How Dark Is the Night Sky?

image via New Scientist

October 4, 2024, 10:45 AM

• •The night sky isn't perfectly dark—it glows faintly, and the source isn't exactly local.
• •The sky itself glows due to airglow, which is caused by oxygen and nitrogen molecules in the upper atmosphere getting energized by ultraviolet light from the sun.
• •Even in space, the sky isn't totally dark. It's much darker above all that airglow, of course, allowing much fainter objects to be seen—which is a big reason we put telescopes in orbit around Earth or the sun.

The night sky isn’t perfectly dark—instead it glows faintly, and the source isn’t exactly local. The sky itself glows! During the day oxygen and nitrogen molecules in the upper atmosphere get energized by ultraviolet light from the sun, and they release that energy as a soft nocturnal glow. Sometimes atoms of these elements collide with each other and combine into molecules, again emitting a faint light. Both processes contribute to what’s known as airglow, which, for example, shows up in photographs from orbit as arcs of color above the horizon. Long photographic exposures taken from the ground can reveal it as well.

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