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Moonlight has a color temperature of 4100K, while sunlight has a higher color temperature of more than 5000K.

But objects illuminated by moonlight don't look yellower to the eye. They look bluer. This holds for indoor scenes (like my hall) and for outdoor. I find it counter-intuitive that moonlight has a lower color temperature. I thought the sun is the yellowest natural source of light we have.

Is that because of the poor color sensitivity of the eye in dim light? In other words, moonlight is actually yellower, but our eyes can't see the intense yellow color?

If one were to use a giant lens to concentrate moonlight to reach the brightness of sunlight, will objects illuminated by this light appear yellower to the eye than the same objects under sunlight? Has anyone done such an experiment? I looked, but couldn't find any.

Alternatively, if I take a long-exposure photo of a landscape illuminated by the full moon, and another one illuminated by sunlight, and equalise the white balance and the exposure, will the moonlit photo look yellower?

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possible duplicate: physics.stackexchange.com/q/15184 – Martin Mar 22 at 13:54
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Isn't it because sunlight comes from emission, while moonlight mainly comes from reflection ? I'd like to see a spectrum of moonlight, maybe it differs significantly from the blackbody one. You might also be interested in this : en.wikipedia.org/wiki/Purkinje_effect – Dimitri Mar 22 at 14:24
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@CarlWitthoft -- The eye's photopic response is described by biophysics, which is on-topic for this SE site. If you instead had blamed the visual cortex, that would be a matter for the cognitive sciences SE rather than biology.SE. – David Hammen Mar 22 at 16:17
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A tangentially on-topic xkcd.com "what-if" discussing using a giant lens to focus moonlight... what-if.xkcd.com/145 – James Mar 22 at 17:32

I refer you to the picture below, taken from Ciocca & Wang (2013). This clearly shows that the spectrum of the moon (normalised to have a similar overall strength as sunlight) is redder than sunlight and so has a lower "colour temperature". This is a fact, not a perception.

EDIT: Just to clear up some confusion - the OP talks about "yellower" because that is how the eye perceives a redder spectrum (in the Physics sense of the word, meaning shifted to longer wavelength - see picture). In this sense yes, moonlight is "yellower" than sunlight because it has a redder spectrum.

The reason for the redder spectrum is that the reflectance of the moon gets larger at redder wavelengths, so as moonlight is reflected sunlight, it must be redder than sunlight.

As for our perception of moonlight, opinions vary. Whilst the light is probably too bright for true scotopic vision, it is likely not bright enough for full colour vision to be operative and therefore inferior mesopic vision takes over, with eye cells that are more sensitive to blue light - a.k.a. the Purkinje effect.

This is exactly what Ciocca & Wang suggest in their paper. However, it must be pointed out that the difference between the solar and moon spectrum is not that big, especially considering that the eye works as a logarithmic intensity detector. It is entirely possible that the difference is not big enough to be perceived by the eye, so that the broad spectrum of the moon basically appears white and that this is enhanced if it is seen against a dark sky. enter image description here

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Interesting. As I wrote in the question, the Purkinje effect and the effect of the blue sky making things look yellow can be neutralised by a long exposure photo taken on a full moon night, with the exposure long enough to make it as bright as daylight, and comparing it with a daytime photo of the same scene with the same color balance. We can crop out the sky to avoid being mislead by its color. Correct? – Kartick Vaddadi Mar 23 at 6:05
    
@KartickVaddadi You asked about moonlight. It does have a lower colour temperature. It is redder than sunlight. I don't know what you mean by yellow. Sunlight is almost white and the full moon looks white. – Rob Jeffries Mar 23 at 6:51
    
@KartickVaddadi If you could somehow make the moon as bright as the Sun, then it might just appear a little redder (lower colour temperature) than the Sun. Because it is. The Purkinje effect makes us perceive faint light as bluer (higher colour temperature) than we would perceive a brigher light with an identical spectrum. Not "yellower". – Rob Jeffries Mar 23 at 7:04
    
@KartickVaddadi An interesting proposal! I'd conjecture that, if you leave white balance the same and only change exposure time, then the night-time photo would look overall pretty reddish, or the daylight scene blueish. Taken on their own, sun and moon would have almost the same colour in their respective picture, but in context the sun might be perceived yellower, due to the effect I discussed in my answer. – leftaroundabout Mar 23 at 11:21
    
@KartickVaddadi: what you'll find if you try to do the long-exposure thing is that camera sensors have all sorts of odd behaviours which will distort their colour rendition for very long exposures. I would expect that sensors used by astronomers have this controlled for. – tfb Mar 24 at 11:20

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.
  • When the sun is low, the same scattering acts as a filter, meaning the sunlight is yellow/orange when it arrives you.
    • 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. (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:

Twice the same grey circle, once over starry, once sky-blue background. The latter should appear yellower. Well... it doesn't work entirely convincingly I have to say – the two whites do look pretty similar. They're in fact the same. Does the blue background make it seem a bit yellower to you? I'm not sure.

But the effect is certainly a lot more pronounced if you're actually surrounded by blue sky. In particular, as I just remembered: this “automatic white balance correction” works with some time delay. A white object on green background may still look white, but stare at a green screen for a few minutes and everything else will look purple-tinted. I think this actually has to do with colour receptors in the eye tiring out. At night, your eyes have a lot of time to adapt to the white point, and thus everything will look bluer, including the moon. Add to this that traditional artificial light sources have a very low colour temperature – the eyes will adapt to those, more than to the remote natural light sources.

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Interesting, thanks. What do you think of my comment on Rob Jeffries's answer? – Kartick Vaddadi Mar 23 at 6:05
    
Is the so-called "midnight blue" rayleigh-scattered moonlight? – Random832 Mar 23 at 20:06
    
Yeah, I suppose so. 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. – leftaroundabout Mar 23 at 20:25
I thought the sun is the yellowest natural source of light we have.

Whatever made you think that? It's pretty much by definition the whitest source of light we have since sunlight is what we judge all colors in.

Now direct sunlight is pretty close to a black body emission spectrum. The moon, not so much. It's reflected light, so assigning an actual color temperature is not going to reflect reality all that much since a color temperature implies a whole spectral distribution, and the moon has the spectrum of the sun modified by its wavelength-dependent average reflection coefficient. Its own temperature is such that its own black body radiation is not going to significantly contribute to the visible part of the spectrum.

The reddish color of a lunar eclipse is attributed to scattered light from the earth's atmosphere rather than "afterglow" from the immediately preceding full moon.

So I doubt that a "color temperature" will make much sense for the moon.

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A colour temperature for the moon makes as much sense as a colour temperature for the Sun. Neither are blackbodies and certainly not after passing through the Earth's atmosphere - see the picture in my answer. – Rob Jeffries Mar 23 at 16:06
    
@RobJeffries I disagree with you. 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. – Aron Mar 24 at 3:47
    
@aron Look at the spectrum. 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 3500K in sunspots to 10,000 K in plages. I.e It is a multi-temperature spectrum with absorption features and is only an approximation to a blackbody with effective temperature of 5770K. – Rob Jeffries Mar 24 at 7:18
    
@RobJeffries That is one interpretation, certainly. 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. Then we can model that process as Brownian motion. Of course its all approximate. But by your reckoning there is no such thing as a Blackbody. – Aron Mar 24 at 7:23
    
@aron it is the correct interpretation. My whole career has been spent fitting detailed (not "approximate") spectra to stars. None of them are blackbodies and if they were, I wouldn't be able to determine their chemical abundances, or anything else apart from a crude temperature and emitting area. – Rob Jeffries Mar 24 at 7:42
up vote 1 down vote accepted

I took a photo during the day, at 2PM:

enter image description here

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:

enter image description here

I adjusted its color balance to match the day photo (temp=5100 and tint=+3 in Lightroom). The moonlit scene has a strong yellowish color:

enter image description here

This proves that moonlight indeed has a lower color temperature. It's not a slight difference in color, but a huge one.

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).
  • eye being unable to perceive colors or perceiving them wrongly (Purkinje effect) when it's dark.
  • sun or moon at horizon (because they are high in the sky for this photo)
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So, to get this clear... the third photo was taken with the same settings as the first, except longer exposure and different time of day? – leftaroundabout Mar 24 at 13:21
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This has nothing at all to do with (a) why the moon has a lower colour temperature (the title of your question) or (b) how the eye perceives moonlight (since you took pictures with a camera). 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. – Rob Jeffries Mar 24 at 13:43
    
@leftaroundabout Yes – Kartick Vaddadi Mar 25 at 3:45
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Even though I gave a +1 , because it is an interesting demonstration, I think it is unfair to the effort people made to answer the original title question to make such drastic change so that it is a new question. 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. – anna v Mar 25 at 4:15
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Yes, completely unfair. 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. – Rob Jeffries Mar 25 at 8:02

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