Photographing the Color of the Night Sky

Words and Images by Glenn Randall  |  September 2024

(Editor’s Note: This astrophotography tutorial is excerpted from Glenn Randall’s book, Dusk to Dawn, Second Edition: A Guide to Landscape Photography at Night.)

On nights with a full moon, the sky and landscape seem to have a bluish tinge. Indeed, the sky when the full moon is up really is blue, and an image taken with a daylight white balance (which sets the camera to record the wavelengths actually present, without altering them) will record it as blue. We can’t see color in the moonless night sky, but it seems logical to assume it must be blue as well.

Not Always Blue

For decades, filmmakers have exploited our belief that the night sky, and therefore the landscape, must always be blue. They use a technique called day for night to shoot “nighttime” scenes during the day. To create the illusion of night they underexpose their footage and shift it blue, either by using a blue filter over the lens or by changing the white balance in post-production.

Our experience and expectations, reinforced by the many movies and TV shows we’ve seen, lead us to the wrong conclusion. The sky is not always blue. For starters, although the color of the moonlight sky really is blue, moonlight itself isn’t blue; in fact, it’s slightly yellower in hue than noon daylight.

The land in a photograph taken under a full moon will actually appear warmer in tone than a photo of the same scene taken at 12 noon. In any case, even the light of a full moon isn’t bright enough to directly excite our cones, the cells in our retinas that allow us to see color.

Brain Games

So why do we see the moonlit world as bluish? Saad M. Khan and Sumanta N. Pattanaik, two researchers at the University of Central Florida, have advanced the theory that this apparent bluish tinge is essentially a perceptual illusion. The light-sensitive cells in our retinas called cones come in three types, each sensitive to a different region of the visual spectrum. One type detects red light; another detects green light; the third detects blue light.

At night our cones become inactive, and the rods in our retinas take over. Rods are much more sensitive to light than cones but cannot distinguish colors. Khan and Pattanaik cite evidence that some rods have neural connections with cones, so that stimulation of the rods by moonlight actually causes some activation in nearby cones as well. They then hypothesize that these interconnected rods interact primarily with blue-sensitive cones.

Your brain interprets this cone activation as bluish light striking your eyes, so the world appears to have a blue tinge even though the light reaching your eyes from the land is not actually bluish. You conclude that if the world looks bluish in moonlight, then the sky on a moonless night must be blue as well.

When you shoot with a daylight white balance you are essentially telling the camera to record the colors actually present in the scene. If you set the white balance to daylight and shoot photographs during a night with a bright moon, you will see that the sky is indeed blue for the same reason a clear daytime sky is blue: Rayleigh scattering.

Moonlight is simply sunlight that has bounced off the moon’s surface and traveled to Earth. Sunlight is originally composed of all wavelengths. When it hits Earth’s atmosphere, however, the blue light tends to scatter out of the beam while the warmer tones tend to travel straight through. The sky looks blue during a clear day and records as blue during a moonlit night because that scattered blue light has traveled from the sky to your eyes or your camera.

On Moonless Nights

Moonless nights are a different story. In fact, you may be startled by the greenish color of the sky in the first shots you take on a moonless night. Even on the darkest night, the sky is never completely black. Instead, it often exhibits airglow, a faint glow caused by a variety of complex processes in the upper atmosphere. (Technically speaking, airglow occurs 24 hours a day, so some authorities use the term nightglow for the glow we see at night.) The most common nighttime airglow color is green, but airglow can also be red, blue, or yellow.

As Scott Bailey, a professor of atmospheric science, explained it, “To get this green line [emission] at night, we need to find a place in the atmosphere where molecular and atomic oxygen are both relatively large in abundance. This rare mixture occurs about 95 km above Earth’s surface in a very narrow layer only 10 km thick. Excited O2 molecules collide with O, exciting the atoms, which relax by emitting green photons.”  This 557.7 nanometer Wizard-of-Oz green light has the same color as the most common type of aurora, but the mechanism of excitation of the atoms is different.

We don’t see the sky on a moonless night as green, of course; in fact, we only see color if we’re looking at an object, such as a bright star or a planet, that’s bright enough to excite the cones in our retinas. If you look closely, you can see that certain stars exhibit color.

Antares, Aldebaran, Arcturus, and Betelgeuse, along with the planet Mars, all exhibit a reddish hue; Rigel and Sirius are blue-white. A few photographers show the night sky as their cameras record it when set to a daylight white balance. While this approach is certainly accurate in showing the colors actually present in the scene, it produces a sky color that most viewers find distinctly odd.

The Bottom Line

I choose to change the color of the sky to restore the deep blue color we imagine the night sky to be. However, I also choose to preserve the star colors captured with a daylight white balance, since those are colors I can actually see. I shift the color of the land slightly toward blue to help preserve a nighttime feel.

The bottom line is this: shooting in color at night is like shooting in black-and-white during the day. What shade of gray best represents a clear sky at noon? Any shade of gray that looks good!

And what shade of blue (or green) best evokes the feeling of gazing awestruck at a sky full of stars? Any hue that satisfies your artistic intentions! Since you can’t see the true color of the night sky regardless of whether there is moonlight or not, your choice of sky color is inherently subjective.

See more of Randall’s work at glennrandall.com. Buy his book, Dusk to Dawn, Second Edition: A Guide to Landscape Photography at Night, here.