Author Topic: Bortle scale accuracy?  (Read 328 times)

coreanoguf

  • Active Astronomer
  • ***
  • Posts: 132
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Bortle scale accuracy?
« on: December 28, 2017, 01:32:40 AM »
I don't normally come over to this forum but i am hoping to clear something up.  I was told once that the clear sky clock Bortle scale has had some areas assigned their respective colors based on info taken when snow was on the ground and is therefore inaccurate.  These areas would be asigned colors that suggest the sites are not as good as they should be.  My primary summer site in lower Michigan is a yellow zone but it seems more like a green zone to me...



Aaron Romano

  • Active Astronomer
  • ***
  • Posts: 109
  • Activity:
    0%
  • Reputation: +1/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #1 on: December 29, 2017, 05:55:44 PM »
Ive found it to be pretty accurate. Im in a grren zone now after observing in nothing but blue zones for a year and it is not quite as dark. The yellow zone near home in florida is not as dark as here. Ill be goong to a grey zone and anticipate it being darker then blue zone. I think where the accuracy problems come in with cleardarksky is with everything else.

Shawn Bush

  • Active Astronomer
  • ***
  • Posts: 112
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #2 on: December 31, 2017, 12:59:55 AM »
Quote
I don't normally come over to this forum but i am hoping to clear something up.  I was told once that the clear sky clock Bortle scale has had some areas assigned their respective colors based on info taken when snow was on the ground and is therefore inaccurate.  These areas would be asigned colors that suggest the sites are not as good as they should be.  My primary summer site in lower Michigan is a yellow zone but it seems more like a green zone to me...

First of all, please don't call it the Bortle scale. BrooksObs, the expert on the subject, is welcome to correct me. But as far as I'm concerned, any connection between these colors and the Bortle scale is very nearly coincidental.

The color zones come from the  World Atlas of Light Pollution, based on satellite measurements taken in the late 1990s. The  Bortle Dark-Sky Scale is a set of criteria for evaluating the actual condition of your skies based on what you can see with your own eyes.

In any case, you are correct about snow biasing the Light Pollution Atlas. The original post in this forum is  here. And here is  my blog on the subject.

The fact that someone in Tennessee finds the color zones accurate confirms this; there's not much snow in Tennessee.

bandretaco

  • Active Astronomer
  • ***
  • Posts: 125
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #3 on: December 31, 2017, 07:56:59 AM »
I would add to Tony's remarks that the combining of the Bortle Dark Sky Scale ratings with zones on light pollution maps was done without any input from me, so I cannot verify any degree of accuracy in this sort of application. In some cases it may prove reasonably accurate, while in other situations the ratings may be off significantly.

I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination  imparted to the surrounding atmosphere by these sources. Such can be apparrent from far beyond the location of the sources themselves.

Even at great distances from intense light sources the sky can be seen noticeably illuminated in the source's direction. Thus, light pollution maps can suggest a site to be one, or two, levels darker than it actually may be. Then, too, this impact varies over the course of the seasons, in many instances becoming enhanced further when the trees have lost their foliage and sometimes greatly so in winter when the ground is snow-covered.

The Bortle Dark Sky Scale is intended for an evaluation of sky darkness and clarity by the observer at the specific time he is at a site, not truly any sort of method for predicting conditions to be anticipated when going there.

BrooksObs

Randy Wiggins

  • Active Astronomer
  • ***
  • Posts: 122
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #4 on: January 04, 2018, 12:22:32 AM »
Quote
I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination  imparted to the surrounding atmosphere by these sources. Such can be apparrent from far beyond the location of the sources themselves.

Actually, that is factored into the Light Pollution Atlas. It uses Roy Garstang's well-known formula for light dispersion and applies it to the direct measurements from the DoD satellites. That's why, for instance, only the center of Adirondack State Park is shown as really dark. The park has huge sections that generate no light at all (no electric wires), but it's surrounded by large and small cities on most sides.

tessacubadc

  • Active Astronomer
  • ***
  • Posts: 104
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #5 on: January 04, 2018, 05:08:05 PM »
The applications of formulae to supposedly account for effects of scatter at a distance is all well and good, but in the real world just how accurately does it do so?. What I've experienced over many years as an observer is that such formulae do not unusually indicate the true situation.

For example, I can tell you folks that the last time I was down at TSP I was rather shocked to note that I could definitely detect the presence of cities like Stockton and even Midland-Odessa, at very great distances from the ranch. And this impact was quite distinct well up into the sky, too! Now I don't believe for a moment that any formula would actually fully predict that to be the case. 

Likewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days.

If you can detect any trace of sky-glow be assured that it's impacting your overall level of sky darkness, whether some plot says it is, or not.

BrooksObs

Lasaro Tourabi

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #6 on: January 09, 2018, 04:32:11 PM »
Quote
The applications of formulae to supposedly account for effects of scatter at a distance is all well and good, but in the real world just how accurately does it do so?

Modestly well -- as well as you could expect. You can read Garstang's papers yourself if you want to know more.

The actual scatter will depend on the conditions on any given night. For instance, high clouds above a distant city make its light travel much farther. Conversely, low fog can suppress the light almost completely.

<p class="citation">QuoteLikewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days.[/quote]

Actually, the Light Pollution Atlas shows the darkest part of the Adirondacks as gray, the second-darkest zone. It shows no black zones at all east of the Mississippi.

Don't be too quick to assume that the glow south of the Adirondacks is primarily from New York City. There's a solid line of medium-sized cities just south of the park. Albany puts out a huge amount of light, as I know to my sorrow.

Michael Litvack

  • Active Astronomer
  • ***
  • Posts: 131
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #7 on: January 10, 2018, 05:01:40 AM »
I'm sorry, Tony, but I don't for a minute buy your explanation of the Adironack skyglow I cited, as originating locally. The individual I'm quoting has a level of experience likely far beyond just about anyone on this forum and certainly knows whether the source of illumination he indicates was relatively near, or very far off. Likewise, I've distinctly seen the glow of the New York metropolitan area from the region well west of Albany myself.

The sad fact is that intense sources of light pollution are subtly evident hundreds of miles away on really clear nights. Darkest local conditions may often coincide with periods of higher humidity that makes the air less transparent, blocking distance light sources. I've already cited that Midland-Odessa's sky illumination is visible in the clear air at TSP. And some years ago I was informed by a professional pilot/amateur astronomer that on the Hawaii-LA run, from 35,000 feet it was possible to home in on the glow of LA from 600 miles out at sea!

Amateur astronomers still don't appreciate half the impact light pollution actually has.

BrooksObs

Jack Jefferson

  • Active Astronomer
  • ***
  • Posts: 118
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #8 on: January 11, 2018, 02:04:28 AM »
Quote
I would add to Tony's remarks that the combining of the Bortle Dark Sky Scale ratings with zones on light pollution maps was done without any input from me, so I cannot verify any degree of accuracy in this sort of application. In some cases it may prove reasonably accurate, while in other situations the ratings may be off significantly.

I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination  imparted to the surrounding atmosphere by these sources.

BrooksObs
It's unfortunate that the misunderstanding still exists, but those who are aware that there is no connection between the Bortle scale and the World Atlas of Artificial Night Sky Brightness try to remove the confusion when aware of it.

No one would expect you to vouch for the accuracy of the other work once they understand that you are neither associated with it nor familiar with it, as you both state and demonstrate above.

handthedemo

  • Active Astronomer
  • ***
  • Posts: 123
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #9 on: January 11, 2018, 10:23:20 AM »
Quote
For example, I can tell you folks that the last time I was down at TSP I was rather shocked to note that I could definitely detect the presence of cities like Stockton and even Midland-Odessa, at very great distances from the ranch. And this impact was quite distinct well up into the sky, too! Now I don't believe for a moment that any formula would actually fully predict that to be the case.  Likewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days. If you can detect any trace of sky-glow be assured that it's impacting your overall level of sky darkness, whether some plot says it is, or not.BrooksObs
I agree.  There are VERY few truly dark spots left in the lower 48.  Here is photo of the sky glow from Seattle in Wenatchee.  Downtown Seattle is about 125 air miles from Wenatchee.  Those mountains in the lower part of the picture are 9,000 feet and Seattle is on the OTHER side of them.Check out the aurora and compare that to the sky glow from over a hundred miles away.  The other point is truly great nights are fairly rare in even dark sky areas.  The two best nights I had were on the Beaverhead National Forest outside of Yellowstone and up at my cabin in the Washington Cascades.  In that case, heavy fog covered the entire state with the house and observatory ABOVE the fog layer which prevented any "light leakage".  My observation has been ANY moisture or aerosols in the atmosphere REALLY affect the sky IF ANY light is visible.


bersrorexnutg

  • Active Astronomer
  • ***
  • Posts: 122
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #10 on: January 11, 2018, 04:44:21 PM »
Quote
Here is photo of the sky glow from Seattle in Wenatchee.  Downtown Seattle is about 125 air miles from Wenatchee.  Those mountains in the lower part of the picture are 9,000 feet.

For the record, Wenatchee is 95 miles from the center of Seattle. It's pretty depressing that the mountains didn't block more of the light, but that's likely due to the fact that Seattle had clouds above it. As I said, high clouds above a city make its light spread much farther.

I have a rather similar picture taken 120 miles north of the center of New York City.

Since the mountains are quite close to Wenatchee and the clouds are directly visible in the photo, they must actually be quite high.

Was this actually taken inside Wenatchee? If so, I'm surprised that the local light pollution wasn't more prominent.

The good side of all of this is that the Seattle skyglow seems to be confined to a fairly modest band along the horizon, not spreading very high.

cokoksmarous

  • Active Astronomer
  • ***
  • Posts: 119
  • Activity:
    0%
  • Reputation: +1/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #11 on: January 12, 2018, 12:12:58 PM »
Quote
Here is photo of the sky glow from Seattle in Wenatchee.  Downtown Seattle is about 125 air miles from Wenatchee.  Those mountains in the lower part of the picture are 9,000 feet.
Wait a sec ... something's wrong here. The city of Wenatchee is almost due east of Seattle, but this photo was taken to the south-southeast of Seattle, possibly near Mt. Rainier. That's the Big Dipper in the upper right corner, and Polaris is just out of the frame to the right.

Sean Lee

  • Active Astronomer
  • ***
  • Posts: 124
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #12 on: January 12, 2018, 06:32:25 PM »
Just took a compass bearing and the left side of the picture is due WEST.  So the sky glow is from Seattle up to Everett and a little beyond.You might be getting thrown by the fact that Wenatchee is significantly farther north than Boston.I measured the distance on an atlas from our vacation home to downtown Seattle at a hundred miles.  Our house in Wenatchee is 12 miles east of our vacation home.  Wenatchee does have considerable sky glow.  The electricity rate is TWO cents a Kilo-watt hour. Talking to people about turning out lights to save money is a non-starter.  We are 5 miles north of downtown and up in elevation.  Most of the town is south due to the rivers.  There is only sage north of us. I really do notice the effect of moisture and aerosols in the air.  It does make a significant difference.  I am about 5.7 to the north.....and considerably less due south when looking over town.The good news is Seattle is the cloudiest part of the United States (outside of Alaska) and Wenatchee according to the Chamber of Commerce gets 300 days of sunshine.  The clouds in Seattle are low in elevation so most of the time we do not get to see Seattle's sky glow.  That picture does represent the worst sky glow from Seattle that I have seen.  You are probably right about the high clouds reflecting the light from Seattle.

veworltonuc

  • Active Astronomer
  • ***
  • Posts: 128
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #13 on: January 13, 2018, 06:45:57 AM »
Going from the maps, we should be starting up an astronomy village in southeastern Oregon.-Rich

brodsandbacksosp

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: Bortle scale accuracy?
« Reply #14 on: January 13, 2018, 09:03:49 AM »
Yep. Somewhere between Adel and Denio along Route 140. One lonely road, though the banjoes can sometimes be distracting.Dark skies.Jack