What the Globe at Night Study Found

The Globe at Night program asks citizen observers worldwide to look at a designated star field and record how many stars they can see with the naked eye. The number of visible stars is a direct proxy for sky brightness — the dimmer and more uniform the sky glow, the fewer stars are visible. Observers have been submitting reports since 2006, creating one of the largest longitudinal datasets of sky brightness available.

The 2011–2022 analysis of 51,351 observations from sites across Europe, North America, and other regions found that average sky brightness increased at approximately 9.6% per year over the study period. Regional rates include approximately 6.5% annually in Europe and 10.4% annually in North America. Compounded over 18 years, these rates produce a dramatic outcome: a child who can see approximately 250 stars from a given location at age zero may see fewer than 100 stars from that same location by the time they are eighteen.

Why Satellites Missed This — and Why It Matters for Outdoor Lighting

The 9.6% annual brightening rate documented by the Globe at Night study is approximately four times higher than what satellite instruments like VIIRS (the Visible Infrared Imaging Radiometer Suite on NOAA's Suomi NPP satellite) had measured. The reason for this discrepancy is directly relevant to outdoor lighting specification choices.

Satellites like VIIRS have two systematic blind spots:

• Blue wavelength insensitivity: VIIRS is significantly less sensitive to blue and blue-green wavelengths — the very wavelengths that white LEDs emit most strongly. As the world's outdoor lighting has converted from yellow-orange HPS and metal halide sources to blue-rich white LEDs, the satellites have under-detected the resulting increase in sky brightness because they are not calibrated to measure it accurately.
• Horizontal light emission: Satellite sensors measure light emitted directly upward. They under-detect the sideward-scattered light from unshielded or partially shielded fixtures that bounces off atmospheric particles and contributes significantly to sky glow. This is the scatter that full-cutoff shielding eliminates — and the scatter that is driving the sky brightening that human observers document but satellites miss.

These two blind spots mean that the shift from HPS to white LEDs — which satellites often measured as a reduction in light pollution because HPS emits more in the satellite-detectable orange band — actually increased sky glow significantly when measured by the human eye or by instruments calibrated to blue wavelengths.

What This Means for Outdoor Lighting Specification

The Globe at Night study confirms what dark sky advocates have argued for over a decade: the replacement of HPS with standard cool-white or neutral-white LEDs does not reduce sky glow in practice — it increases it. The energy efficiency gain is real. The sky glow reduction that municipalities and developers assumed would accompany the LED transition has not materialized, because the fixtures being installed emit more blue-spectrum light, more sideward light, or both.

The Scale of the Problem

The Globe at Night study's findings are consistent with other measures of the light pollution problem's scope. More than 80% of humanity now lives under light-polluted skies. The Milky Way is invisible to roughly one-third of the global population — including approximately 60% of Europeans and nearly 80% of North Americans. These are not distant, astronomical concerns. They are the documented consequence of outdoor lighting decisions made one parking lot, one streetscape, and one sports field at a time.

The 9.6% annual rate documented in the Globe at Night data is not inevitable. The study authors are explicit on this point: sky glow depends on the quantity, direction, spectrum, and timing of outdoor light — and can be reduced by specifying fixtures that keep light on the ground rather than scattering it upward. The problem is man-made. The solution is in the specification.

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