Night Skies Data Report: Photometric Assessment of Night Sky Quality at Organ Pipe Cactus National Monument
White JM . 2025. Night Skies Data Report: Photometric Assessment of Night Sky Quality at Organ Pipe Cactus National Monument. Science Report. NPS/SR—2025/338. National Park Service. Fort Collins, Colorado. https://doi.org/10.36967/2314920
This report characterizes night sky conditions in Organ Pipe Cactus National Monument (ORPI) using measurements made in the park unit and models of regional conditions based on satellite data. Calibrated night sky imagery was obtained to characterize the night sky at 4 sites. These ground-based observations were collected on 4 nights from 2023-02-11 to 2023-02-15. Satellite data from 2023 was used to create a map of predicted night sky conditions in and around the park. The sky overhead remains near pristine with an average zenith brightness of 21.54 mag/arcsec2. We estimate more than 91% of naked eye stars were visible throughout the data collection period, providing a unique opportunity to view the natural night sky from the park during a cloudless moonless night. The whole sky over ORPI is 41%–55% (mean = 46%) brighter than average natural levels, indicating impaired dark sky conditions on average, especially along the horizon but with near natural levels overhead. Based on visibility of astronomical objects and surrounding light domes, we classified the sky between Bortle Class 2–4, from a truly dark site in some remote parts of the park to a rural/suburban transition in others. The average naked eye limiting magnitude (NELM) is 6.65 which is approaching near pristine under average conditions. Our Sky Quality Meter (SQM) measurements averaged 21.45 mag/arcsec2, indicating the zenith is nearly as dark as we can accurately measure with an SQM. The excellent night sky quality overhead provides opportunities for visitors to observe most visible stars, deep sky objects and the Milky Way. The main impacts on ORPI’s night sky quality were the light domes from Sonoyta and Puerto Peñasco, Mexico, and the Phoenix metropolitan area, Tucson, and Yuma, AZ. These light domes along the horizon, especially from Sonoyta, diminish night sky quality in the park by obscuring natural features, and hindering visitor dark adaptation. However, overall conditions are very good, and many areas of the park remain undisturbed by light pollution allowing for a true natural night sky experience.
- Type
- Published Report
- Authors
- White, Jeremy
- Date of Issue
- 2025-07
- Publisher
- National Park Service
- DOI
- 10.36967/2314920
- Units
- NRSS , ORPI
- Keywords
- brightness, light pollution, night skies, night sky, photometric assessment, sky quality