Novac's Next Generation Light Pollution Effort

The Novac Next Generation Light Pollution Map

Here is one of Novac's past light pollution efforts created by member, Bill Burton.

NOVAC needs your help! Almost five years ago the club produced a map showing star counts and limiting magnitudes in the Washington, D.C. area, as part of a cooperative effort with the Washington Post dubbed "Project Orion". Sky and Telescope writer Fred Schaff called this project the best light-pollution study ever conducted in the United States. The map clearly shows the severe impact of the Washington-area nighttime sky glow on our ability to see fainter stars and deep-sky objects, and outlines "islands" of darker skies within the urban region. Now, using a slightly refined technique in cooperation with a different organization, we are aiming to do it again, with potentially farther-reaching impact.

Our cooperator this time is NOAA's National Geophysical Data Center (NGDC), in Boulder, Colorado. Working with the Defense Meteorological Satellite Program (DMSP), Chris Elvidge and others at the NGDC have produced a map of the United States showing nighttime visual and near-infrared emissions. The map is similar to the familiar one of the US at night displayed by the International Dark-Sky Association (IDA) and other organizations, but at much higher resolution: 30 arc seconds, or about half a kilometer. Elvidge and others have taken great pains to assure that the DMSP data represents cloud-free, moonless conditions.

The DMSP data for the DC area looks quite similar to the Project Orion map published in the June, 1996 issue of Sky and Telescope-remarkable, since the two maps were produced using completely different techniques. So, if the whole country is now covered by this DMSP nighttime-lights database and map, why do we need to make a new light-pollution map? Because we don't know what the DMSP values mean exactly for the average sky-gazer and amateur astronomer in terms of sky brightness and limiting magnitude (LM). The goal of the new project, then, is to translate DMSP visual and near-infrared nighttime emission values, measured from orbit and represented by different colors and brightnesses on the map, into equivalent LM for ground-based observers. This "ground-truthing" by NOVAC members and other amateur astronomers will then make the DMSP data more meaningful for skygazers around the country, who could then easily determine from the map the limiting magnitude for their area. A map that displays the degree and extent of light pollution in an area can make a compelling argument for good lighting practices, as recent experience by NOVAC members at DC-area hearings has shown. Successive versions of the DMSP database will be used to monitor the advance--or, hopefully, retreat!--of light pollution in the US in terms of changes in LM.

Evidence that the DMSP imagery will be able to monitor changes in light pollution can be shown in the accompanying FIGURE, which compares the intensity of nighttime lights between Tucson and Albuquerque. Despite its larger population, Tucson emits less light in its downtown area, probably because of its progressive light-pollution-control policy and extensive use of full-cutoff lighting.

For this project we need accurate determinations of LM, under cloud-free, moonless conditions, for accurately determined locations. In the beginning participation in the project should probably be limited to experienced amateur astronomers, in order to assure high-quality data. (For Project Orion, which involved the general public, much screening of unreliable data was necessary.) Once we have a good database solidly in hand we can open it up to more people. I will collect the observations from participants and send them on to Elvidge at NGDC, who will use them to produce a spatially-oriented digital "layer" to compare with the digital DMSP data.

The three principle activities for this project are:

1. Determining your location;
2. Determining limiting magnitude; and
3. Submitting your data.

1. Determining location

In order to provide data that can be usefully integrated with the DMSP data, the location of the LM observation needs to be known to better than 30 seconds of arc, or about half a kilometer, or .008 degrees of latitude and longitude. This is well within the capabilities of most standard global positioning satellite (GPS) receivers, as well as MapBlast, a Web-based map utility that will give the coordinates of any residential address in thousands of a degree. A test of MapBlast for my house with a high-accuracy military GPS receiver showed a difference between the two of only .001 degree. Therefore NOVAC participants doing LM determinations from their homes for this project can get the necessary lat&long coordinates from the Web. Presumably MapBlast works for non-residential addresses as well. The URL for the site is http://www.mapblast.com. You can also do what I plan to do, and go to a wide range of locations, both urban and rural, with a GPS device to do LM determinations. If you are submitting LM data with coordinates obtained with a GPS, please convert them to thousands of a degree, and give the accuracy (plus or minus meters), if possible. Our NGDC cooperators would particularly like accurately-located LM measurements from sites that show up as isolated bright spots on the DMSP map, such as small outlying towns, which will help them spatially register the two data sets.

2. Determining limiting magnitude

The preferred method for determining LM in this project is to do star counts for selected areas of the sky. These areas are bounded by triangles or quadrilaterals made up of the brighter stars of familiar constellations. Charts have been made for these areas that correlate number of stars visible within the area to limiting magnitude, to a tenth of a degree. 20 charts that span the sky are available through the NOVAC website. On the bottom of the home page click on "Links"; on the Links page go to the "How To" column, and then click on "Limiting Magnitude". Select the charts you need for the time of year you are working in, and print them out. Use areas as close as possible to zenith for your determination.
Procedures originally outlined for the Project Orion star counts apply equally well here. Pick a perfectly clear, moonless night-preferably a cool one with low humidity, such as in late fall or winter following the passage of a cold front. Outside, avoid any direct glare from lights shining into your eyes, and spend at least 10 minutes allowing your eyes to get dark-adapted. Use a red flashlight to illuminate the charts while you make your star count. Counts from two or more areas may yield a more accurate LM determination, if the areas are in equally dark patches of sky.

3. Submitting your data

After you have made your LM determination and accurately located yourself, e-mail the LM (to tenths of a magnitude), location (to thousands of a degree), locating technique (e.g. GPS or MapBlast), and date and time of observation to me. You may also include notes on conditions such as temperature and humidity, but I will assume that all submitted measurements were taken in meteorological conditions that were as ideal as possible. Of course the degree of light pollution will be another factor, but that, after all, is what we are measuring!