lulz,
This subject comes up from time to time. At the end of last year I was writing a long tutorial describing the differences between dark sky mapping Websites and how to use both styles of maps to best advantage. My tutorial illustrated a method for using Google Earth to seek out the best sites near any given vicinity. (It turned out that Cloudy Nights' TOS prevented me from presenting the tutorial here so I started writing an more detailed and complete Web page for my Website. However, Google Earth then underwent major changes and my methods needed a major revamping to be compatible with the newer versions.)
In any case, here is a long excerpt from my unpublished tutorial that you may find useful in understanding the differences between the maps presented at the two Websites you note in your first post.
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Three Aspects Of Light Pollution
Before we begin, there are a couple of things to understand concerning the nature and types of light pollution as it relates to astronomy. There are three key aspects to light pollution which we should consider. First of all, we would like to observe from a site which has no straight-line visibility to a bright local light source. Glare from such sources is not conducive to either visual or photographic astronomy. Street lights or lighting from our neighbors' homes are examples of the types of direct sources we should always try to avoid.
Secondly, we would like to have the skies at the zenith as dark as possible. Overhead sky brightness is generally expressed with a measurement of brightness per square area of sky. The usual units are MPSAS or magnitude per square arc-second. A perfectly dark sky as seen from the Earth's surface has an accepted value of 22 MPSAS. (It will not get generally darker than this due to background air glow originating from natural phenomena). Viewing from the center of a large city will result in sky brightness readings at the zenith of approximately 17.8 MPSAS or worse. This range of brightness corresponds to a logarithmic scale so comparisons are often done using what is known as the Bortle scale. The Bortle scale divides the common range of sky brightness into 9 dark sky classes or zones. When using the Bortle scale, an inner city observing site would be rated as a class 9 site while the darkest places on earth would be rated a class 1 site. The Bortle scale is color-coded from black being the darkest possible site to white being assigned to class 9 skies at the center of larger cities. There is a good description of each expanded Bortle zone's characteristics on the cleardarksky.com website and also on David Lorenz's Expanded Bortle Scale light pollution webpage. Many amateur astronomers now routinely use the Bortle scale and its color coding to describe their observing conditions -- as in "My club's observing site is in a Bortle 4 zone" or "we typically observe from a blue zone."
Finally, the last aspect of an ideal observing site is the lack of any significant light domes visible around the horizon. Light domes appear because of the light emanating from distant sources such as a city many miles away. The combined light from those sources tends to illuminate particulates in the air over and around the lighting source. We see the combined light from these distant sources as a brightening of the sky at the horizon. The brightening takes the shape of a dome of light centered over the distant source. In the absence of a perfect observing site, amateur astronomers seek out potential observing areas from which the light domes do not reach more than a few degrees into the sky.
Mapping The Problem Of Light Pollution
Light pollution conditions around the world have been mapped in a way that makes it easy for amateur astronomers to search out dark sky observing sites. However, there are two types of light pollution maps available on the Internet which some amateur astronomers may find confusing as they do not look the same. While the two types of maps show differing information, they are both extremely useful in our search for the darkest skies nearby once we understand what is being displayed.
The first type of mapping is derived directly from overhead views of the Earth's surface at night taken by orbiting satellites. Thousands of images from the satellites have been combined to give us an image which depicts a cloudless Earth as seen from space at night. This composite image is commonly called the Black Marble. The intensity of light being emitted upward into the sky can be directly measured from analysis of the Black Marble image. The resulting radiance of emitted light (measured in nanoW/cm2sr) from the ground can be color coded and provided as an overlay on a map of the Earth. As we will see, such a map can be very useful in our search for dark skies. It must be noted, however, that such a map provides information about an area on the ground as it would appear from space. That information will prove useful as we will see later but does not tell us anything directly about how bright the overhead sky will appear as seen by an observer on the ground at the mapped location.
The second type of light pollution mapping is indirectly derived from the Black Marble images. In this case, the effect of light pollution on the sky is mapped. The rather involved technical analysis process is described in detail in a technical paper that accompanied the release of the most recent maps. The resulting maps are called the New World Atlas of Light Pollution. The article describing the generation of this data may be found online in the archives of Science Advances 10 June 2016. In essence, the methodology takes each point of the 2012 Black Marble Image and models its effect on the surrounding sky taking into account atmospheric scatter at various angles as well as other factors. The process then continues by summing the total effect on the sky of all such nearby points in the Black Marble image. The resulting map shows the overall brightness of the sky at the zenith from any point in the world as seen by an observer at ground level looking up. The map data is presented in color codes which directly correspond to the Bortle scale with measurements of MPSAS in the sky.
Note now that even though both forms of maps are derived from the same source satellite imaging, they each depict a different aspect of the light pollution problem. One map type shows the light pollution emanating upwards towards an observer in space. It shows where the light pollution is coming from. The second map type shows the effect of that light on the sky as seen by an observer on the ground looking upwards. These will both be very useful in our search for dark skies though they represent two completely different things.
An analogy may help make the difference clear. Imagine a very large nighttime dinner party being held under a large canopy. Each of 100 tables has a candle to provide a little light for the dining guests. Each candle illuminates the table but also throws light upwards onto the canopy above. Now let's imagine a scenario in which we move the candles around. Let's move all of the candles onto just two tables near the center of the dining area under the canopy. If we now stand on a ladder and look down at those two tables, we see them very brightly lit. However most of the tables away from the center show no light at all. We could say that other than the center tables, the other tables are completely dark. (They are emitting no light).
Now let's step down from the ladder and look upwards at the canopy above us. The area of canopy directly over the two center tables with all the candles will appear quite well illuminated. While the areas of canopy away from the center tables will be darker, they will still be illuminated somewhat by the oblique lighting from the center tables. Even when we view the canopy from what we previously considered dark table areas, we see considerable light on the canopy above.
This is similar to the situation we have with the two forms of light pollution mapping. When looking downward from above, we can clearly see where the polluting light is coming from. Looking up from ground level, we can see the effect of lights and can see that even areas well away from the lighted tables are still illuminated and not completely dark.
As we will see, using the two complimentary types of light pollution maps together will give us a more realistic view of conditions at any given site. Several websites offer interactive light pollution maps of one or the other type discussed above. For the first form of light pollution map showing the sources of light pollution as seen from space, see the website lightpollutionmap.info. (Note that the color coding of this site represents the light intensity coming from the ground. It does not match the Bortle light pollution color codes which indicate sky brightness at the zenith.) The website blue-marble.de directly presents an interactive image of the Black Marble data.
An example of a website which presents the second form of light pollution map is darksitefinder.com. On this website, the integrated sky brightness (as seen by a ground based observer) is shown color-coded to match the Bortle light pollution scale. The light pollution maps displayed on the website cleardarksky.com also map integrated sky brightness using the Bortle color coded light pollution scale.
Unfortunately, I am not aware of any website that attempts to show both forms of light pollution mapping in a way that would help amateur astronomers search for good dark sky observing sites. This need is what led me to try to find a way to better predict conditions at a prospective dark sky site.
The Beginnings Of A Methodology
Web sites like darksitefinder.com and lightpollutionmap.info can show you (within the limits as discussed above) what to expect at a site but do not show you exactly where suitable sites may exist for public use. By using both to get a full picture of the light pollution conditions, these websites can work very well for evaluating a site you already know to exist. An arguably better solution for the amateur who needs to both find a specific site and then learn what to expect before arriving exists in the guise of the program Google Earth Desktop. Google Earth Desktop is the perfect tool for finding potential observing sites and evaluating them before visiting in person. I learned to use Google Earth Desktop in this manner several years ago when I started my own search for a dark sky site to purchase for use in both astronomy and as a weekend nature retreat.
Over a period of about a year and a half, I evaluated over a hundred potential plots of land and developed a methodology that allowed me to skip visiting many sites suggested by the realtors I worked with. Once a site met my general criteria and checked out favorably using Google Earth Desktop, I would visit the site at night and take panoramic images of the sky conditions at the site. This allowed me to finally find a site which met nearly all my requirements. The on-site panoramic photos of the night sky at each site served to validate and improve my pre-evaluation methodology.
In this tutorial, I will relate my methodology for using Google Earth Desktop and show the correlation between my Google Earth predictions and actual results obtained on site. At the end of this article, I will also briefly go over my criteria for choosing a personal dark sky site. My purchase criteria may not match your own but many of my dark site needs are general enough that they can apply both to a purchase or can simply be used as additional criteria for finding the better observing sites. The same methods I developed for my dark sky land evaluations can be applied to searching for and evaluating any potential observing locations.
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Hopefully, this will help to clarify the differences. Some day when I get more time, I will rework the full now-outdated article describing how to predict both zenith darkness of an arbitrary prospective observing site as well as the location and extent of light domes on the horizon using the newest version of Google Earth.
John
