Night skies data report: Photometric assessment of night sky quality at Niobrara National Scenic River

White, Jeremy

doi:10.36967/2310882

This report characterizes night sky conditions in Niobrara National Scenic River (NIOB) 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 2 sites. These ground-based observations were collected on 2 nights from 2024-08-29 to 2024-08-30. Satellite data from 2022 was used to create a map of predicted night sky conditions in and around the park. The sky overhead remains pristine and unaffected by light pollution with an average zenith brightness of 21.62 mag/arcsec2. We estimate more than 99% of stars were still visible for most of the night, providing a unique and unfettered opportunity to observe the natural night sky from the park. The whole sky over NIOB is 9%–18% (mean=14%) brighter than average natural levels, indicating excellent dark sky conditions on average. In NIOB, we classified the sky as Bortle Class 2: typical truly dark site, based on the visibility of astronomical objects. The average naked eye limiting magnitude (NELM) is 6.75, which is approaching near pristine under average conditions. Our Sky Quality Meter (SQM-L) measurements average 21.44 mag/arcsec2, slightly higher than average natural conditions, elevated by Milky Way and airglow luminance. Natural airglow is apparent along the horizon. M33, the Triangulum Galaxy, is seen rather easily with direct vision. The summer Milky Way is highly structured to the unaided eye and visible down to the horizon. Its brightest regions look like veined marble when viewed with ordinary binoculars. The main impact on NIOB’s night sky quality is the light dome from Valentine, NE, with smaller impacts from Ainsworth, NE, Springview, NE, Sicangu Village, SD and Winner, SD. Of these Valentine is the largest and brightest. From Fort Niobrara, the Valentine light dome is the dominant feature of the landscape, significantly brighter than the Milky Way and any other natural feature, although limited to a small portion of the horizon. From Norden Bridge, the Valentine light dome is still the main source of anthropogenic light, and just slightly brighter than the Milky Way, although its footprint is much smaller at that distance and its impact on the overall scene is minimal.

Night skies data report: Photometric assessment of night sky quality at Niobrara National Scenic River

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