In moonlight the other retinal cells called rods are doing most of the work. Rods detect relative lightness and darkness, but they are entirely color-blind.
Moonlight is simply the white light of the sun reflecting off the gray surface of the moon. There’s nothing in that interaction to give the light a bluish or greenish quality. In fact, scientific instruments have shown that the light from the moon is very slightly redder in color than direct sunlight.
These facts added together suggest a mystery at the heart of how we as artists choose to portray moonlight in paintings. If moonlight is just gray-colored light, and if it’s close to the minimal threshold of our color receptors anyway, then why do so many artists paint moonlight as bluish or greenish? Do we really see it that way? Is it some kind of illusion, or perhaps is it just an artistic convention?
Let’s look at some paintings by master painters of moonlight. As you look at them, consider your own perception of the colors at night, and ask yourself which of the paintings best convey your own experience.
Here’s a painting by J.M.W. Turner. It’s fairly gray, with just a hint of warm color around the moon. Notice that there isn’t much detail in the shadow area. All you really see clearly are the silhouettes of the sail and the boat on the water.
Russian seascape painter Aivazovsky painted this night scene lit by a golden moon. The sky, the water, and the shadows all sink into blue-green tones. He doesn’t show very much detail, and he stops well short of black in the shadows.
This lightening of darks was also a feature of Remington’s nocturnes. The cast shadow to the left of the pony’s nose is composed of dull umbers and greens. These luminous shadows lighten and liven the obscurity. Except for the light saddle cover, Remington has left most of the edges soft and undefined, especially on the donkey on the right.
This famous nocturne by Whistler of the Battersea Bridge uses a fairly saturated blue-green color, especially in the water and in the silhouetted figure. The detail is blurred throughout, even in the areas where the bridge appears against the sky, setting up for the tiny sparkles of light in the distance.
One of the reasons for softening the edges is that we depend on the cones for fine discrimination of edges. Unfortunately the cones are located on the fovea, the centerpoint of vision, and with them off-line in the darkness, we just can’t sort out small details.
If you take a book or newspaper outdoors in moonlight, you can see that there is writing on the page, and you might be able to read headlines or other large type, especially when you glance around with your peripheral vision. But reading normal size text is almost impossible. When you look directly at the words, the blind spots get in the way.
I said earlier that our cones are barely functioning in moonlight. In fact, contrary to what some authorities have claimed, most people’s cones can make basic color judgments by the light of a full moon. But how much variation in color can we really see?
Maxfield Parrish rendered this moonlight scene with quite a bit of color saturation. He painted the yellow moonlight, the reddish cupula on the barn, the deep blue of the sky, and the orange color on the shadow side of the house. Did he really see such colors in moonlight, or did he invent them for pictorial effect? Too bad he’s not here to ask.
Direct plein-air painting is virtually impossible in moonlight. Every artist has to work from memory and imagination. We may try to convey our actual optical sensations, but we’re not scientists. Each of us is also trying to make a subjective aesthetic statement intended to evoke a particular mood or emotion. Any moonlight painting is an attempt to translate a “rod experience” into a “cone experience,” an image that will be seen in a brightly lit environment.
Here’s how you can test how your cones actually respond to color in moonlight. Paint a set of separate, matching, unmarked color swatches or find some construction paper at about the same value. Take them into full moonlight (this Tuesday) and let your eyes adjust (it takes about 30 minutes). Shuffle the cards, and while you’re still outdoors, mark on the back what colors you think they are.
I have used Photoshop to manipulate a photo of the swatches (actually shot in daylight) to simulate how they appeared to me under the full moon: dulling, darkening, and blurring them. Both Jeanette and I could easily identify the basic hue family of each swatch. But beyond that basic classification, we weren’t sure, and the gray swatch confused us both.
When I looked at the same swatches in the much dimmer light of a half-moon, or in a moon shadow, I found my cones went sub-threshold and shut down completely, and the swatches became completely monochromatic.
Although the rods of the eye can’t actually see color, scientists have shown that they are most sensitive to greenish wavelengths of light. As a result blue-green hues appear lighter in tone in dim conditions. There’s a name for this: the Purkinje Shift. It’s a different phenomenon from, and often mistaken for, the perception of moonlight as blue.
You can demonstrate the Purkinje Shift by comparing a red and green swatch that start out indoors at the same value. If you take them outdoors in moonlight, the greenish one will seem much lighter in tone. Many observers have noticed that red roses look black in the moonlight.
If you scroll back up to my Photoshopped version of the moonlight color swatches, you can see I’ve adjusted the values to simulate the way the red and green looked to me as a result of the Purkinje Shift.
Here, Remington shows a scene with Indians in moonlight. We see their flesh tones and some clear red touches in their costumes. Throughout, the edges are much crisper than his other painting.
This nocturne of old Cincinatti by contemporary artist John Stobart has a distinctly bluish cast. He introduces much more detail than we’ve seen in the other examples, reminiscent of the “day-for-night” film shoots in old westerns. You can even read the name “Bonanza” on the shadow side of the ship.
In addition to the moonlight, there’s a secondary source of yellow-orange lamplight. In this case, one could argue that the blue cast to the picture may be a complementary color induced in opposition to the color of the lamplight.
Atkinson Grimshaw was famous for his poetic moonlight studies. Here the shadow masses at the left are fairly soft and impenetrable, but the bricks and branches show up very clearly. The moonlight on the road is an intense yellow-orange, assuming this reproduction is accurate. The shape of the patch of light points to the lovers standing in silhouette at left.
Russian landscape realist Ivan Shishkin, painted this haunting image of a winter night in the wild north. The snow in moonlight is relatively brilliant, with a soft halation along the edge at left, but it’s not yellowish. The cast shadow gradates in tone, getting lighter as it catches more sky fill and bounced light. There’s quite a bit of detail in the tree form, but he has kept the foreground and background description to a minimum.
So, to get back to the question posed earlier, why do we see moonlight as blue?
Saad M. Khan and Sumanta N. Pattanaik of University of Central Florida have proposed that the blue color is a perceptual illusion, caused by a spillover of neural activity from the rods to the adjacent cones.
A small synaptic bridge between the active rods and the inactive cones touches off the blue receptors in the cones, kind of like an insomniac turning over in bed and rousing his sleeping spouse.
This influence of rod activity on the adjacent cones tricks the brain into thinking we’re seeing blue colored light, even though we’re really not.
As the authors put it: “We hypothesize that the rod cells predominantly synapse onto the S-cone (cone cells sensitive to bluish light) circuitry resulting in the visual cortex perceiving a tinge of blue.”
So moonlight isn’t blue; our eyes are just playing tricks on us.
Unfortunately, this tantalyzing hypothesis remains untested. I contacted Dr. Khan and he told me that because of other projects he hasn’t had time to prove the hypothesis in controlled conditions. I hope that he can shed more light—of whatever color—on this elusive topic.
Until then, moonlight remains a mystery at the meeting point of art and science.
- Khan and Pattanaik’s summary article in Journal of Vision, 2004. Link.
- Related discussion on the NASA web site. Link
- "The Eye and Night Vision," from American Optometric Association. Link.
- More on Remington’s nocturnes at David Apatoff's blog. Link
Tomorrow: Elegant Graphics