Is There True Darkness?

Stars and airglow contribute enough light to a "pitch black" sky to easily see your hand as well as find your way around at night. Illustration: Bob King, photo from the Hubble Space Telescope.

The night sky’s aglow with more than comet tails. Some of you have been lucky enough to experience truly dark skies without any city lights in sight. The best places, and I’ve only been to a few of them, are a full 360 degrees of darkness, unblemished by the glow of distant towns. Even under these circumstances, you can always see your hand when it’s held up to the sky. Why is that?

Stars contribute about half of the light at night. The rest comes from a phenomenon called airglow. Yes, the air actually glows all night long. During the day, energetic ultraviolet light from the sun breaks apart molecules of oxygen and nitrogen, and jiggles around the electrons in sodium atoms in our upper atmosphere. At night the excited molecules "calm down" and recombine, emitting red and green light in the process.

Streaks of glowing air, caused by light emitted from molecules in Earth’s upper atmosphere, stripe the sky near the horizon. Credit: Space Physics Research Group, Univ. of California, Berkeley. Please click here to see a spectacular all-sky airglow photo.

Combine airglow and starlight and throw in some snowcover, and you’ve got enough light, even in Earth’s darkest places, to recognize a face or find your way along a road. Cosmic rays also contribute a small fraction of light to the sky. These are subatomic particles careening around the galaxy like kamikazes, ready to zap the atmosphere of any planet that gets in their way. When they strike, they excite atoms and molecules just like the sun’s rays. As the electrons and atoms fall back into place, they release particles of light called photons that contribute to the nighttime illumination.

Although airglow is colorful, it’s far to faint to stimulate our eye’s color receptors, so it looks like faint white streaks and glows. As with so many dim things in the sky, only long exposure photography makes the colors pop.

Airglow is present everywhere on Earth, but can only be seen from very dark places, where it can’t be confused with man-made sources. If you live where darkness rules, allow your eyes a good 20 minutes to "dark adapt", and then scan about one to two outstretched fists above and all around the horizon. Does the sky look a little paler there? If so, it’s likely the glow of busy molecules reuniting 60-80 miles over your head.

Green and red airglow across the constellation Orion (three Belt stars at center right) photographed by space shuttle astronaut Don Pettit from orbit. Credit: NASA

Several times on otherwise cloudless nights, I’ve seen airglow overhead. At first I thought the very faint stripes and streaks I saw were clouds, but once I learned about airglow, I realized what I was really seeing. The aurora also involves excited atoms and molecules emitting light, but there’s so much more energy involved, a good display can be seen right from town.

The green airglow light shows up beautifully from orbit in this time exposure photograph. The blurred lights in the foreground are cities. Credit: NASA

Astronauts get the best view of all of the phenomenon. As they peer out the windows of the International Space Station and space shuttle, the glow reveals itself as a thin green band enveloping the Earth. In a wonderful coincidence of nature, the color of light emitted by excited oxygen is the same as the plants that produce it in the first place: green

Another view of airglow and the Northern Lights taken from aboard the space shuttle. Credit: NASA

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