Sky brightness
Sky brightness describes the scattering and diffusion of light in the sky.
Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources such as the Moon and light pollution were removed from the night sky, airglow and starlight would still contribute to the sky's glow.
Lore & Background
The sky's brightness varies greatly over the day, with direct scattering of sunlight dominating during daytime. During twilight, the situation is more complicated, divided into three 6° segments based on the Sun's position below the horizon. When the Sun's depth exceeds 18°, the sky attains its maximum darkness. Sources of night sky brightness include airglow, indirect scattering of sunlight, scattering of starlight, and light pollution. It is typically comparable to or brighter than starlight and includes emissions from oxygen, sodium, and hydroxyl bands.
Reader's Guide
Sky brightness is a fundamental concept in observational astronomy and atmospheric physics, quantifying how light from various sources—solar, stellar, and artificial—is scattered and diffused by the Earth's atmosphere. Its study has practical implications for light pollution management, astronomical observation, and understanding atmospheric chemistry. The division of twilight into civil, nautical, and astronomical phases provides a standardized framework for planning observations and activities. Airglow, first noted in the auroral spectrum, reveals ongoing photochemical processes in the upper atmosphere, with emissions from oxygen and sodium layers at different altitudes.
Did You Know?
- Airglow is typically comparable to or brighter than the collective glow of starlight.
Frequently Asked Questions
What is sky brightness in astronomy?
Sky brightness is the term describing how light is scattered and diffused through the atmosphere, producing the visible glow we perceive in the sky. It applies to both the bright daytime dome and the faint residual illumination present even on the darkest night.
Why isn't the night sky perfectly black?
Even if you removed the Moon and all artificial lighting, the sky would still carry a faint glow from airglow and the combined light of countless stars. These natural sources guarantee the sky never reaches absolute darkness.
How does natural sky brightness differ from light pollution?
Natural sky brightness is the inherent scattering and diffusion of light in the atmosphere, whereas light pollution is the extra glow added by human-made sources. Removing all artificial lighting still leaves a measurable natural baseline that defines the sky's true background.
Why do astronomers track sky brightness at their observing sites?
Knowing the exact background glow lets observers subtract it from their data to isolate faint celestial signals. It also provides a quantitative way to assess how much artificial light is degrading a particular dark-sky location.
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