Ranking light pollution

Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

--
Craig Franck

Cortland, NY

Yes, there is a system but it is not based on limiting magnitude alone (nor
could any accurate system be). Check Sky & Telescope's website and look under
the Light Pollution subsection. There you will find the Bortle Dark Sky Scale,
which evaluates a number of factors in deriving a sky darkness value. The
system has been utilized to evaluate the sky darkness at various sites by the
U.S. National Park Service as well as being in wide use by observers
throughout Europe.

JB

Yes, there is a system but it is not based on limiting magnitude alone (nor
could any accurate system be). Check Sky & Telescope's website and look under
the Light Pollution subsection. There you will find the Bortle Dark Sky Scale,
which evaluates a number of factors in deriving a sky darkness value. The
system has been utilized to evaluate the sky darkness at various sites by the
U.S. National Park Service as well as being in wide use by observers
throughout Europe.

JB

Craig Frank posted:

Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

I would say that this is probably in the moderate category of light pollution.
There are worse, but there are also considerably better conditions.
I came up with an idea of gauging sky quality a while back which divides it
into several levels: Severe, Moderate, Mild, Dark Sky, and Pristine. I would
call Severe light pollution a situation where (even in the absence of direct
lighting), only stars magnitude 3.5 and brighter were visible to the unaided
eye with considerable skyglow in all parts of the sky. Areas like some places
in New York City or central Los Angeles could fit this classification.
Moderate light pollution is even more difficult to quantify, but some
limited deep-sky observations are possible in such an environment
(double stars, brighter open clusters, brighter planetary nebulae, ect.). I
would probably put the moderate light pollution at a limiting magnitude of
between 3.6 to 4.5 or so with varying amounts of skyglow in different
directions, although no part of the sky would appear very dark. A "mild"
light pollution level would probably be at the point where a person could see
4.6 to 5.5 or so in some areas of the sky, and where the brighter portions of
the Milky Way high above the horizon might be visible at times. Various areas
around the horizon would show skyglow, but overhead and perhaps in a few other
areas, the sky brightness would appear noticably darker. I think that
probably most of the Messier Objects would be visible in a good telescope over
4 inches in aperture with mild to moderate light pollution levels, although the
detail visible in them might be a bit limited. The number of faint galaxies
visible in a 6" or 8" takes a big jump when you get down to the mild light
pollution level. Some light pollution filters can help combat the effects of
low to moderate light pollution to a degree.
I would consider a "dark sky" site to be a place where you can see stars
around 6th magnitude or so (occasionally even fainter) with averted vision
routinely, but where there may be a few limited light domes from nearby
cities visible. At such sites, detail in faint deep-sky objects becomes a
good deal easier to see, and the Milky Way shows up prominently. My dark sky
site routinely gets me to +6.5 naked eye, and occasionally fainter. Skies
which routinely allow naked-eye sightings fainter than +6.5 I would consider
"pristine". Usually, these places are located well away from any major
cities, and at higher altitudes. Little or no light pollution is visible
over the entire sky at such pristine sites. For example, the Nebraska Star
Party site is deep in the sparsely-populated Sandhills at 3100 feet
elevation (the nearest sizable "city", North Platte, Nebr., pop. 24,509, is
100 miles to the south). At that site, I have seen stars as faint as +7.5,
and others have gone to 8.0. Similar such feats are possible in many areas,
especially in the Rockies, and in the high desert areas of the American
southwest.
In summary, the following are the approximate unaided-eye visual magnitude
limits (Zenith Limiting Magnitude) and overhead light-pollution ratings for
judging light pollution:
SEVE only mag. 3.5 or brighter stars are visible with fairly bright skyglow
over most of the sky.
MODERATE: mag. 3.6 to 4.5 visible (variable skyglow).
MILD: mag. 4.6 to 5.5 visible (some notable darker areas visible).
DARK SKY: mag. 5.6 to 6.6 visible (dark with occasional light domes).
PRISTINE: mag. 6.7 and fainter visible (little or no light pollution).

If you want to get the most out of Deep-Sky with a telescope, get to as dark
a site as you reasonably can (even if there is some light pollution), and
then work with what you have. You may be surprised as to how well you do,
even from a non-optimal location! Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Craig Frank posted:

Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

I would say that this is probably in the moderate category of light pollution.
There are worse, but there are also considerably better conditions.
I came up with an idea of gauging sky quality a while back which divides it
into several levels: Severe, Moderate, Mild, Dark Sky, and Pristine. I would
call Severe light pollution a situation where (even in the absence of direct
lighting), only stars magnitude 3.5 and brighter were visible to the unaided
eye with considerable skyglow in all parts of the sky. Areas like some places
in New York City or central Los Angeles could fit this classification.
Moderate light pollution is even more difficult to quantify, but some
limited deep-sky observations are possible in such an environment
(double stars, brighter open clusters, brighter planetary nebulae, ect.). I
would probably put the moderate light pollution at a limiting magnitude of
between 3.6 to 4.5 or so with varying amounts of skyglow in different
directions, although no part of the sky would appear very dark. A "mild"
light pollution level would probably be at the point where a person could see
4.6 to 5.5 or so in some areas of the sky, and where the brighter portions of
the Milky Way high above the horizon might be visible at times. Various areas
around the horizon would show skyglow, but overhead and perhaps in a few other
areas, the sky brightness would appear noticably darker. I think that
probably most of the Messier Objects would be visible in a good telescope over
4 inches in aperture with mild to moderate light pollution levels, although the
detail visible in them might be a bit limited. The number of faint galaxies
visible in a 6" or 8" takes a big jump when you get down to the mild light
pollution level. Some light pollution filters can help combat the effects of
low to moderate light pollution to a degree.
I would consider a "dark sky" site to be a place where you can see stars
around 6th magnitude or so (occasionally even fainter) with averted vision
routinely, but where there may be a few limited light domes from nearby
cities visible. At such sites, detail in faint deep-sky objects becomes a
good deal easier to see, and the Milky Way shows up prominently. My dark sky
site routinely gets me to +6.5 naked eye, and occasionally fainter. Skies
which routinely allow naked-eye sightings fainter than +6.5 I would consider
"pristine". Usually, these places are located well away from any major
cities, and at higher altitudes. Little or no light pollution is visible
over the entire sky at such pristine sites. For example, the Nebraska Star
Party site is deep in the sparsely-populated Sandhills at 3100 feet
elevation (the nearest sizable "city", North Platte, Nebr., pop. 24,509, is
100 miles to the south). At that site, I have seen stars as faint as +7.5,
and others have gone to 8.0. Similar such feats are possible in many areas,
especially in the Rockies, and in the high desert areas of the American
southwest.
In summary, the following are the approximate unaided-eye visual magnitude
limits (Zenith Limiting Magnitude) and overhead light-pollution ratings for
judging light pollution:
SEVE only mag. 3.5 or brighter stars are visible with fairly bright skyglow
over most of the sky.
MODERATE: mag. 3.6 to 4.5 visible (variable skyglow).
MILD: mag. 4.6 to 5.5 visible (some notable darker areas visible).
DARK SKY: mag. 5.6 to 6.6 visible (dark with occasional light domes).
PRISTINE: mag. 6.7 and fainter visible (little or no light pollution).

If you want to get the most out of Deep-Sky with a telescope, get to as dark
a site as you reasonably can (even if there is some light pollution), and
then work with what you have. You may be surprised as to how well you do,
even from a non-optimal location! Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

You can check on http://cleardarksky.com to find your region then they have
a light pollution image that they claim represents the levels around the
USA. I doubt their assessment for my area because according to their chart
my limited mag. is limited by pollution that would exclude me seeing M33
naked eye -- but I can see it readily.

Dusty
"Craig Franck" wrote in message
...
Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

--
Craig Franck

Cortland, NY

You can check on http://cleardarksky.com to find your region then they have
a light pollution image that they claim represents the levels around the
USA. I doubt their assessment for my area because according to their chart
my limited mag. is limited by pollution that would exclude me seeing M33
naked eye -- but I can see it readily.

Dusty
"Craig Franck" wrote in message
...
Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

--
Craig Franck

Cortland, NY

"Craig Franck" wrote in message ...

Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

As John Bortle says, limiting stellar magnitude alone is a very poor
way to communicate sky quality from one person to another. The
reason is that people vary wildly in how faint the stars are that
they can see under identical circumstances. I have personally
witnessed two different experienced amateur astronomers classify
the same sky as magnitude 6.5 in one case and "not quite mag 5.0"
in another. For me, that's the difference between barely acceptable
and nearly pristine.

I have numerous quibbles with Bortle's classification system,
but it's by far the best thing out there, because it is based on
multiple criteria. As he says, you can find it under
http://skyandtelescope.com/resources/darksky.

Another way to approach this is through the global light-pollution
study at http://www.lightpollution.it/worldatlas/pages/fig1.htm.
As you can see from the North America map, Cortland is a tiny
blob of rather intense light pollution between Syracuse and
Binghamton. It also shows that it is lunacy for you to observe
from your apartment; just a 5-mile bicycle ride would reduce
your skyglow dramatically, and a 50-mile drive would take you
to one of the best oases of dark sky in the Northeast.

In my experience, the color yellow on that map corresponds to what
I call "reasonably dark skies" -- probably Class 4 in the Bortle
Scale. According to the authors of the map, it means 1-3 times
the natural sky brightness -- an explanation that doesn't explain
much to me, since natural sky brightness varies easily by a factor
of 2 or 3 depending on the part of the sky, the time of night,
latitude, and solar activity. I suspect that they actually mean
mag 24 per square arcsecond, the figure that Garstang takes as a
baseline in his famous paper on light pollution.

My guess is that your skies are what I could call "typical suburban",
meaning that on a clear night I would be able to see mag 5.0 stars
pretty easily, and that the summer Milky Way should be visible
overhead with minor effort. Somewhere around Bortle Class 6.

In my opinion, what we have now is a virtual Tower of Babel
situation; it is nearly as hard for one astronomer to convey
the quality of his/her skies to another as it is for one person
to feel another person's toothache. And I think that the *only*
way that will ever be cured is if someone can build a cheap,
widely available light meter that is sufficiently sensitive
to measure natural skyglow.

- Tony Flanders

"Craig Franck" wrote in message ...

Is there a standard system for ranking light pollution based on
limiting magnitude? The Southern exposure of my apartment
complex lets me see stars down to 4.5. I was wondering what
this would correspond to as light pollution goes.

As John Bortle says, limiting stellar magnitude alone is a very poor
way to communicate sky quality from one person to another. The
reason is that people vary wildly in how faint the stars are that
they can see under identical circumstances. I have personally
witnessed two different experienced amateur astronomers classify
the same sky as magnitude 6.5 in one case and "not quite mag 5.0"
in another. For me, that's the difference between barely acceptable
and nearly pristine.

I have numerous quibbles with Bortle's classification system,
but it's by far the best thing out there, because it is based on
multiple criteria. As he says, you can find it under
http://skyandtelescope.com/resources/darksky.

Another way to approach this is through the global light-pollution
study at http://www.lightpollution.it/worldatlas/pages/fig1.htm.
As you can see from the North America map, Cortland is a tiny
blob of rather intense light pollution between Syracuse and
Binghamton. It also shows that it is lunacy for you to observe
from your apartment; just a 5-mile bicycle ride would reduce
your skyglow dramatically, and a 50-mile drive would take you
to one of the best oases of dark sky in the Northeast.

In my experience, the color yellow on that map corresponds to what
I call "reasonably dark skies" -- probably Class 4 in the Bortle
Scale. According to the authors of the map, it means 1-3 times
the natural sky brightness -- an explanation that doesn't explain
much to me, since natural sky brightness varies easily by a factor
of 2 or 3 depending on the part of the sky, the time of night,
latitude, and solar activity. I suspect that they actually mean
mag 24 per square arcsecond, the figure that Garstang takes as a
baseline in his famous paper on light pollution.

My guess is that your skies are what I could call "typical suburban",
meaning that on a clear night I would be able to see mag 5.0 stars
pretty easily, and that the summer Milky Way should be visible
overhead with minor effort. Somewhere around Bortle Class 6.

In my opinion, what we have now is a virtual Tower of Babel
situation; it is nearly as hard for one astronomer to convey
the quality of his/her skies to another as it is for one person
to feel another person's toothache. And I think that the *only*
way that will ever be cured is if someone can build a cheap,
widely available light meter that is sufficiently sensitive
to measure natural skyglow.

- Tony Flanders

"Dusty" wrote in message ...

You can check on http://cleardarksky.com to find your region then they have
a light pollution image that they claim represents the levels around the
USA. I doubt their assessment for my area because according to their chart
my limited mag. is limited by pollution that would exclude me seeing M33
naked eye -- but I can see it readily.

Hmm, I just took a look at the Clear Sky Clock's light-pollution
stuff. IMHO, the underlying map is quite accurate, but I know for
a fact that Danko's callibration against the Bortle scale is *way*
off. Almost all of the Boston suburbs show as red on that map, and
in most of that area, the Milky Way is *readily* visible overhead.
In better spots, the Sagittarius Milky Way is obvious nearly down
to the horizon. That would make it Bortle class 5 at worst.

My country home shows as yellow -- although probably the good side
of yellow -- and I would say that it straddles Bortle class 3 and 4.
Green probably covers Bortle classes 2 and 3.

I'm still rather baffled, though, by *exactly* what the original
authors mean when they say that light pollution is 2x "the natural
sky brightness." I rather suspect that they actually mean mag 24
per square arcsecond, the figure that Garstang uses as a baseline
in his famous paper on light pollution. But in fact, natural
sky brightness varies by at least a factor of 2 or 3 depending
on distance from the zodiac, time of night, solar activity, and
latitude. Using something so variable as a fundamental unit seems
like a really bad idea! It also illustrates the fact that by the
time you get to the shade blue on that map, artificial light
pollution is negligible compared to natural factors.

On the other hand, the map is really pretty useless for measuring
urban and suburban light pollution, because that all shows as
one big blob of red or white, masking some extremely important
distinctions.

- Tony Flanders