Natural Lightscapes Monitoring & Data
Night Sky Assessments
National parks are preserves of biotic diversity and natural processes, as well as the "crown jewels" of America's most scenic lands. Humans have gazed awe-struck into the universe for millennia, and ecosystems have adapted to the natural rhythms of the moon and stars. Unspoiled natural lightscapes have natural, cultural and scenic importance, and the National Park Service is charged with their protection. Resource inventories constitute a critical first step; they inform park managers about the nature of the resources held in trust. Inventories of night sky quality will contribute to Air Quality Related Value assessments being completed servicewide. These night sky baseline assessments will eventually become the foundation for a monitoring program to detect long-term changes, provide scientifically sound data for park stewardship, and track this park feature that has scenic, ecological, and cultural implications. Many of the 32 NPS Inventory & Monitoring Networks have identified night sky quality as a vital sign – a physical or biological element of a park that represents the overall condition or is a particularly valuable attribute.
Our national parks are some of the last places left in America where stunning views of the night sky and the cosmos beyond are available. Many park visitors expect to view the night sky under pristine conditions and "sleep under the stars." The enjoyment and appreciation of these are dependent on the weather, the clarity of the air, and the amount of light pollution present. In recent years light pollution has encroached upon lands that were once remote from large cities. Quantitative data are required for our understanding of light pollution and for management decisions. Advances in technology have now made possible an efficient and accurate method of measuring the impact of light pollution. The National Park Service is in the process of gathering one of the most complete light pollution data sets. Currently, these data span a period from 2001-2005. While these data were collected with a variety of equipment combinations, great care has been taken to calibrate each instrument so that each data set is comparable. At this time, the collection, processing, and analysis of night sky quality data is reproducible and scientifically rigorous.
The goals of the data program are listed below.
- Adapt established methods of astronomical photometry to a portable field instrument that can quantify sky brightness (combination of natural and artificial light), image the entire sky in high resolution, and do so quickly.
- Develop a scientifically sound method for collection and analysis of night sky brightness.
- Inventory the present condition in national parks, first targeting many of the presumed pristine and highest quality night skies.
- Enable identification of light pollution sources, and separate natural sky phenomena from artificial light.
- Correlate numerical data to human visual perception of the night sky, and provide visual descriptions of night sky to compliment instrumentation.
- Provide baseline data that will serve as a core for future monitoring and tracking of night sky conditions. Allow synthesis of data to meet management information needs, assess restoration efforts, and inform other park concerns such as sustainability, ecological disruption, or cultural resource impact.
- To further the science of light pollution studies, share the data with the public, and promote collaboration between NPS, other government agencies, public entities, and private citizens.
Procedures
The data collection procedure uses a CCD (charged coupled device) camera— essentially a research grade digital camera. It is attached to a robotic mount and laptop computer. The computer choreographs the entire system, pointing the camera to pre-determined areas of the sky and capturing a series of short exposures. These images are stitched together to form a mosaic of the entire sky that can be displayed in either a panoramic or hemispheric (a.k.a. fish-eye) view. Data are calibrated to stars of known brightness, allowing absolute brightness measures to be extracted from the images. The camera uses a green filter, rejecting all other light from the infrared to the ultraviolet. This green or "V-band" filter approximates human night vision sensitivity. Data is displayed in V magnitudes, an astronomical brightness system, but can be transformed to many other measurement units.
Sites are selected that have a good view of the horizon with minimal obstructions. Small parks generally have one data site, whereas large parks may have several. Data is collected for 1-6 hours per night, and on multiple nights throughout the year. Weather conditions are limiting, as it has to be at least 95% clear to capture sky brightness conditions properly. Only nights with no moon are suitable. Natural light sources such as the Milky Way, zodiacal light, airglow, and aurora can result in a 50% or more variation in natural sky brightness. Artificial light sources will vary due to humidity, air pollution particles, and wind-blown, natural dust particles and also vary throughout the night as outdoor lights are turned on and off. Interestingly, higher extinction/lower visual range tends to amplify nearby light sources and attenuate far light sources. Clean, clear dry air has the opposite effect.
Detailed methods are slated for publication and will eventually be posted.
Locations and Data
The data for the different sites is available from the map. Use the links on the map to access site and data information by park. Currently, only a portion of the data collected is presented here. Data collected from 2002 to 2005 will be posted incrementally, as will future data.
