9 Dec 2016

Did You Know That Moonlight Is Actually Yellower than Sunlight?

Moonlight has a color temperature of 4100K, while sunlight's is greater than 5000K. This means moonlight is yellower than sunlight. That's not what we see, is it? We see sunlight being yellow, while moonlight is white. But the reality is the opposite! Apparently, this is because moonlight is so dim that we can't perceive its color, just as we lose color vision at night. So we think moonlight is white.

If we could concentrate moonlight to make it brighter, we should be able to see that it's yellower. How do we prove this to ourselves? One way is to build a giant magnifying glass. But that's not a practical option. What's an experiment we can do with the resources we do have?

The answer is to use a camera, and take a long exposure, like 30 seconds. This concentrates the sunlight, but over time rather than spatially. One problem is that cameras adjust colors to make them look the way the eye perceives them. That comes in the way here, so we have to turn it off, by using the same settings for both photos, so that we can compare them.

With that in mind, I took a photo of my garden during day time:

And then at night:

I adjusted the night photo's color balance to match the day photo (temp=5100 and tint=+3 in Lightroom), and behold:

See that the moonlight is much yellower than the sunlight? This is how it would look if our eyes were more sensitive, if we could see the world as it really is! It's amazing that something we take for granted since childhood — sunlight is yellow, moonlight is white — is actually the opposite in reality!

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