Why does moonlight look blue

Pattanaik, University of Central Florida [ More ]. The authors of the study went on to propose a bio-electrical explanation--that signals from rods can spill into adjacent blue-sensitive cones under conditions of full-moon illumination see the diagram, right. This would create an illusion of blue. So there are still some mysteries in the moonlight. Look for them on Oct. Caveat Lunar: This story makes some generalizations about what people can see at night but, as with all things human, there are exceptions: Some people can read by moonlight; others have no trouble seeing the red petals of a moonlit rose.

These people have "moonvision," boosted by an extra-helping of rods or unusually sensitive cones. Are you one of them? The blueshift is sometimes attributed to the spectral response of rods. Although rods are nominally color blind, they do not respond equally to all colors: Rods are more sensitive to blue-green photons and less sensitive to red photons.

You can see this in your moonlit rose. By day, the red flower dominates the green leaves. At night, the situation is reversed. The green leaves are more vivid than the red flower. No matter which part of the rose stands out most, however, the ensemble is still gray. This is because the rods have no mechanism for separating colors.

Shades of gray are all we get. Cones are able to separate colors because they come in three varieties: red-sensitive, green-sensitive, and blue-sensitive. The brain can sort out the color of an object by noting which kind of cone it stimulates most. Rods, on the other hand, come in one variety only--monochromatic, which brings us back to the mystery of the blue shift. If rods can't separate colors, how does the brain register a blue rather than gray landscape?

I took photo at night 4AM of the same scene, lit by the full moon. This is a long exposure photo 30 seconds with roughly the same exposure:.

The moonlit scene has a strong yellowish color:. This proves that moonlight indeed has a lower color temperature. It's not a slight difference in color, but a huge one. Python Javascript Linux Cheat sheet Contact.

Why does moonlight have a lower color temperature? In addition to the Purkinje effect, another thing that contributes to the different perception is contrast to ambient light : When the sun is high in the sky, the ambient light is that of the sky.

Rayleight-scattering blue. Against that, anything directly luminated by the sun appears slightly yellow. Incidentally, that's also true for the moon when it's close to the horizon: it looks much redder then!

When the sun is low, the ambient light has also a much lower temperature, meaning any self-emitting object will appear bluer by comparison — including the moon, if it's higher in the sky. 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. 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. Do we really see it that way? Is it some kind of illusion, or perhaps is it just an artistic convention? 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. 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. 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. 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. 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?

Direct plein-air painting is virtually impossible in moonlight. Every artist has to work from memory and imagination. Each of us is also trying to make a subjective aesthetic statement intended to evoke a particular mood or emotion. 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. 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. 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.

As a result blue-green hues appear lighter in tone in dim conditions. 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. 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. 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.

Which it generally needs to be, to be visible by day. At night, the ambient light is not dominated by Rayleigh-scattered moonlight, but by starlight or, in urban regions, rather by sky glow. This is not blue, therefore it doesn't lead to the moon being perceived yellower than it really is. To test this hypothesis somewhat, I've made this picture: Well What do you think of my comment on Rob Jeffries's answer?

Rayleigh scattered moonlight is mostly notable at full moon, midnight, and then it is, in fact, blue — obviously. But that light doesn't dominate the night sky, you still see stars and often also skyglow. That infamous dress color picture resulted in me saying "blue and gold".

Add a comment. The moonlit scene has a strong yellowish color: This proves that moonlight indeed has a lower color temperature. This experiment excludes the effect of the: blue sky making things on the ground look yellow in comparison because there's no sky in this photo, and the color balance is set to the same for both photos.

Your conclusions are also incorrect. A camera tells us nothing about the Purkinje effect, because that is an effect of perception and the physiology of your eye. The fact that your photo appears yellow merely confirms in a rather unquantified way that moonlight has a lower colour temperature than the Sun. But we knew that already. It is like pulling a rug.

In your original question you were not questioning that the moon had a lower temperature, you give it as a datum and ask a different question. The fair thing to do is to leave the original question, and make a new one and answer it with this beautiful demonstration. The question was always about the inconsistency between what I read and what my eyes tell me, and the original version of the question mentioned the experiment of using a long exposure to verify that the moonlight indeed has a lower color temperature.

If you have a different question - namely, how do I verify that moonlight and sunlight have different colours, then ask it as a different question. Your edit completely changed the meaning of the question. Show 3 more comments. So I doubt that a "color temperature" will make much sense for the moon. Neither are blackbodies and certainly not after passing through the Earth's atmosphere - see the picture in my answer. The sun might generate energy through fusion, which produces a defined photon energy spectrum non-thermal.

But those photons are created in the core. It takes many thousands of years for the photon to make it to the photosphere, where we finally can observe it, by which time it will have thermalized. Plus your understanding of the physical process is completely wrong. The visible photons we see from the Sun come from the photosphere and from different depths at temeratures ranging from about K in sunspots to 10, K in plages.

However the photosphere gets its energy from photons from the core. We can model the process as a photon making its way out from the core scattering constantly.