Skyglow

Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to sky luminance arising from artificial lighting, skyglow may also involve any nighttime sources of diffuse light, including natural ones like the zodiacal light, starlight, and airglow. Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to sky luminance arising from artificial lighting, skyglow may also involve any nighttime sources of diffuse light, including natural ones like the zodiacal light, starlight, and airglow. In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely gas) lighting used for illumination and advertisement, and from gas flares. Light propagating into the atmosphere directly from upward-directed or incompletely shielded sources, or after reflection from the ground or other surfaces, is partially scattered back toward the ground, producing a diffuse glow that can be seen from large distances. Skyglow from artificial lights is most often noticed as a glowing dome of light over cities and towns, yet is pervasive throughout the developed world. Light used for all purposes in the outdoor environment contributes to skyglow, by sometimes avoidable aspects such as poor shielding of fixtures, and through at least partially unavoidable aspects such as unshielded signage and reflection from intentionally illuminated surfaces. Some of this light is then scattered in the atmosphere back toward the ground by molecules and aerosols (see § Mechanism), and (if present) clouds, causing skyglow. Research indicates that when viewed from nearby about half of skyglow arises from direct upward emissions, and half from reflected, though the ratio varies depending on details of lighting fixtures and usage, and distance of the observation point from the light source. In most communities direct upward emission averages about 10% - 15%. Fully shielded lighting (with no light emitted directly upward) decreases skyglow by about half when viewed nearby, but by much greater factors when viewed from a distance. Skyglow is significantly amplified by the presence of snow, and within and near urban areas when clouds are present. In remote areas snow brightens the sky, but clouds make the sky darker. There are two kinds of light scattering that lead to sky glow: scattering from molecules such as N2 and O2 (called Rayleigh scattering), and that from aerosols, described by Mie theory. Rayleigh scattering is much stronger for short-wavelength (blue) light, while scattering from aerosols is less affected by wavelength. Rayleigh scattering makes the sky appear blue in the daytime; the more aerosols there are, the less blue or whiter the sky appears. In many areas, most particularly in urban areas, aerosol scattering dominates, due to the heavy aerosol loading caused by modern industrial activity, power generation, farming and transportation. Despite the strong wavelength dependence of Rayleigh scattering, its effect on sky glow for real light sources is small. Though the shorter wavelengths suffer increased scattering, this increased scattering also gives rise to increased extinction: the effects approximately balance when the observation point is near the light source. For human visual perception of sky glow, generally the assumed context under discussions of sky glow, sources rich in shorter wavelengths produce brighter sky glow, but for a different reason (see § Dependence on light source). Amateur astronomers have used the Bortle Dark-Sky Scale to measure skyglow ever since it was published in Sky & Telescope magazine in February 2001. The scale rates the darkness of the night sky inhibited by skyglow with nine classes and provides a detailed description of each position on the scale. Amateurs also increasingly use Sky Quality Meters (SQM) that measure in astronomical photometric units of visual (Johnson V) magnitudes per square arcsecond. Professional astronomers and light pollution researchers use various measures of luminous or radiant intensity per unit area, such as (nano-)Lamberts, magnitudes per square arcsecond, or (micro-)candela per square meter. All-sky maps of skyglow brightness are produced with professional-grade imaging cameras with CCD detectors and using stars as calibration sources.