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General Astronomy >> Light Pollution

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George N
Carpal Tunnel
*****

Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3872177 - 06/18/10 12:32 PM

The new map looks more accurate to what Ive experienced living in rural NY, with a camp in the central Adirondacks, plus frequent trips to Cherry Springs and once-a-year to Stellafane.

Snow is bad news for observational astronomy! Ive been getting about .3 to .5 brighter SQM readings with snow cover, and qualitatively the sky just looks brighter not to mention the annoying light from the ground while trying to observe.


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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: George N]
      #3873391 - 06/19/10 02:02 AM

I'm posting because I have discovered a relatively minor problem with the 2001 DSMP satellite data used to make the new atlas. Apparently, in some locations the light sources in the 2001 data are displaced from where they should be. The maximum displacement is about 4 to 5 km in the east/west direction and 2 to 3 km in the north/south direction. I have determined the origin of this problem is the DSMP satellite data online because there are also independent population and land-use data in the same location as the DSMP satellite data. These are on the same projection/grid and there is no horizontal displacement in these datasets. I dicovered this problem by overlaying the maps in google earth and zooming in on small towns.

I think I might be able to come up with an algorithm to fix the problem.

This doesn't affect the what we have talking about above, and you would never notice it if you didn't zoom in on google maps. Nevertheless I'm sorry I didn't check things out more closely before I posted.

-Dave


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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3875110 - 06/20/10 05:21 AM

I've come up with an algorithm to fix the displacement errors in the 2001 light data. The new data is now available on the webpage. The fix is not perfect but it's definitely better than before. Most people will not notice, unless you overlay the maps in google earth and zoom way in.

The adjustment is not uniform across the map and some area have no adjustment. The biggest adjustment is about 5 pixels (the size of a pixel is 1/120 degrees). I'll add more detail on the adjustment on the web site soon.

-Dave


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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3876804 - 06/21/10 02:17 AM

You can read about the small position errors in the 2001 light data and how I corrected for them here. None of this effects the brightness of the lights, etc.

The corrections are small (I thought about reposting the figures in this thread, but I can't see a difference after I apply the corrections). You will only notice the improvement when you compare the light pollution atlas to small towns when zoomed in in Google Earth or something similar.

-Dave


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csa/montana
Den Mama
*****

Reged: 05/14/05

Loc: montana
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3877174 - 06/21/10 10:32 AM

Dave, thanks for all the work you have put into this! It is greatly appreciated!

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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: csa/montana]
      #3879058 - 06/22/10 05:40 AM

Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. The image should be semi-transparent so that you can see the info from google maps underneath (I'm hoping all web browsers show it the same way).

-Dave


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George N
Carpal Tunnel
*****

Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3879770 - 06/22/10 01:49 PM

Quote:

Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. ...-Dave




It works great Dave, and at least in my area of NY/PA it makes more sense than the old map.


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Matt Lindsey
sage
*****

Reged: 06/06/08

Loc: Baltimore, MD, U.S.A.
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3891603 - 06/28/10 07:48 PM

Thank you Dave for this valuable tool! I used to observe around Wildcat Mountain S.P. in Wisconsin and the skies seemed much darker than a green zone. Indeed, it's actually in the GRAY! Likewise, my dark observing sites in Maryland just got darker.

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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Matt Lindsey]
      #3892118 - 06/29/10 12:14 AM

Your welcome!

I usually observe in southwest Wisconsin (although not at Wildcat Mountain unless I'm camping there too). It turns out that all my green dark sky sites are actually blue and my blue dark sky sites are gray. These new colors seem to make more sense to me when I compare my sites to ones further south.

-Dave


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mountain monk
Carpal Tunnel
*****

Reged: 11/06/09

Loc: Grand Teton National Park
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3895592 - 06/30/10 05:55 PM

Thanks for all great work. Glad to see that my neck of the woods--NW Wyoming/Jackson Hole is getting darker!

Dark skies.

mm


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mountain monk
Carpal Tunnel
*****

Reged: 11/06/09

Loc: Grand Teton National Park
Re: New Light Pollution Atlas w/o Snow Cover new [Re: mountain monk]
      #3895650 - 06/30/10 06:29 PM

Really thanks for your work. I was in green, now I am in blue and gray is 15 minutes away. Black is 30-40 minutes. You made my day.

Dark skies.

mm


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: New Light Pollution Atlas w/o Snow Cover new [Re: mountain monk]
      #3923479 - 07/15/10 03:23 PM

The issue of sky brightness' lack of correlation with the satellite light maps is well-known.
In the southwest, my neck of the woods, light pollution seems to relate extremely well to humidity in the lower atmosphere. We have the greatest amount of dust in the lower atmosphere when it is dry and windy, yet the brightest skies are when the humidity near the ground creeps up.
So a site that is in, for instance, a red zone, might get a reading of 16.8 when the humidity is high, and a reading of 17.5 when the humidity is low.
This idea corresponds well to what I see at my home--Polaris is an averted vision object when the sky is very hazy and humidity is 80% or more, while magnitude 4 stars are easily visible when the humidity is near zero.

It's possible it is not solely humidity but the interaction between airborne water vapor and smog or dust. But I have taken measurements at my home for over 5 years and the transparency of the sky varies right along with the humidity. Air SHOULD be just as transparent with water vapor in it, but it's not.

That factor seems to make any sort of absolute light pollution map unlikely unless you have annual averages for each site correlated to a color map.


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Dan G
Carpal Tunnel
*****

Reged: 06/27/06

Loc: Minisink, NY, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3924133 - 07/15/10 09:20 PM

Great work. Thank you.

Dan in NY


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Tony Flanders
Postmaster
*****

Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3924721 - 07/16/10 07:22 AM

Quote:

The issue of sky brightness' lack of correlation with the satellite light maps is well-known ... That factor seems to make any sort of absolute light pollution map unlikely unless you have annual averages for each site correlated to a color map.




Oh sure, but that's a separate issue. Obviously you're going to get significant variation from one night to another, from one time of night to another, and (in places with deciduous trees and/or snow) from one season to another.

But one would hope that on average, a red zone in one part of the world would be roughly -- vaguely -- equivalent to a red zone in another part of the world.

However, it now seems pretty clear that this is not true. In the original Light Pollution Atlas, a red zone in a band stretching from New England and the Maritimes across the Upper Midwest seems to be on average roughly equivalent to an orange zone in a band stretching from CA to the Old South.


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3925266 - 07/16/10 12:57 PM

So... any idea if and when Clear Sky Chart will be updated with the new values?

Mike


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925280 - 07/16/10 01:02 PM

I have noticed that at my dark sky site, often the sky is more transparent when it is very dewy at groundlevel, and less transparent when it is not so dewy. It seems as if when it is less dewy, the moisture is still hovering above in the atmosphere, but when the ground is very dewy, the moisture has settled down and the sky has cleared. So for transparent skies, heavy dew can be a good thing. Am I understanding this correctly?

Mike


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925317 - 07/16/10 01:20 PM

My dark sky site on the Eastern Shore of Maryland is a yellow zone according to the Clear Sky Chart. However, I have noticed that at the dark site, the Milky Way has a definite bulge into Ophiuchus. In fact, on relatively good nights I can follow the MW down to the vicinity of Cebalrai and up to Sabik. According to CSC, in a yellow site there are "some dark lanes in milkyway but no bulge into Ophiuchcus," in a green site "Milkyway shows much dark lane structure with beginnings of faint bulge into Ophiuchus," while in a blue zone the "Milky way shows bulge into Ophiuchus." There is definitely something wrong here.

Mike


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Tony Flanders
Postmaster
*****

Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925355 - 07/16/10 01:40 PM

Quote:

I have noticed that at my dark sky site, often the sky is more transparent when it is very dewy at groundlevel, and less transparent when it is not so dewy.




I suspect you have that precisely backward; it's the good transparency that causes the dew.

Dew forms when radiative cooling cools surfaces below the ambient air temperature. A surface at the same temperature as the air can get wet from fog, but it can't form dew, properly speaking.

Water vapor in all its forms is quite opaque to infrared, which is why there's so much interest in putting infrared scopes in Antarctica and atop high, dry mountains. So haze is a very good inhibitor of radiative cooling. Conversely, very clear nights maximize radiant cooling and promote dew formation.


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3926424 - 07/16/10 09:39 PM

So my observation is correct, that transparency does correlate with dewing, but my understanding of the causal relationship is incorrect? I just assumed that dew does not spontaneously generate as in the old beliefs about flies and meat, or mice and wheat, that there must be some reason behind it, and that it had something to do with the transparency of the sky.

Mike


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3927203 - 07/17/10 10:12 AM

Don,

Quote:

In the southwest, my neck of the woods, light pollution seems to relate extremely well to humidity in the lower atmosphere. We have the greatest amount of dust in the lower atmosphere when it is dry and windy, yet the brightest skies are when the humidity near the ground creeps up.
So a site that is in, for instance, a red zone, might get a reading of 16.8 when the humidity is high, and a reading of 17.5 when the humidity is low.




In Maryland, I have the exact opposite experience. At my dark zone site, when the "humidity near the ground creeps up" the sky tends to be more transparent. Now, when you say "humidity near the ground creeps up," do you mean dewing? When we have heavy dew - and a prediction of high humidity - the skies are usually more transparent. For instance, when we have heavy dew there are galaxies that I can see in my 10" Newt that I cannot see when we do not experience as much dew. As long as I keep the optics free from dewing, I will see dimmer objects on a dewy night. At least, so far, that is my experience.

Mike


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