Real Girls: Is the night sky actually black?


	November 10, 2024

Original photo by oscargutzo/ Shutterstock

The sky is never totally black, even at night.

Stargazers who live in major cities know all about light pollution, but this lack of complete darkness goes beyond geography. It turns out the sky is never totally black no matter where you live, even at night. We can mostly chalk this up to the luminous effects of moonlight and starlight, as well as to our atmosphere, which, in addition to making the planet's air breathable, scatters all the light that passes through it. (This is what makes the sky appear blue during the day, as the atmosphere scatters blue light more than other colors.)

In highly populated areas, the effect of light pollution is known as skyglow, and it's what gives the night sky its milky, sometimes yellowish complexion. But even if you find yourself in an extremely remote location, the sky would be closer to a navy blue than black. As previously mentioned, this is partially because of the moon, but it's also because the atmosphere itself emits a faint light of its own (in addition to the light it scatters), known as both airglow and nightglow. Depending on where you are, that glow could be any combination of red, green, purple, and/or yellow. If you'd like to see a completely black sky, you'd have to go to the moon, which has no atmosphere and thus nothing to scatter or emit light.

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		Numbers Don't Lie

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The darkest shade of black absorbs 99.995% of light.

For several years, the "blackest black" was something called Vantablack — a super-black coating whose name comes from the vertically aligned nanotube arrays that helped create it. Pictures of it look genuinely unreal, as though a portion of the image has been cropped out and replaced with a blank background. Vantablack's reign came to an end in 2019, when MIT engineers announced they'd created a shade 10 times blacker. It absorbs 99.995% of all light and looks even more like a void than its predecessor. Made from vertically aligned carbon nanotubes (CNTs), the material could have practical applications beyond aesthetics, such as reducing glare in telescopes and other optical instruments, which could help them detect planets beyond our solar system.

Today's edition of Interesting Facts was written by Michael Nordine and edited by Brooke Robinson.