Why is moonlight white not yellow

Why does moonlight have a lower color temperature?

I refer you to the picture below which is from Ciocca & Wang (2013). This clearly shows that the moon's spectrum (normalized to have an overall strength similar to sunlight) is redder than sunlight and therefore has a lower "color temperature". This is a fact, not a perception.

EDIT: Just to get rid of confusion - the OP speaks of "yellow" because this way the eye perceives a redder spectrum (in terms of physics, this means shifting to a longer wavelength - see picture). In this sense, moonlight is more "yellow" than sunlight, because it has a redder spectrum.

The reason for the redder spectrum is that the reflectivity of the moon increases at reddish wavelengths. Since moonlight is reflected sunlight, it must be redder than sunlight.

Opinions differ when it comes to our perception of moonlight. While the light is likely to be too bright for true scotopic vision, it is likely not bright enough to allow full color vision, and hence inferior mesopic vision with eye cells that are more sensitive to blue light - also known as the Purkinje effect .

This is exactly what Ciocca & Wang suggest in their work. It must be noted, however, that the difference between the solar and lunar spectra is not that great, especially considering that the eye acts as a logarithmic intensity detector. It is entirely possible that the difference is not big enough to be perceived by the eye, so the broad spectrum of the moon appears basically white and this is amplified when seen against a dark sky.

Vaddadi Kartick

Interesting. As I wrote in the question, the Purkinje effect and the blue sky effect that makes things look yellow can be neutralized by a long exposure photo taken on a full moon night, with the exposure long enough to make it so bright how to make daylight and compare it to a daytime photo of the same scene with the same color balance. We can cut out the sky so as not to be misled by its color. Correct?

Rob Jeffries

@KartickVaddadi You asked about moonlight. It has a lower color temperature. It's redder than sunlight. I don't know what you mean by yellow. The sunlight is almost white and the full moon looks white.

Rob Jeffries

@KartickVaddadi If you somehow make the moon as bright as the sun could , then he appears possibly slightly redder (lower color temperature) than the sun. Because it is. The Purkinje effect allows us to perceive weak light as blue (higher color temperature) than we would perceive a brighter light with an identical spectrum. Not more "yellowish".

to the left

@KartickVaddadi An interesting suggestion! I would guess that if you left the white balance the same and just changed the exposure time, the night photo would be a total of quite reddish or the daylight scene looks bluish. Taken by themselves, the sun and moon would have almost the same color in their respective image, but in context the sun might be perceived as more yellow due to the effect I discussed in my answer.


@KartickVaddadi: If you try to do the long exposure, you will find that camera sensors have all sorts of weird behaviors that distort their color rendering with very long exposures. I would expect sensors used by astronomers to control this.