Greta Jimenez
Light pollution is the presence of unwanted, excessive artificial lighting. It is a significant problem in many parts of the world, including North America, Europe, and Asia. To measure light pollution, scientists use various tools and methods, including satellite imagery, ground-based photometers, and smartphone applications. One of the most widely used instruments is the Sky Quality Meter (SQM), which measures night sky brightness. Citizen science programs, such as Globe at Night, also contribute to light pollution measurement by inviting people to measure and submit their night-sky brightness observations. These collective efforts help researchers better understand and address the issue of light pollution.
| Characteristics | Values |
|---|---|
| Understanding the magnitude of light pollution | Measure the brightness of the night sky |
| Tools | Sky Quality Meter (SQM), SQM-LU, SQM-LU-DL, SQM-LE, SQM-L, Dark Sky Meter app, Loss of the Night app, Globe at Night, Lightmeter, photometers, cameras, satellite imagery |
| Units | Magnitudes per square arcsecond |
| Scales | Bortle scale, Light pollution map |
| Databases | National Park Service Night Sky Program monitoring database |
| Standards | Common standard for recording measurements, CLIC workshop proposal for a new standard format |
| Citizen science programs | Globe at Night, Cities at Night, International Dark Sky Places Program nominations, Dark Sky Meter advocacy |
| Other methods | Photography, light pollution filters, remote sensing |
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What You'll Learn
Using a Sky Quality Meter (SQM)
The Sky Quality Meter (SQM) is a device that measures the luminance of the night sky in magnitudes per square arcsecond (MPSAS). The SQM scale ranges from 16.00–22.00, with 16.00 representing the brightest sky and 22.00 the darkest. The higher the MPSAS reading, the darker the sky. The Unihedron Sky Quality Meter-Lens (SQM-L) is one of the most popular SQMs. It has an additional integrated lens, offering a narrower measurement range of 20° compared to the 84° range of the standard SQM model. SQMs are not waterproof and must be protected from moisture using a housing, generally provided by the manufacturer.
There are several models of SQM, offering different fields of view and various automatic measurement and data logging or data communication capabilities. The values reported by the SQM are in units of magnitudes per square arcsecond (mag arcsec−2). As astronomical magnitudes are a negative logarithmic scale, smaller values indicate a brighter sky. SQM response can be influenced by ambient temperature variations, so it is important to verify these effects. Radiance measurements taken by SQM-LU devices are stable within the temperature range of −15 °C to 35 °C, with variations smaller than the 10% systematic uncertainty state.
When SQMs are installed permanently outdoors for long-term monitoring, their sensitivity can degrade over time due to environmental exposure. A 2021 study found that ageing effects, such as reduced sensor sensitivity and optical degradation, can lead to the systematic darkening of measurements. Researchers have proposed a correction method using twilight sky brightness as a natural calibration source.
To measure light pollution, you can participate in citizen science campaigns such as Globe at Night, which involves submitting observations of sky glow through a smartphone, tablet, or computer. You can also provide data from an SQM device. Another similar project is Cities at Night, which involves mapping and identifying photos of cities taken from the International Space Station.
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Utilising smartphone apps
There are several smartphone apps available to help measure light pollution. These apps are designed to work with standard smartphone cameras and sensors available on devices from the past 5-7 years. They are a great way to help understand the magnitude of light pollution and its impact.
The Dark Sky Meter app, available for $4.99 on iOS devices, uses the smartphone camera sensor to measure the brightness of the night sky. The app provides calibrated readings in standard astronomical units, making measurements comparable across different devices and locations. Users can track light pollution trends over time, compare measurements between different locations, and contribute data to research projects that are studying the environmental impacts of artificial lighting.
For Android users, there is an app called Loss of the Night that can also be used to measure the brightness of the night sky. This app turns your eyes into a light meter, allowing you to become a citizen scientist and report how bright the night sky is in your location.
Another app that can be used to measure light pollution is Light Pollution Map, available on the App Store. The map displays VIIRS/World Atlas/Clouds/Aurora/IAU observatories and the user SQM measurements. It also includes overlays that concern light pollution, such as SQM/SQC, World Atlas 2015 zenith brightness, clouds, aurora, and IAU observatories. If you have a permanent SQM reader installed, you can add it to the map.
There is also a proposed citizen science project that consists of an app that you can download onto your smartphone. With this app, you simply take pictures of any of the light sources that you see. The app then measures the spectrum of that light and estimates the amount of different colours. This is important because not all biological processes are sensitive to all colours of light equally. For example, certain blooms that only happen in periods of strong moonlight can be reduced by human interference.
By using these apps, anyone with a smartphone can become a citizen scientist and help researchers understand the impact of artificial light on the environment and astronomy.
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Participating in citizen science programs
Globe at Night
The GLOBE at Night citizen science project enables individuals to quantify skyglow using star maps that illustrate different levels of light pollution. This project provides valuable data that can be used to track lighting changes worldwide. Observations can be made with the naked eye, and these are stable due to the slow pace of human eye evolution. Additionally, handheld sky quality meter (SQM) devices can be used to take measurements on cloudy nights, which cannot be done with the naked eye.
Dark Sky Meter App
This smartphone application uses the iPhone camera to record the brightness of the night sky, contributing to the understanding of sky glow and its impact.
Loss of the Night App
This application guides users through the sky as measurements are made using the human eye as a sensitive tool.
Cities at Night
This project relies on citizen scientists to map and identify photos of cities taken from the International Space Station. The data helps researchers assess light pollution globally.
SciStarter Ambassador Program
This program offers free training to volunteers who want to promote citizen science in their communities and make a positive impact.
Other Opportunities
There are also other ways to get involved with citizen science initiatives. For example, individuals can participate in projects that utilise distributed networks of personal computers for massive computations, such as the Search for Extraterrestrial Intelligence (SETI@home). Additionally, citizen scientists have contributed to classifying galaxy morphologies, predicting protein structures using games like Foldit, and providing solutions to problems in comparative genomics. These diverse projects demonstrate the breadth of citizen science programs available for those interested in addressing light pollution and beyond.
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Taking photographs
The NPS camera measurements are used to calibrate a continental-scaled sky-glow model using NASA’s satellite imagery of the Earth at night. The images are also used to assess night sky quality and light pollution levels in over 125 park units.
Another widely used instrument for measuring light pollution is the Sky Quality Meter (SQM). SQMs are compact devices that measure night sky brightness (NSB) in magnitudes per square arcsecond. They are deployed worldwide by both professional observatories and citizen scientists, providing high-resolution data that complements remote sensing approaches.
There are also smartphone apps and the Globe at Night project that allow enthusiasts to participate in citizen science programs and contribute to long-term monitoring.
When taking photographs to measure light pollution, it is important to keep a few things in mind. Measurements should be taken under a clear, open sky without cloud cover, and when the moon is below the horizon. This ensures that the data collected accurately reflects the natural darkness of the location. Additionally, only take readings under conditions of astronomical darkness, meaning that the sun is at least 18 degrees below the local horizon.
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Referencing the Bortle scale
For example, a Class 1 Bortle sky means that one can observe the Triangulum Galaxy (M33) as a direct-vision naked-eye object. A Class 8 or 9 sky, on the other hand, indicates a high level of light pollution, with the zodiacal light, gegenschein, and zodiacal band all visible to the unaided eye.
There are a number of tools and resources available to help determine the Bortle scale class of a particular location. Light pollution maps, for instance, use colours to represent the amount of artificial light in an area, with white to red indicating high levels of light pollution and green to blue indicating low levels. These maps can provide an approximate reading for any location on Earth. Additionally, smartphone apps such as Clear Outside and Dark Sky Meter can provide more precise readings based on your current GPS location.
It is worth noting that there can be drastic differences between some classes on the Bortle scale, such as the jump from Class 4 to 5. This information is valuable for astronomers and astrophotographers when discussing observations and posting pictures.
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Frequently asked questions
Light pollution is the presence of any unwanted, inappropriate, or excessive artificial lighting.
There are several ways to measure light pollution. One way is to use a Sky Quality Meter (SQM), a compact device that measures night sky brightness. SQMs are deployed by both professional observatories and citizen scientists worldwide. You can also use smartphone apps such as Dark Sky Meter or Loss of the Night.
The Bortle scale is a nine-level measuring system used to track how much light pollution there is in the sky. A Bortle scale of four or less is required to see the Milky Way, while one is "pristine", the darkest possible.
You can help combat light pollution by participating in citizen science programs such as Globe at Night, which collects data on sky brightness to raise awareness about light pollution. You can also support organizations dedicated to reducing light pollution, such as the International Dark-Sky Association.
