Why We Can See In The Dark

In search of a pitch black night?  Don't expect to find it on Earth. Thanks to starlight, zodiacal light, and especially airglow, true darkness doesn't exist.

Milky Way night light

Starlight and starlit dust in the plane of the galaxy contribute to nighttime illumination.
Bob King

Pitch black nights don't exist. Not on Earth, the Moon, Mercury, Mars, or anywhere else in the solar system where you can gaze up into the night sky. Find the darkest place on Earth, hold your splayed hand up against the sky and you'll see it in silhouette. Chances are, once your eyes have become properly dark adapted, you'll be able to carefully pick your way across the landscape without a light.

What makes your hand visible, anyway? Ignoring human-made light pollution and focusing only on natural sources, there are several contributors to nighttime illumination. The stars, of course, including the unresolved ones plus starlight reflecting off interstellar dust in the plane of the Milky Way. This amounts to at most one-third of nature's night light, making it more feeble than one might suppose.

Touch of dusty lingering dusk

Sunlight reflecting from dust in the plane of the solar system — the zodiacal light — is a significant source of what makes the night bright even from rural skies. It's most visible in mid-northern latitudes in the western sky on spring evenings and eastern sky on fall mornings.
Bob King

Another major player is the zodiacal light, sunlight reflecting off comet and asteroid dust concentrated in the solar system plane. Zodiacal light emissions vary over time depending on your latitude, seasonal variation of the ecliptic's angle to the horizon, and solar activity.

But the most widespread contributor to night sky brightness comes from airglow. Look at any nighttime photo taken from the International Space Station and you'll see the arc of Earth encapsulated in a thin green shell of glowing air. Unlike the aurora, which concentrates in ovals centered on Earth's geomagnetic poles, airglow pervades mid-latitudes, equator regions and polar skies alike.

Bubble of green

Airglow from oxygen emission visible as a "bubble shell" surrounding the Earth. It appears brighter along the outer edge because we're looking through the greatest thickness of glowing air.
NASA

If the green color reminds you of the aurora borealis, it's because similar processes are at work. Both involve the excitation of atoms and molecules — in particular oxygen — at altitudes of around 60-65 miles (100 km). But different mechanisms get them jazzed.

Celestial anatomy lesson

Like dreamy ribs extending from the "vertebrae" of the summertime Milky Way, airglow emission covers a good portion of the southeastern sky on July 18–19, 2015, as seen from north of Duluth, Minn. Human-made light pollution is visible at lower right. Details: 20-mm lens, f/2.8, ISO 4000, 30-seconds.
Bob King

In auroras, electrons and protons from the Sun physically crash into oxygen and nitrogen atoms and molecules at high speed, energizing electrons within the atoms to higher energy levels. When the atoms return to their rest states, they emit photons of green and red light. With gazillions of atoms and molecules at play, the amount of light released can create staggering auroral displays.

Airglow fingerprints

Green light from excited oxygen atoms dominates the light of airglow. The atoms are 56-62 miles high in the thermosphere. The weaker red light is from oxygen atoms further up. Sodium atoms, hydroxyl radicals (OH), and molecular oxygen add their own complement.
Les Cowley

Airglow, which is present both day and night, arises from the Sun's ultraviolet light. UV light is powerful stuff as anyone who's experienced a nasty sunburn can attest. Solar UV galvanizes several different processes in the upper atmosphere that lead to airglow emission. These include excitation, where an energized atom returns to its ground state either by itself or by smacking into a nearby atom, and photo-ionizaton, where UV radiation knocks the electron right out of an atom. When it recaptures another, the satisfied atom releases a photon of light.

In still another reaction, UV cleaves oxygen molecules apart into separate atoms that are then free to combine with nitrogen to form NO (nitrous oxide), a process that also emits photons.The brightest emission, the one that typically shows in ISS and ground-based photos, originates from excited oxygen atoms beaming light at 557.7 nanometers, or yellow-green.

Christmas in July

A mixture of red and green emissions from airglow on July 18–19, 2015. This view faces east with the Andromeda Galaxy above center. Green derives from oxygen emission at around 60 miles in altitude; red from oxygen higher up. To see airglow, find a place well away from city light and allow your eyes to become fully dark adapted (about 45 minutes).
Bob King

With today's digital cameras working at high ISOs, airglow frequently shows up in time-exposure photos taken from dark sky sites. Years ago, I'd notice streaks of pale light across the darkest of skies when no clouds were about. Back then I couldn't figure it out. Now, thanks to my camera, it's clear I was seeing airglow. When in doubt, I'll make a 30-second exposure at ISO to 3200 with the lens wide open then check the LCD screen for telltale green streaks. I use the camera both to confirm what I see and to hunt for patches and plumes I may have overlooked.

Green-striped night

Multiple layers of airglow cut perpendicular to the Milky Way band in the north-northeastern sky possibly caused by gravity waves.
Bob King

Airglow is visible across the seasons and best visible about 10–20° high along a line of sight through the thicker atmosphere. If you look lower, its feeble light is absorbed by denser air and dust. Looking higher, the light spreads out over a greater area and appears dimmer. That said, on certain nights, I've seen the green sheen up to 50°. Too dim to register color, it takes the form of streaks, featureless smears, and plumes.

Dippers in a sea of green

Large amounts of the northern sky (Big Dipper at lower left) were bathed in airglow emission July 18–19, 2015 seen from Duluth, Minnesota. Much of this was faintly visible to the naked eye as patches of very thin cloud.
Bob King

Auroras put in appearances from my latitude of 47° North but they assume different forms, move around, and are generally much brighter. Airglow is visible whether or not aurora is present and appears all over the sky — north, south, east and west. Airglow varies with solar activity and season, becoming more pronounced during solar maximum. Once identified, you'll see it nearly every moonless night from dark skies. Allowing your eyes to fully dark adapt is key to seeing the phenomenon.

Based on my own viewing experience, airglow is more obvious and widespread in the spring and summer and less so in the winter. It also varies in shape, extent, and brightness during the night. Patches can disappear or multiply, and faint streaks may brighten up and then slowly fade. What will you find?

Two nights ago, I saw and confirmed many streaks, including a remarkable series of nearly parallel "bars" perpendicular to the band of the Milky Way, likely the work of gravity waves. Unlike the more familiar gravitational wavesgravity waves are created by jet stream shear, wind flowing over mountain ranges, and even thunderstorms in the lower atmosphere. Wave disturbances propagate upward into the ionosphere to spawn and shape multiple layers or streamers of airglow.

A layer cake of colorful emissions

Excited oxygen at higher altitude creates a layer of faint red airglow. Sodium excitation forms the yellow layer at 57 miles up. Airglow is brightest during daylight hours but invisible against the sunlight sky.
NASA (annotations by Alex Rivest)

Airglow comes in multiple colors depending on whose doing the emitting or recombining:

* Green — The most common emission occurs when UV light breaks molecular oxygen or O2 into individual atoms about 60 miles (95 km) overhead. Rife with excess energy, they radiate green photons to return to their rest states.

* Red — I’ve never seen it, but long-exposure photos often reveal red/pink mingled with the more common green caused by excited oxygen atoms at 90-185 miles (150-300 km) emitting light as they return to the rest state. Excited OH- (hydroxyl) radicals can also radiate deep red light in a process called chemoluminescence when they react with oxygen and nitrogen. Another chemoluminescent reaction takes place when oxygen and nitrogen molecules are split apart by ultraviolet light high in the atmosphere and recombine to form nitric oxide.

* Yellow — Due to sodium atoms around 57 miles (92 km) high. Sodium, a component of meteorites, "salts" the upper atmosphere when meteoroids vaporize as meteors.

* Blue — Weak emission occurring at approximately 59 miles (95 km) altitude when two separate oxygen atoms reunite to form an oxygen molecule (O2).

Airglow is brightest in the daytime, but the glare of daylight masks its presence. The nighttime variety is a thousand times fainter in comparison. Good thing or we'd never know a dark sky!

CATEGORIES
Astronomy Blogs, Explore the Night with Bob King, Observing
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Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. Check out my forthcoming book "Night Sky with the Naked Eye" (on Amazon and BN) about all the great things you can see at night without any special equipment.

9 thoughts on “Why We Can See In The Dark

    1. Frank-ReedNavigation.comFrank-ReedNavigation.com

      Bob King: Absolutely fascinating. I was aware of “airglow” in general terms, but I learned so much detail from this article. I had no idea that a modern camera under very dark skies could detect actual patterns in the airglow. Rather than seeing it as a mere nuisance, is this not something that we, as backyard astronomers, should try to image, as part of the pageant of natural sky phenomena?

      I would like to add, Bob, that your contributions to the S&T web site, in every article, over the past couple of years (how long has it been?) have greatly enhanced the value of the content here. You articles are novel, always interesting, fact-filled and delivered with eloquent prose combined with the energy and excitement of someone who’s really passionate about the subject. Bravo!!

      Frank Reed
      Conanicut Island USA

      1. Anthony BarreiroAnthony Barreiro

        Hear, hear!

        One thing I greatly appreciate about Bob King’s articles is that they always make me want to get outside and look at the sky, with new appreciation.

      2. Bob KingBob King Post author

        Frank,
        Thank you so much for your kind words. I appreciate that very much. I agree with you that airglow has been rather overlooked, but given that it requires dark skies, it’s understandable. Tracking trends in airglow’s distribution, form and intensity over a year would make a great amateur project.

    2. Bob KingBob King Post author

      John,
      The darkest place I’ve ever been is in a mine a half mile down. They turned out the lights, and absolutely nothing was visible.

  1. Peter WilsonPeter Wilson

    Two normally invisible phenomena actually outshine the stars by 2-to-1? Thanks, I’d never guess that!

    1. MaiaMaia

      Yes, by far the biggest obscuration is human-caused: we over-light everything so that most of us never get to even SEE the Milky Way. I find that extremely sad. But there are so many other things to miss due to human lighting systems, most of it unnecessary. Save Our Stars is one of many anti-light pollution orgs to support. But we also need to “save our Night”, ie the darkness itself. Night is majestic and bracing, puts everything into proper perspective.

  2. Hugo MarracoHugo Marraco

    I used to get observing time at Cerro Tololo Interamerican Observatory. After dark adapting our eyes (about 20 minutes) the astronomers could drive the VW cars, that were in use in the 1970 decade, from the mountain top to the dormitories (half a mile) without turning on any light, of course VERY slowly. I am referring to moonless nights.
    One cloudy night I was waiting for the miracle of a midnight clearing (sometimes happens) in the Library at the lower level of the 5 meter telescope dome. We used dimmed red lights, in order to get quickly adapted back to the darkness as soon as possible. I decided to get outside to look for possible sky clearing.
    I walked on the mountain top several yards outside over the courtyard that is located between the several domes. Then I waited to my eyes to become dark adapted … waited … waited … but nothing was visible even after 20 minutes. There was a heavy overcast and it was a TOTAL DARKNESS. I turned to the left and then to the right and become totally disoriented! In order to come back to the 5 meter telescope dome I had to turn ON my flashlight.

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