A vocabulary of landscape — a landscape of words
1469 entries
nightglow
Nightglow is the faint luminescence of the upper atmosphere seen after dark, and it is the nighttime component of airglow. It is why the night sky is never fully black even with no moon, no cities, and no starlight. Oxygen and other molecules broken apart by sunlight during the day release the stored energy as light hours later when they recombine, in a layer around 55 to 60 miles up.
Etymology
The word is a plain compound built as a subdivision of airglow, which was coined around 1950. The parallel terms are dayglow and twilightglow, which are much harder to observe because there is brighter light in the way, and which are studied mostly with instruments rather than by eye.
Notes
Green is the commonest color, produced by atomic oxygen, with red from a higher layer and a yellow line from sodium. Long-exposure photographs from dark sites often show it as faint green banding running across the frame, which photographers frequently take for a fault in the sensor. Astronauts see it from orbit as a thin glowing band on the edge of the earth.