An obscure nocturnal phenomenon prevents the night sky from becoming completely dark. Even if the moon, all artificial lights, and auroras were blotted out, the sky would still emit a soft luminosity all over the globe. This is “airglow,” and it is created by the light of atoms excited by solar UV radiation. Airglow is contained within Earth’s extremely variable ionosphere and produces a dominant green light approximately 50 to 60 miles above the planet. Oxygen atoms are the main component of airglow, but — through chemiluminescence and the decay of excited atoms and molecules — sodium atoms, hydroxyl radicals, and molecular oxygen add to the light emitted. The ionosphere is an electrified layer of the upper atmosphere that stretches roughly 40 to 400 miles above Earth’s surface. Courtesy of NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith. Airglow differs from auroras. Both share similarities with respect to the heights at which they occur and the fact that their glow comes from the light of excited atoms. Airglow, however, is produced by a chemical excitation from daytime shortwave solar radiation (day-to-day sunlight). Auroral excitation is produced by collisions with high-energy particles plunging down from Earth’s magnetosphere. Jack Jewell, a laser scientist hooked on nighttime photography, captured the atmospheric phenomena while taking time-lapsed photos in Bryce Canyon National Park of the Milky Way rising. “Despite no city lights anywhere near, the sky did not appear especially dark, and I saw some bands,” Jewell said. “The photos all had a green hue due to strong airglow that night.” Image of green airglow in Bryce Canyon, Utah. Courtesy of Jack Jewell. The successive frames that he captured were 12 seconds each and shot on a tripod-mounted Nikon D800E at 24 mm focal length. Although airglow is actually quite colorful, our unaided eyes do not typically see its color because its light appears below the threshold of our color perception. So the next time you look to the night sky, know a faint but majestic luminescence separates us from the stars, and admire the beauty of the upper atmosphere. To watch a brief video made by Jewell showing airglow in action, visit: https://vimeo.com/73254759.