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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 |
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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/ |
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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 |
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Thanks Tony. I shall look for the gegenschein around Jupiter the next =
couple of nights! -Florian |
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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 |
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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 |
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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 |
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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 |
#10
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