Calvin: Why does the sky turn red as the sun sets?
Calvin’s Dad: That’s all the oxygen in the atmosphere catching fire.
Calvin: Where does the sun go when it sets?
Calvin’s Dad: The sun sets in the west. In Arizona actually, near Flagstaff. That’s why the rocks there are so red.
Calvin: Don’t the people get burned up?
Calvin’s Dad: No, the sun goes out as it sets. That’s why it’s dark at night.
Calvin: Doesn’t the sun crush the whole state as it lands?
Calvin’s Dad: Ha ha, of course not. Hold a quarter up. See, the sun’s just about the same size.
Calvin: I thought I read that the sun was really big.
Calvin’s Dad: You can’t believe everything you read, I’m afraid. -Bill Watterson
But seriously, why is the Sun yellow?
This is particularly troubling, because when light shines on white things, they look white, not yellow! So what’s going on with the Sun that makes it look yellow in the sky?
Let’s remember how good the atmosphere is at tricking us about color, first. As example #1, I give you… the Moon!
When the Moon is very, very low on the horizon, it has to pass through a huge amount of atmosphere! And what the Earth’s atmosphere does — thanks to a process called Rayleigh Scattering — is to let the redder wavelengths of light through while scattering away the bluer ones.
This is why not only the Moon appears redder when it’s close to the horizon and whiter when it’s high in the sky, but the Sun does as well!
The more atmosphere you pass the Sun through, the more blue light gets scattered away, and so the redder it appears! White light occurs when you get a roughly even mix of all the colors, but take some of the bluest blues away, and it’ll turn yellow, orange, and eventually red as you progressively take more and more away.
How to demonstrate this? This last picture gives me an idea.
Notice how the sunlight appears orange-ish, and the reflected sunlight also appears orange? This is because both the directly light from the sun and the reflected light from the water both pass through roughly the same amount of atmosphere.
But what if we went to space? We could compare the light reflected off of the Earth’s oceans — which passes through the atmosphere twice — with the light directly from the Sun! Let’s have a look:
The direct sunlight? Nearly perfectly white! But the light that passes through the Earth’s atmosphere, reflects off the ocean, and heads back through the atmosphere to you? Yellow!
So that tells us that the atmosphere definitely makes the Sun look yellow to us. But what if we went to space, and took a good look at our Sun, and compared it with the other stars out there?
The Sun, although your eye isn’t great at discerning it, is almost, but not perfectly white!
To an astronomer, if we want to know what colors come out of something, we imagine heating up something that’s perfectly black to a ridiculously high temperature, and calculate what light would get emitted from it. We call this radiation, imaginatively, black body radiation. This has applications that range from estimating the temperature of lava,
which is about 1300-1500 Kelvins, to the temperature of different stars! It turns out that each temperature has its own particular color, as illustrated by these three bulbs.
The one at 5500 Kelvin turns out to be very close to our Sun, but is only blue-ish by comparison with the other two bulbs. Our Sun, in reality, is actually at an average surface temperature of around 5700 Kelvin. You can see, by looking at the blackbody spectrum for a few different temperatures, why the cooler ones look redder and why the hotter ones look bluer.
If a higher percentage of your visible light is red as compared to blue, you look redder, and vice versa! This means there are two ways to change your color, to either add more intensity of one type of light, or subtract out some light, like the atmosphere does to blue light!
In other words, it isn’t just about how much red or blue light you have, it’s how much you have relative to the other types!
If you were a perfect blackbody, the temperature would be the only thing that mattered in determining your color. And you would live, in color temperature world, on this black line in chromaticity space.
Of course, we don’t. The Sun, after all, isn’t black, and is powered by an awfully complicated process. It turns out that the real Sun has more blue light and more red light than a perfect blackbody, but less yellow and green light!
Why, you ask? We don’t know. It might have something to do with how heat is generated in the Sun, the number of collisions it undergoes until it reaches the surface, uneven heating of the Sun’s crust, or a multitude of other factors, but we don’t know the whole story.
There are plenty of stars that are bluer than the Sun, and many, many more that are redder, but as far as we can tell, the true color of the Sun is almost perfectly white, but slightly more yellow/red than blue.
But — and this is important — not from the surface of the Earth! Our atmosphere turns direct sunlight slightly yellow! The effect is worse near sunrise or sunset, and is least at high noon on the summer solstice (when the Sun is nearest its zenith), but it’s always present, and it always yellows the Sun slightly.
So if you want to see the whitest sunlight possible, space is your only option! Either than, or trade your eyes for an owl’s (who’ll see a blue Sun) or for a bee’s (who’ll see an orange one)!
Until then, enjoy the yellow one you’ve got, and thank our atmosphere for it!