Re: Ranking light pollution
From: Tony Flanders (tony_flanders_at_yahoo.com)
Date: 06/25/04
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Date: 25 Jun 2004 12:56:26 -0700
"Craig Franck" <craig.franck@verizon.net> wrote in message news:<UzGCc.2507$L8.2247@nwrdny02.gnilink.net>...
> 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
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