How Dark Is It, Really?

Thursday, May 14 2009 @ 03:51 am EDT

Contributed by: dgrosvold

You can download a very high-resolution version of this North America map as a zipped file here (2MB).
Over the past few months as we've begun investigating various possible sites for the relocation of Coleman Observatory, we've had several discussions about whether the potential locations had adequately dark skies or not. The best way to determine this, of course, is to spend quite a bit of time at each location at night, and evaluate the sky conditions.

To be accurate, this would normally need to be done over several week-long stretches during every major season of the year, to be sure that the evaluation included the diversity of weather that could be expected at each site, as well as how things like seeing and transparency at that specific location will impact the quality of the dark sky. The demands in terms of time and energy make it impractical for AOAS to undertake such an intensive evaluation of every possible site. So what can we do?

There are several sources of information regarding the general brightness of artificial lighting in North America. One of the best is the World Atlas of Artificial Night Sky Brightness. This is an excellent resource which can give a fairly good idea of the level of sky glow and light pollution for any location on the globe. There are a number of high-resolution individual maps that show the brightness intensity bases on a color code established to match the Bortle Dark Sky Scale. The Bortle Scale was originally created and published by John E. Bortle in an article in Sky and Telescope magazine in February, 2001. The Bortle scale is based on nine classes of dark sky, with Class 1 being the best, or black on the atlas maps, and Class 9 or white on the atlas maps.

There are very few areas of Class 1 (black) quality dark skies left in the continental United States, and none of them are in Arkansas or Eastern Oklahoma. In fact there are but a very few Class 2 (grey) places left in Arkansas and Eastern Oklahoma. The best we can probably expect is Class 3. However, how do we know for sure if a selected site is Class 3, 4, or even worse? Even the highest resolution maps from the World Atlas of Artificial Night Sky Brightness leaves us with quite a bit of doubt regarding specific locations because there just isn't enough information on the maps to make more than a general guess about the exact location of a site.

Google Earth with the Artificial Night-Sky Brightness Layer activated..
Fortunately, there is a solution. There is now an Artificial Night-Sky Brightness Layer for Google Earth that lets you find out exactly what the actual Artificial Night Sky Brightness is at any location in North America. Using this layer in Google Earth, we can see that the site of the Mulberry Mountain Star Party is in a Class 3 (blue) zone. You can download the Artificial Night-Sky Brightness Layer and save it to your PC. It is located in our Downloads section. If you have Google Earth installed, you can simply double click on the KMZ file after you've downloaded it, and it should automatically open in Google Earth.

Using this tool, we can evaluate possible future locations for the Coleman Observatory, or you can evaluate your own observing sites for night-sky brightness. For comparison, the existing Coleman Observatory site on Wildwood Road in Van Buren is in a Class 4 (green) zone. The Diamond Bar G Ranch observing location the club uses is in a Class 3 (blue) zone, near the edge of a Class 4 (green) zone to the north.
The Mulberry Mountain Star Party site is also in a Class 3 (blue) zone, but it's near the edge of a Class2 (grey) zone to the east. Since each zone is based on a range of brightness levels, it's highly likely that the Mulberry Mountain location has a darker sky than the Diamond Bar G location, even though they are both Class 3 (blue) locations.

Using Google Earth with the Artificial Night-Sky Brightness Layer, and a thorough understanding of the Bortle Dark Sky Scale we can arrive at a pretty good approximation of the quality of the night sky at any location we are likely to want to evaluate. We must remember that although the zones are fixed, night sky brightness varies within them. We can extrapolate that based on how close we are to adjacent zones as well as local topography and vegetation. So — how dark is your site, really?

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