NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 9 2.4 Astronomical and Scientific Values of Night Skies More than 2,000 public and private astronomical observatories are in the U.S. and Canada, including many in the western states on BLM-managed lands (Danko 2019). Professional and amateur astronomers value dark night skies for clear views of faint astronomical objects, such as distant nebulae and galaxies (Crawford 2000). The ability to see faint astronomical objects is decreasing with the general brightening of the sky, also known as skyglow (see Section 3.1 to learn more about light pollution). Astronomers also need skies free from light pollution for spectroscopy, the study of the spectra (light signature) of light emitting bodies, in this case celestial bodies. Spectroscopy is used to derive the chemical composition, distance, temperature, and many other properties of stars and galaxies. Particular types of lights (e.g., certain LED sources) emit wavelengths of light that mask spectral information from celestial bodies (Kornreich 2015). Dark night skies are so important for astronomy that professional observatories are typically built on remote mountaintops and islands to avoid impacts from light pollution (Henderson 2010; Luginbuhl et al. 2009). For the same reason, many amateur astronomers routinely travel hundreds or even thousands of miles to experience pristinely dark night skies. 2.5 Biological Values of Natural Darkness and Night Skies More than 60 percent of invertebrate and one third of vertebrate animals are not diurnal (primarily active during the day) (Hölker et al. 2010). These organisms are crepuscular (primarily active at twilight), nocturnal (primarily active at night), or both. Through evolutionary processes, these animals have adapted their physiology and behavior to function under low or extremely low light conditions, bats being an obvious example. Many animal species rely on natural cycles of daylight and darkness to trigger and regulate behaviors such as hunting, hiding from predators, mating, nesting, navigating, and communicating. Regardless of whether they are diurnal, crepuscular, or nocturnal, most animals are highly dependent on light for vision and for regulation of physiological processes and behaviors critical for survival. Natural cycles of daylight and darkness are essential to the health of most animals, including humans. Plants also depend on natural light cycles to regulate physiological processes such as flowering, leaf senescence, and growth form (Bennie et al. 2016). Night sky over Kitt Peak National Observatory located outside Tucson, Arizona. KPNO/NOIRLab/NSF/AURA/R.T. Sparks Many animals are active at night and dusk, such as this northern spotted owl.
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