- The visibility of certain celestial features can be used to make simple qualitative appraisals of the night sky. For example, the ability to see the Milky Way in the night sky (our own galaxy seen edge-on) indicates a moderate level of sky quality. These estimates can be done quickly by a dark-adapted observer but tend to be biased from one person to another. The most popular qualitative assessment is the Bortle Dark Sky Scale.
- Another simple method is using "star counts." A defined area of the sky, such as the constellation Orion, is examined and the number of stars are either counted or the constellation is compared to a series of images. Each image shows an increasing number of stars;the more light pollution, the less contrast is afforded the observer and the less stars are seen. This estimate can be made by a dark-adapted observer with a basic understanding of the night sky in about 20 or 30 minutes. Star counts can also be biased from one individual to another and depend on the visual awareness and patience of the observer. Worldwide star counts allow the public to participate;one popular count is the Globe at Night program. You can try this yourself in your own neighborhood or park by exploring How Dark Is Your Night Sky.
- A more sophisticated solution is to use a light sensor that measures illuminance (the light falling upon a surface) or luminance (the brightness of a surface). These are typically photodiodes, but even a simple solar panel can be adapted to measure the relatively faint nighttime light striking it. These can be readily filtered and calibrated to estimate brightness as the human eye sees it, or can be tuned to what a specific wildlife species "sees." A recent innovation of this type is inexpensive sky quality meters. These are similar to a photographer's light meter but are tuned for measuring the luminance of the night sky as seen by human night vision. These provide a single quantitative measure of the night sky, typically pointed straight up at the zenith. The most common device is the Unihedron Sky Quality Meter.
- Another approach entirely is to put the camera in space onboard a satellite and look downward upon Earth's surface. This method has been used to look at the global span of light pollution, trace the development of cities, and reconstruct the growth of light pollution over time.
NPS has pioneered portable instrumentation and techniques for measuring sky glow conditions in parks
Metrics captured in national parks across the USA express observed and estimated artificial sky brightness
The report provides metrics data and visual indicators of sky quality.
Last updated: September 13, 2023