, dissused railway
tunnel without a torch. A great test of nerve for a kid when the walls
are slimy, wet and black from a century of smoke soot so you don't want
to touch them! ;-)

Someone once mentioned that seeing stars reflected in the sea was a
particularly good test of real darkness. But I keep forgetting to check
this in my garden fish pond on suitably dark nights....Sploosh! :-)

Chris.B

Ï Ýãñáøå óôï ìÞíõìá
oups.com...
[snip]
Someone once mentioned that seeing stars reflected in the sea was a
particularly good test of real darkness.

The only location I've seen this during my life was on the Antiparos island,
in the middle of the Aegean at my x-girlfriend's summer resort. It was
actually quite fascinating. Not *stars* being reflected on the sea, but the
*milky-way*.
It was the time I made my bino 20x100 astro report:
http://users.forthnet.gr/ath/jgal/as...100Report.html

The milky way around the Sagittarius area was visible as a faint glow
against the sea. If I could spend this life doing something very pleasurable
besides what I am doing, this would be #1. But alas, all good things come to
an end :-)

Chris.B
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
Eventually, _everything_ is understandable