Brian Barton
Light pollution is a growing problem that has significantly impacted our view of the night sky. To address this issue, various methods have been developed to measure light pollution, including satellite imaging, ground-based studies, and citizen science projects. Satellite imaging, such as the Operational Linscan System (OLS) and Visible Infrared Imaging Radiometer Suite (VIIRS), captures data on artificial lighting and light pollution sources. Ground-based studies utilize instruments like Sky Quality Meters and CCD cameras to measure sky brightness and light sources. Citizen science projects, such as Globe at Night, encourage people to observe and report night sky brightness using their naked eye or electronic devices. While these efforts provide valuable data, the lack of standardized measurement formats and tools limits the ability to compare data from different locations and develop long-term global databases. Standardization and collaboration are crucial for effective light pollution measurement and management.
| Characteristics | Values |
|---|---|
| Measurement methods | Satellite imaging, ground-based studies, citizen science projects, instruments |
| Citizen science projects | Globe at Night, Loss of the Night app, Light Pollution Interactive Map of the World |
| Instruments | Sky Quality Meter, DigiLum luminance meter, Mark Light Meter, CCD camera |
| Units | Magnitude per arcsecond square (mag/arcsec2), candela per meter square (cd/m2), magnitudes per square arcsecond (MPSAS) |
| Scales | Bortle Dark-Sky Scale, Light Pollution Map |
| Magnitude | 1 (darkest sky) to 9 (brightest sky) |
| SQM | 16.00 (brightest sky) to 22.00 (dark sky) |
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What You'll Learn
Sky brightness
Night sky brightness (NSB) is one of the key indicators of light pollution. The brightness of the night sky is measured using a combination of satellite-based observations and ground-based networks of photometers. The most widely used instrument for measuring NSB is the Sky Quality Meter (SQM), a compact device that measures NSB in magnitudes per square arcsecond. SQMs are used by both professional observatories and citizen scientists, providing high-resolution data that complements remote sensing approaches.
There are several other methods for measuring sky brightness. One simple method for estimating the darkness of a location is to look for the Milky Way, which from truly dark skies appears bright enough to cast a shadow. The Bortle scale is another standard way to measure sky brightness, ranging from Class 1 (pitch black) to Class 9 (heavily light-polluted). Lower numbers on the Bortle scale indicate better sky quality and more stars visible. The Bortle scale can be determined using a light pollution map or an astronomy app that uses your coordinates.
Smartphone apps such as Dark Sky Meter and Loss of the Night can also be used to measure sky brightness. The former uses the iPhone camera to record the brightness of the night sky, while the latter uses the human eye to make measurements. Citizen science projects such as Globe at Night also encourage people to participate in estimating the extent of light pollution by counting the number of stars visible within specific sky patches.
Long-term monitoring of sky brightness is also conducted using permanently connected SQM devices such as the SQM-LU, SQM-LU-DL, and the SQM-LE. These devices have led to the development of continuous sky glow monitoring stations, although there is currently no common standard for recording measurements from these devices. Astronomical spectroscopy is another method used to study the spectra of the night sky.
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Satellite imaging
In 1972, the U.S. Department of Defense declassified its Defense Meteorological Satellite Program (DMSP), which had been launched to gather weather information in support of American military priorities. During each orbit, DMSP satellites spend part of their journey in the shadow of Earth, on the night side of the planet, where they can observe the lights of cities. This data was made available to the civil scientific community, who realised its value in measuring light pollution.
Since then, satellites have continued to play an important role in measuring light pollution. In 2001, a 'World Atlas' of artificial light at night was published, made from individual images taken on cloud-free nights without moonlight. This atlas yielded the first global measurements of the amount of human-caused light. However, because the purpose of the DMSP was to forecast weather, the cameras aboard DMSP satellites couldn’t see fine details on the ground. Many cities were seen simply as bright patches of light, without great detail.
More recently, in 2011, the U.S. National Oceanic and Atmospheric Administration (NOAA) launched the Suomi National Polar-orbiting Partnership (Suomi NPP) satellite, which carried to orbit a camera called the Visible Infrared Imaging Radiometer Suite (VIIRS). This camera is equipped with some of the best sensors and lenses ever flown on civilian space missions, and has transformed how we see Earth at night.
Other satellite missions that have contributed to our understanding of light pollution include SDGSAT-1 and Qimingxing-1, which were launched after 2021 and offer high-resolution (10-40m) and multispectral (3-8 bands covering 400nm to 1000nm) nighttime light images. NASA’s Black Marble project has also delivered high-quality nightly images along with comprehensive metadata that includes viewing zenith angle and cloud conditions.
In addition to satellite imaging, other technologies such as drones and wearable devices are also being used to measure light pollution from new perspectives.
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Ground-based studies
Another method is the use of ground-based photometers, which are essential for monitoring light pollution over time. Photometers can be used to determine which light sources are most responsible for light pollution, but interpreting the data can be complex due to spectroscopic differences between light sources. All-sky cameras are also used in ground-based studies, providing data and metrics to model and remove celestial contributions and measure artificial light contributions.
Imaging technologies, including drones, are becoming increasingly important in ground-based light pollution studies. Drone imagery can locate light sources, and geolocation data can pinpoint every instance of light in a scene. This information is then validated through traditional ground-based methods.
To measure the impact of light pollution on the human experience and wildlife, direct measurements of night sky brightness are obtained on the ground. This involves looking at the night sky from the Earth's surface with various radiometric sensors to characterise the nighttime photic environment.
Standardisation of measurement techniques is an ongoing challenge in ground-based studies. At the Cabauw Lightmeter InterComparison (CLIC) workshop in 2012, researchers proposed a new standard format for recording sky glow measurements to facilitate comparisons of data from different locations and detectors.
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Citizen science projects
Another citizen science initiative is the Dark Sky Meter app, which enables users to measure night sky brightness by using their iPhone cameras as photometers. The app also provides information about sunset, twilight, and moon phases, as well as weather conditions. This data is valuable for environmental monitoring projects and can be used to study the impact of light pollution on wildlife, such as bats.
The Loss of the Night app is another citizen science tool that guides users through the sky as they make measurements with their eyes. The Cities at Night project also relies on citizen scientists to map and identify photos of cities taken from the International Space Station, helping researchers assess light pollution globally.
In addition to these smartphone apps, more rigorous, long-term monitoring efforts are also being conducted through permanently connected SQM devices and the International Year of Astronomy Lightmeter. These devices have led to a network of continuous sky glow monitoring stations, providing valuable data for light pollution research. However, the lack of a common standard for recording measurements from these devices has made it challenging to compare data from different locations and develop comprehensive databases.
Citizen scientists have also contributed to understanding the rapid reductions in star visibility due to light pollution. By collaborating on multidisciplinary teams, citizen scientists and researchers can address the critical implications of light pollution and work towards solutions.
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Standardisation of measurements
One approach to standardisation is the use of common units and terminology across different disciplines. This enables collaboration and a unified understanding of the issue, allowing for more effective solutions. It also helps to avoid errors in measurement methodologies, ensuring the accuracy and reliability of data.
The spectral response characteristics of detectors, for example, photometers and digital cameras, should be standardised to enable consistent measurements. This standardisation allows for comparisons between different measurement sites and facilitates the creation of global light pollution databases.
In addition, the introduction of permanently connected SQM devices, such as the SQM-LU and SQM-LE, has led to a growing number of continuous sky glow monitoring stations. However, the lack of a common standard for recording measurements from these devices has hindered the ability to compare data from different locations. To address this challenge, researchers have proposed a new standard format for recording sky glow measurements, aiming to improve the consistency and comparability of data.
Furthermore, the use of satellite data and remote sensing technologies has provided valuable insights into light pollution. By observing the Earth at night, researchers can identify areas with significant light pollution and monitor its growth over time. This data can be used to create light pollution maps, such as the work by Falchi et al. (2016), who created colour-coded maps to visualise the extent of light pollution globally.
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Frequently asked questions
Light pollution is the presence of artificial light in a natural environment, such as light from streetlights, billboards, and advertisements.
Light pollution is measured by quantifying the brightness of the night sky, also known as night sky brightness (NSB). This can be done through satellite imaging, ground-based studies, or citizen science projects.
Tools such as Sky Quality Meters (SQM) and cameras can be used to measure the brightness of the night sky. Citizen science projects often involve the use of smartphones, tablets, or computers to record observations and submit data.
