Atlas of Light Pollution vs. Experience

I'm curious how people's observing experiences jibe
with their location in the World Atlas of Light
Pollution (http://www.lightpollution.it/dmsp).

For people in North America, Attila Danko's Clear
Sky Clock (http://cleardarksky.com/csk) is probably
the most convenient interface to the Atlas. Find your
state or province on the homepage, then click on a
Clock near to you, then click on "Light Pollution
map", one of the Nifty Links below the Clear Sky info.
That gets you a closeup from the Atlas centered on
the clock that you picked. You can probably get a
pretty good idea where things lie just by correlating
the bright areas in the Atlas with the towns that you
know nearby. But if you're in doubt, you can check
"link roadmap" or "link topomap", then go back to
the Atlas and click any point that interests you.

Below the map, you'll find a key to the color coding.
There are eight zones: black, gray, blue, green,
yellow, orange, red, and white. They're organized
so that each represents three times as much artificial
skyglow as the previous. That sounds reasonable, but
the net result is that the gradations are much too
fine in the darker zones and too coarse in the brighter
zones. That's because you have to factor in natural
skyglow as well, and natural skyglow is in fact quite
bright -- as anybody who's been to a "dark" site knows
quite well. That's why nebula filters work so well even
under pristine skies.

So black and gray together represents a *total* skyglow
(natural plus artificial) anywhere from 1.00 - 1.11 times
natural, and blue represents 1.11 - 1.33 times natural.
It's a fair bet that the difference between the darkest
black and the bright edge of the blue zone is barely
perceptible. The gegenschein isn't usually thought of
as a major source of light pollution, but the center
of the gegenschein as seen from the blackest black
zone is considerably brighter than the zodiacal pole
as seen from the bright edge of the blue zone.

I would say that black, gray, and blue (and possibly
the dark edge of green) correspond to Bortle classes
1 and 2 (http://skyandtelescope.com/resources/darksky).

In my experience, if you're observing anywhere inside
the green zone, you've got little to complain about.
In fact, the first time I ever saw the gegenschein,
I was at the bright edge of the green zone, near the
border with yellow. It wasn't hard to see, either --
it's the kind of thing that you'll never notice until
somebody points it out to you, and after that, it's
obvious. That site did have one large obtrusive light
dome, and views in that quadrant of the sky were
significantly compromised. But everywhere else, things
looked pretty good. There was a little loss of
crispness in the dark lanes that adorn the Milky
Way, but not much. This corresponds pretty clearly
to Bortle class 3.

Things start to deteriorate significantly in the yellow
zone, corresponding to levels of light pollution that
are (according to the Atlas) anywhere from 2X to 4X
the natural skyglow. What I find suffer the most are
dark nebulae. At the bright edge of the yellow zone,
the Milky Way has lost a lot of the luster and fine
detail that result from the innumerable tiny dark lanes
that adorn it when seen under pristine skies. Objects
that produce light, by contrast, are relatively little
affected. I always find M33 a difficult naked-eye
target; I can't even come close to seeing it with
direct vision under pristine skies. But I can still
find it with averted vision at the bright edge of
the yellow zone, as long as it's not too low.
I would say that yellow matches Bortle class 4
pretty well.

There's an immense change over the orange zone, which
is supposed to correspond to anywhere between 4X and
10X the natural sky background. The dark edge of this
zone is really quite acceptable; for instance, the dark
lanes in M31 show through a telescope with no trouble
at all, and the spiral arms of M33 are still visible.
But at the bright edge of the orange zone, things are
grim. That's where my astronomy club's observing field
is located (cleardarksky.com/lp/EdKnghtObMAlp.html).
As we always say when we're there, it's not bad for a
suburban site, but you can never for a moment forget
the skyglow, even at the zenith. You can still see
the spiral arms of M51, but that requires a pretty
big telescope, and galaxies with lower surface
brightness lose their entire outer disks. The
summer Milky Way and the Great Rift are obvious,
but the winter Milky Way is subtle even near the
zenith. M34 and M35 are obvious to the naked eye,
and M13 is detectable, but M33 is completely gone.

By the time you get to the bright edge of the red
zone, the Milky Way is nearly gone. You can still
see the Cygnus Starcloud when it's overhead, but
you have to look carefully. Most galaxies display
little more than their cores in telescopes, though
there are always exceptions (e.g. M82).

Orange and red match Bortle classes 5 and 6, but
it's hard to make detailed correspondences,
because the changes over this range are so vast.

In my area (Boston, MA), the white zone corresponds
to the city and the inner suburbs, where all the
streets are lit and lined continuously with houses.
The border between red and white aligns neatly
with Route 128, the inner ring superhighway.
Indeed, the match is so good that I can't help
wondering if headlights on Rte. 128 aren't
largely *responsible* for the white zone.

Fortunately, there are some pretty big parks within
this region, making it possible to get a fair
distance away from the streetlights. There's
actually quite a big variation in the astronomical
experience within this zone. At the dark edge
(Bortle class 7), M31, M41, and the Double Cluster
are still fairly easy naked-eye targets, although
you would never see them unless you were explicitly
looking. But in downtown Boston (Bortle class 9),
only the Pleiades remain.

In case people are wondering why I haven't talked
about naked-eye limiting stellar magnitude, it's
because I find this nearly useless to convey sky
brightness from one observer to another -- it
depends much more on individual visual acuity
than on sky brightness. To give an extreme example,
I can easily see 6 stars in the Pleiades from my city
home, as long as my eyesight is properly corrected.
And I can still see precisely those same 6 stars,
and no others, under pristine skies. Other people
report seeing 10 or 15 in identical conditions.

- Tony Flanders

Hi Tony,

My home is in an orange to red zone. However on good nights i can see =
the summer Milky Way just barely. I think the reason i can is that south =
of my home are mountains and open desert with very few lights. Of course =
within a mile or so i could be at a car dealership and not see anything =
at all. But i can also go 50 miles east and be at the boundary of blue =
to black zones. This is good! Also you mentioned the gegenschein. I've =
never seen it. (Zodiacal light yes, but not gegenschein.) I know the =
gegenschein is basically opposite the sun but is there a good site or =
method to show where the gegenschein is on any particular night? I'll be =
camping in the Anza-Borrego desert the next two nights and would like to =
try and see it! ;-)

-Florian
Palm Springs, Calif.
http://www.stargazing.com/

On 29 Mar 2005 12:27:16 -0800, wrote:

I'm curious how people's observing experiences jibe
with their location in the World Atlas of Light
Pollution (http://www.lightpollution.it/dmsp).

I'm in a blue zone, but should be in gray. My sky background
measurements are generally about 0.3 magnitudes darker than the atlas
would suggest. I've measured other dark sites in Colorado and found that
the atlas similarly overestimates the brightness. I suspect the model
used is not fully accounting for the effects of altitude. I've had some
communication with one of the study's authors about just this issue, but
don't know where things currently stand as far as revising the model.

_________________________________________________

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

Florian wrote:

I know the gegenschein is basically opposite the sun but is there a
good site or method
to show where the gegenschein is on any particular night?

Sure. The Gegenschein is *precisely* opposite the Sun, and since the
Sun is now at
RA 0:35, Dec. 4N, the Gegenschein is centered at RA 12:35, Dec. 4S.
That happens to be
almost on top of Jupiter (in other words, Jupiter is almost at
opposition). My guess
is that Jupiter's bright enough to blot out the Gegenschein, unless you
can rig up
some way to block it.

Look for a large region of the sky that's brighter than it ought to be.
It doesn't
have anything remotely resembling a sharp edge, so it's very hard to
perceive it
as an entity in its own right. But it's actually respectably bright --
probably
brighter than the faint part of the Milky Way between Orion and Gemini.

- Tony Flanders

Thanks Tony. I shall look for the gegenschein around Jupiter the next =
couple of nights!

-Florian

Tony Flanders:
I'm curious how people's observing experiences jibe
with their location in the World Atlas of Light
Pollution (http://www.lightpollution.it/dmsp).

For people in North America, Attila Danko's Clear
Sky Clock (http://cleardarksky.com/csk) is probably
the most convenient interface to the Atlas. Find your
state or province on the homepage...

Red - White border (about mid-way between Baltimore and Washington.
Gegenschein? Hell, I'm lucky to see moonschein!

My sky rarely, if ever, appears darker than medium gray to a
dark-adapted eye.

Nonetheless, I get along as best I can. I'll always carry the memory of
observing from the Australian Outback some years ago.

I have a more localized problem than what I call the Great East Coast
Light Dome -- my neighbour's dazzling "security" lights
http://davidillig.com/ast-spotlight.shtml that illuminate my
observatory.

We're carrying on a civil discourse, and I expect that we'll be able to
resolve the issue (he hasn't seen that photo yet). I may have to pay
for the fix, however.

I've been meaning to post a rant about light bulbs. Used to be I could
go into a store and buy a 10-watt bug light for my deck. That's all we
need when we're on the screened-in porch late on a summer night. Dim
like a candle, kind of romantic and all. Forget it. Nobody sells
anything less than 60 watts anymore. (Remainder of light bulb rant
reserved due to risk of preaching to choir.)

Davoud

--
usenet *at* davidillig dawt com

Chris Peterson said:

My sky background measurements are generally about 0.3
magnitudes darker than the atlas would suggest.

I wouldn't take those figures too seriously. My sky-brightness
measurements (near sea level in urban and suburban areas)
also run somewhat darker than the atlas suggests, although
I haven't callibrated my measurements accurately enough to
say which is right.

In any case, everywhere I've measured -- city, suburb, and
semi-rural -- is consistently about 0.3 magnitudes brighter
in the evening than in the pre-dawn hours. Moreover,
everything gets another 0.2 - 0.3 magnitudes brighter
when the leaves fall from the trees. And fresh snow can
easily increase the brighness by 0.4 - 0.5 magnitudes.
So it's really not possible to assign a single number to
the zenithal sky brightness, even assuming excellent
transparency.

In a dark area, you've also got to worry about the zodiacal
light -- the single largest factor in natural skyglow -- and the
fact that it varies quite a bit from one part of the sky to
another.

Frankly, if the atlas can get within half a magnitude, in detail,
across the entire world, that's a staggering achievement.
Regardless of the actual numbers attached, I'm assuming
that the zones are more or less self-consistent, i.e. anything
in the middle of a green zone is roughly comparable. Of
course, that assumption may also be false. For instance,
if the North America data was based on satellite data from
the winter and Europe from the summer, that would introduce
a systematic bias into the atlas.

- Tony Flanders

Tony Flanders wrote:

(a good analysis of sky brightness via Clear Sky Clock) and:

It's a fair bet that the difference between the darkest
black and the bright edge of the blue zone is barely
perceptible...

I'm going to have to go ahead and disagree with you on that one, Tony.
I observe quite often from Vekol Ranch, about 50 miles from the center
of Phoenix, Arizona (pop 3 million). Clear Sky Clock puts it firmly in
the blue zone (http://www.cleardarksky.com/lp/VklRdAZlp.html). Quite a
few times per year I observe from Sentinel
(http://www.cleardarksky.com/lp/SntnlAZlp.html), which registers as
dark gray. The difference in zenithal sky brightness may be only a
couple tenths of a magnitude, but the difference in the overall
observing experience is very significant, justifying a doubling of my
drive to two hours.

At Vekol Ranch, the glow of Phoenix harms dark adaptation. I never
point the telescope below an altitude of 60 degrees when pointed north.
From Sentinel, Phoenix is still bright, but doesn't light up the
desert in the opposite direction. You can confidently point the
telescope to an altitude of 30 degrees in that direction with little
loss due to light pollution.

The best example of a "black" site on Clear Sky Clock that I can come
up with is Oregon Star Party, which is clearly darker than either
Sentinel or Vekol Ranch, albeit too far north for my tastes.



I would say that black, gray, and blue (and possibly
the dark edge of green) correspond to Bortle classes
1 and 2 (http://skyandtelescope.com/resources/darksky)...

If that were really the case, then the Bortle scale has little meaning
on the dark end. In fact, I think you would find that a blue site is
Bortle class 3 or 4. Of course, altitude matters, and I bet Chris
Peterson is right on about his site.


In my experience, if you're observing anywhere inside
the green zone, you've got little to complain about...

That describes the East Valley Astronomy Club's Florence Junction site,
and I have done plenty of complaining about it! Half of the sky is
useable, but even it is not really "true dark."


I think the Clear Sky Clock does an excellent job on the dark end of
the scale. It may be argued that telescopic views at the zenith would
not be much different between dark gray or black sites, but the
difference between either of them and blue, and certainly green, is
striking.

Tokm

I wrote:

It's a fair bet that the difference between the darkest
black and the bright edge of the blue zone is barely
perceptible...

Tom Polakis responded:

I'm going to have to go ahead and disagree with you on that one,
Tony.
I observe quite often from Vekol Ranch, about 50 miles from the
center
of Phoenix, Arizona (pop 3 million). Clear Sky Clock puts it firmly
in
the blue zone (http://www.cleardarksky.com/lp/VklRdAZlp.html). Quite
a
few times per year I observe from Sentinel
(http://www.cleardarksky.com/lp/SntnlAZlp.html), which registers as
dark gray. The difference in zenithal sky brightness may be only a
couple tenths of a magnitude, but the difference in the overall
observing experience is very significant, justifying a doubling of my
drive to two hours.

At Vekol Ranch, the glow of Phoenix harms dark adaptation. I never
point the telescope below an altitude of 60 degrees when pointed
north.
From Sentinel, Phoenix is still bright, but doesn't light up the
desert in the opposite direction...

Thanks, Tom; those are very useful data points. I've observed at a
fair number of dark to very dark sites, but not on a regular basis,
as you have. That makes it hard for me to generalize, as you can.

In fact, it occurred to me after posting my note that I was
oversimplifying things drastically by talking only about the
zenith. Skyglow is typically at least three times stronger at
altitude 30 toward the brightest light source than it is at
the zenith. So if the zenith is 33% brighter than it "should"
be, the worst direction can be pretty bad -- as in the case of
Vekol Ranch. If you're in a blue area with the major light source
to the south, you've got a real problem!

Two more thoughts. Arizona is somewhat of a special case. The
light-pollution map is based on satellite data from 1996-1997,
and it's probably seriously outdated in the case of Phoenix,
one of the fastest-growing cities in the U.S. Also, AZ is
unlike most dark places I've been in having vast flat areas.
In my experience, the skyglow of Phoenix is visible almost
across the entire state, which it wouldn't be if Phoenix
were ringed with mountains as, say, Los Angeles is. I suspect
that "baffling" effect of mountain ranges is quite strong,
and I'm sure that this wasn't modelled in the l-p atlas.

- Tony Flanders

wrote:
[snip]

In fact, it occurred to me after posting my note that I was
oversimplifying things drastically by talking only about the
zenith. Skyglow is typically at least three times stronger at
altitude 30 toward the brightest light source than it is at
the zenith. So if the zenith is 33% brighter than it "should"
be, the worst direction can be pretty bad -- as in the case of
Vekol Ranch. If you're in a blue area with the major light source
to the south, you've got a real problem!

Hi Tony,
I put my new SBM on a remote az/el mount here in Tucson while
running tests before installing it at the telescope.

For this location I obtained a factor of 1.8 from zenith to 30 deg
elevation. I think it depends on the amount of haze and the altitude
of the first major inversion. allot of the times its between 9 and 11
thousand feet but some time lowers to less than half of that during a
strong High pressure system.

When time permits we will do a night sky road trip and measure between
Phoenix and Tucson to see how it compares with the map.

dan



Two more thoughts. Arizona is somewhat of a special case. The
light-pollution map is based on satellite data from 1996-1997,
and it's probably seriously outdated in the case of Phoenix,
one of the fastest-growing cities in the U.S. Also, AZ is
unlike most dark places I've been in having vast flat areas.
In my experience, the skyglow of Phoenix is visible almost
across the entire state, which it wouldn't be if Phoenix
were ringed with mountains as, say, Los Angeles is. I suspect
that "baffling" effect of mountain ranges is quite strong,
and I'm sure that this wasn't modelled in the l-p atlas.

- Tony Flanders