Why Is the Night Sky Never Truly Dark? The Air Is Glowing Faintly
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The upper atmosphere emits a faint light of its own, all night and everywhere, unrelated to aurorae or to any pollution. It is the reason a moonless sky still shows a horizon.
Where the light comes from
Sunlight during the day breaks apart molecules in the upper atmosphere, splitting oxygen and nitrogen into individual atoms and ionising some of them. Those fragments are unstable and gradually recombine, and recombination releases energy as light. Because the air at that altitude is extremely thin, collisions are rare and the process is slow, so the energy absorbed during the day is released gradually through the whole night. The emission comes chiefly from a layer around ninety to a hundred kilometres up and produces a faint continuous glow across the entire sky.
How it differs from an aurora
The two are confused and are quite distinct:
- •This occurs everywhere on Earth, not only near the poles
- •It is present every night rather than during magnetic storms
- •It is driven by sunlight absorbed earlier, not by particles from space
- •It is faint and even rather than structured into curtains and rays
- •It shows no rapid movement
- •It contributes to the sky brightness at every observing site
Why it matters to astronomers
The glow sets a floor on how dark any ground-based night sky can be, which has practical consequences for observation. Even at the best sites on Earth, with no artificial light and no moon, this emission is the dominant source of background brightness, so a faint object must be detected against it rather than against true darkness. It also varies, brightening and fading over minutes and hours and forming slow-moving bands as waves pass through the upper atmosphere, which means the background is not merely bright but unsteady. Long exposures therefore require the background to be measured and subtracted, and astronomers select filters partly to avoid the wavelengths where its emission is strongest.
What observers can do about it
Amateur observers encounter the effect as a limit and there are partial remedies. Choosing a target high in the sky helps, since looking towards the horizon means looking through a much longer path of the emitting layer and the background brightens accordingly. Narrowband filters that pass only the wavelengths a nebula emits reject most of the glow and transform what can be recorded from a bright site. Taking many short exposures and combining them lets the varying background be modelled and removed. And checking solar activity is worth doing, since the emission brightens with the solar cycle and can double between minimum and maximum.
What it looks like from above
Photographs taken from orbit show the phenomenon far more clearly than anything visible from the ground, and they are the images most people have seen without knowing what they were. A thin band of green or orange light traces the curve of the Earth's limb in night-side photographs from the International Space Station, sitting well above the visible surface and below the stars. That band is exactly the emitting layer, seen edge on through a long path, which concentrates a glow that is nearly invisible when looked at from below through a short one. The colour comes principally from oxygen, which also produces the green of many aurorae.
The takeaway
Sunlight splits molecules in the thin upper atmosphere during the day and they recombine slowly through the night, releasing the energy as light from a layer around ninety to a hundred kilometres up. It happens everywhere every night, unlike an aurora, and it sets the floor on how dark any ground-based sky can be. Photographs from orbit show it as a band of green tracing the Earth's limb.