The night sky's brightness is due to a combination of natural and artificial light sources.
While we perceive the night sky as dark, it actually possesses a faint, intrinsic brightness resulting from multiple sources. These sources, as outlined below, contribute to the phenomenon we observe.
Sources of Night Sky Brightness
Here's a breakdown of the factors that contribute to the night sky's luminosity:
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Airglow: This is light emitted by the Earth's atmosphere itself. Chemical reactions, driven by sunlight during the day, continue to occur at night, releasing photons and causing a faint glow.
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Indirect Scattering of Sunlight: Even after sunset, sunlight can still be scattered by dust and particles high in the atmosphere. This scattered light reaches our eyes, adding to the sky's brightness.
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Scattering of Starlight: Starlight from countless distant stars is scattered by particles in the atmosphere, contributing to the overall background glow. While individual stars might appear faint, their collective light has a measurable effect.
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Light Pollution: Artificial light from cities and towns shines upwards into the atmosphere. This light is scattered by air molecules and aerosols, creating a skyglow that can significantly brighten the night sky, especially near urban areas. This is perhaps the most significant contributor to night sky brightness for many people.
| Source | Description | Impact on Brightness |
|---|---|---|
| Airglow | Light emitted by chemical reactions in the upper atmosphere. | Adds a constant, faint glow. |
| Indirect Scattering of Sunlight | Sunlight scattering off dust and particles in the upper atmosphere after sunset. | Increases brightness, especially shortly after sunset/before sunrise. |
| Scattering of Starlight | Light from distant stars scattering in the atmosphere. | Adds a faint background glow. |
| Light Pollution | Artificial light scattered by the atmosphere. | Can dramatically increase brightness, especially near cities. |
