Methodology

Duskmeter tracks light pollution at dark-sky sites, observatories, and urban areas by reconciling two fundamentally different types of measurement: satellite radiance data and citizen-science star counts. Each data source captures a different aspect of the night sky, and neither one alone tells the full story.

The satellite sees the light that escapes upward from the ground, but it is blind to the blue wavelengths that modern LEDs emit in large quantities. Citizen scientists see the sky as it actually appears to the human eye, but their observations are sparse and variable. The divergence between these two measurements is itself the signal of modern LED light pollution, which the satellite alone cannot detect.

The Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band is a sensor aboard the Suomi NPP and NOAA-20 satellites. It measures upwelling radiance at approximately 500-meter resolution across the entire globe, producing a uniform picture of artificial light escaping upward into the atmosphere.

The sensor is most sensitive to wavelengths between approximately 500 and 900 nanometers. This range was well-suited to the high-pressure sodium streetlamps that dominated outdoor lighting for decades, since those lamps emit almost all of their light at 589nm, right in the middle of the detection window.

However, the sensor is spectrally blind to light below approximately 500nm. Since the global transition to white LED streetlights began in the early 2010s, a large share of outdoor lighting now emits strongly in the blue part of the spectrum, around 450nm. This blue light scatters through the atmosphere far more efficiently than amber sodium light, making the sky brighter, but the satellite simply does not see it. As a result, satellite-only estimates systematically underestimate the true growth of light pollution in the LED era.

Globe at Night is an international citizen-science campaign run by NOIRLab. Participants report their naked-eye limiting magnitude, which is the faintest star they can see in a specific constellation on a given night. This measurement directly captures how the night sky appears to a human observer, including the effects of all wavelengths of light pollution, even the blue light that the satellite misses.

The data is inherently sparse and noisy. Observations depend on weather, moon phase, observer experience, and local conditions. A single report from a single location on a single night carries significant uncertainty. However, when thousands of reports are aggregated over time and across many observers, the statistical signal becomes clear.

Between 2011 and 2022, Globe at Night collected more than 51,000 observations from citizen scientists around the world. Kyba and colleagues analyzed this dataset and published their findings in Science in 2023, reporting a 9.6% annual increase in sky brightness over that period.

Every site in Duskmeter is assigned a status based on its corrected brightness trend and its current Bortle class. The Bortle scale is a nine-level measure of the night sky's brightness, from Bortle 1 (pristine dark sky) to Bortle 9 (inner-city sky where only the brightest stars are visible).

• Stable: The corrected trend is at or below 1.5% per year and the site is at Bortle 6 or darker. The site is not currently at risk of crossing into a brighter Bortle class within the next decade.
• Watch: The corrected trend is between 1.5% and 3% per year, or the site shows any positive trend at a naturally dark location (Bortle 2 or darker). The site warrants monitoring because it may approach a brightness threshold within the next 10 to 20 years.
• Critical: The corrected trend exceeds 3% per year, or the site is already at Bortle 7 or brighter (severely polluted). The site is either degrading rapidly or has already crossed into a brightness class where astronomical observation is significantly impaired.

Professional astronomy software and observatory control systems use dim red interfaces to preserve the observer's dark-adapted vision. The human eye takes 20 to 30 minutes to fully adapt to darkness, and even a brief exposure to white or blue light can reset that adaptation.

Night Vision mode in Duskmeter shifts the entire interface palette to low-brightness reds and dark backgrounds. It is designed to be usable at a telescope eyepiece or under a dark sky without compromising your night vision. This is a functional tool for observers in the field, not a cosmetic dark theme.

Duskmeter currently runs on synthetic mock data that reproduces the statistical ranges and trends observed in real-world measurements. This allows the tool to be developed and tested without depending on live data pipelines.

A production deployment would ingest two primary data sources:

• VIIRS annual composites from the Earth Observation Group (EOG) at the Colorado School of Mines, distributed through NASA's LAADS DAAC. These provide cloud-free, stray-light-corrected radiance measurements at approximately 500-meter resolution.
• Globe at Night observations from NOIRLab's Community Science and Data Lab. These provide naked-eye limiting magnitude reports from citizen scientists worldwide, along with metadata about observing conditions.

Both datasets are publicly available and are updated annually. The VIIRS composites are released each spring for the previous calendar year, and Globe at Night accepts observations on a rolling basis.