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General Astronomy >> General Observing and Astronomy

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andyrud
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Reged: 05/21/07

Loc: Philo, California
Blue, green, red, blue, black Skys? new
      #5512578 - 11/10/12 01:02 AM

Hi Everyone,
In many of the posts I see people talking about different seeing conditions and giving them names like blue, green etc. I searched in vane for the definitions to no avail. Could somebody direct me to some place that has the definitions and how somebody would grade their viewing spots?

Thanks in advance
Andy R


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Maverick199
Postmaster
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Reged: 02/27/11

Loc: India
Re: Blue, green, red, blue, black Skys? new [Re: andyrud]
      #5512580 - 11/10/12 01:07 AM

Google Bortle scale and you should get it.

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andyrud
member


Reged: 05/21/07

Loc: Philo, California
Re: Blue, green, red, blue, black Skys? new [Re: Maverick199]
      #5512604 - 11/10/12 01:34 AM

Quote:

Google Bortle scale and you should get it.




Thanks Maverick, That did it.

Andy R


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MEE
super member


Reged: 07/10/10

Re: Blue, green, red, blue, black Skys? new [Re: andyrud]
      #5512658 - 11/10/12 03:46 AM

This is supposedly the more accurate of the several color coded maps that represent the Bortle Scale:

https://sites.google.com/site/3davel/home/light-pollution/lp2001/gmap

Even it is out of date.

This has been the subject of much debate, but here's my take on it:
Each color zone is supposed to estimate what the sky would look like, based upon the Bortle Scale. (based upon the amount of light estimated to be given off by towns and cities based upon their populations (the 2000 census, in this case, I believe))

The black zones are supposed to have Bortle Class 1 conditions. The gray zones are supposed to have Bortle Class 2 conditions, and so on.

But that isn't always the case. You might go to a black zone sky and NOT see what is described for a Bortle Class 1 sky. You might go to a green zone and NOT see what is described for a Bortle Class 4 sky. Why?

Sometimes it's obvious. If you go to a green zone sky, and it's cloudy, or if the moon is out, or if there's obvious smoke or haze, or if you're standing right next to several large bright lights in a campground parking lot, then it's obvious you are NOT going to experience Class 4 skies.

Other times it's not so obvious. You might go to a green zone sky and: the sky is clear, no moon, no obvious smoke, haze, or pollution, and no lights in your immediate vicinity. Yet: the sky does not have the features of a class 4 sky; instead, it seems only to have the features of a class 5 sky. Why? (We're assuming here that you've adapted your eyes to the darkness properly)

--it could be a bit hazy. Though the haze or pollution may not be immediately obvious, it still may be there in enough thickness to slightly degrade the quality of your sky.

--there could be more local light pollution than you realize. (there are lots of farmhouses, oil derricks, and other sources of light pollution in the countryside, especially in certain areas)

--it's been a while since your last eye exam. Sure, this is biased by the fact that I have a girlfriend who's an optometry student, but I got glasses after she did an eye exam on me last year, and my vision is certainly sharper. I'm seeing more stars than I previously was able to.

Good news, though: sometimes you can do BETTER than what the color zone may indicate. You may go to a green zone (Class 4) location, yet experience Bortle Class 3 skies. Why?

--altitude: this can make the sky a bit darker (not always)
--exceptional clarity: there may be LESS haze and/or pollution in the air than normal.

Moral of the story: if you're ever reporting an observation, don't just say "I went to a green zone" or something like that. Memorize the full description of the Bortle Scale (or at least have it with you for reference). Look carefully at the sky (after your eyes are adapted to the darkness). Look at the Milky Way. How does it appear? What about the zodiacal light? Light pollution domes? Sure, you may be in a green zone, but what Bortle Class sky are you REALLY seeing? That's the more important thing to report.

Edited by MEE (11/10/12 03:46 AM)


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REC
Post Laureate
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Reged: 10/20/10

Loc: NC
Re: Blue, green, red, blue, black Skys? new [Re: MEE]
      #5512911 - 11/10/12 10:12 AM

The google map is not bad for my area, shows I live right at the edge of a red/orange zone. Facing east for me is where much of the sky glow is, but I just wait for rising objects get higher in the sky before I start to observe them. On good nights od seeing like lately with no humidity, I probably gain an extra magnitude.....it appears?

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Blue, green, red, blue, black Skys? new [Re: andyrud]
      #5512955 - 11/10/12 10:47 AM

The color zones come from the World Atlas of Artificial Light Pollution, published in 2001. The website has everything you could possibly want to know about it -- and more. The Atlas estimated skyglow based on satellite measurements.

In the same year, John Bortle published a Dark Sky Scale based on subjective criteria such as the visibility of the Milky Way.

The two later became conflated or confused by sequence of events too complicated to explain, but they've never been a very good fit.

I have written about the subject extensively in print and online. You might be interested in my article Rate Your Skyglow.

Tony Flanders
Associate Editor, Sky & Telescope


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roscoe
curmudgeon
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Reged: 02/04/09

Loc: NW Mass, inches from VT
Re: Blue, green, red, blue, black Skys? new [Re: MEE]
      #5518792 - 11/13/12 10:08 PM

Quote:

This is supposedly the more accurate of the several color coded maps that represent the Bortle Scale




I like this one!! it puts me almost in a blue zone!
But, don't try to magnify it too much, or it gets all confused, and at least in NW Mass, puts some definitely downtown areas in the greatly expanded blue zone.

My standard test is to look at Ursa minor - the little dipper - the 4 stars in the 'dipper' are approx mag 2-3-4-5, and if the dim ones nearby can be seen, it's almost mag 6. In my version of green zone, that's a once-a-month at best sky!

Russ


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GpB311
sage


Reged: 05/29/12

Loc: Los Angeles, CA
Re: Blue, green, red, blue, black Skys? new [Re: roscoe]
      #5518817 - 11/13/12 10:25 PM

Im apparently in a white zone, but I feel like the skies from my backyard arent really that bad...especially since getting colder.

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Blue, green, red, blue, black Skys? new [Re: roscoe]
      #5519304 - 11/14/12 10:21 AM

Quote:

Quote:

This is supposedly the more accurate of the several color coded maps that represent the Bortle Scale




I like this one!! it puts me almost in a blue zone!
But, don't try to magnify it too much, or it gets all confused, and at least in NW Mass, puts some definitely downtown areas in the greatly expanded blue zone.




Where? I'm not seeing that.


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Maverick199
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Reged: 02/27/11

Loc: India
Re: Blue, green, red, blue, black Skys? new [Re: Tony Flanders]
      #5519442 - 11/14/12 12:06 PM

I have read Tony's article and it's really awesome for people living in the States, Canada and Mexico. Unfortunately, there seems to be no representation for India and from the World Atlas of Artificial Light Pollution, it looks outdated to say the least. Sure there are rural areas here which have dark skies but the cities have horrible light pollution and sky glow.

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Blue, green, red, blue, black Skys? [Re: Maverick199]
      #5519611 - 11/14/12 02:34 PM

Quote:

I have read Tony's article and it's really awesome for people living in the States, Canada and Mexico. Unfortunately, there seems to be no representation for India and from the World Atlas of Artificial Light Pollution, it looks outdated to say the least. Sure there are rural areas here which have dark skies but the cities have horrible light pollution and sky glow.




True. Lighting in India's major cities has changed beyond belief since the year 2000. They're still much dimmer than U.S. cities of the same size, however.

My very first memory of India was flying into Delhi 25 years ago and seeing no bright lights at all, just dim reddish glows stretching on into the distance.


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mountain monk
Carpal Tunnel
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Reged: 11/06/09

Loc: Grand Teton National Park
Re: Blue, green, red, blue, black Skys? new [Re: Tony Flanders]
      #5520747 - 11/15/12 08:40 AM

When I first flew into Shanghai in 1980 there were no lights on at the airport. We circled, they turned on the lights, and we landed. When we we took off, the lights went off again.. In those days it was possible to fly over rural China (not to mention Tibet) and not see a light. No more.

Dark skies.

Jack


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