19 January 2011
Is The Sun White or Yellow?
Posted by Dan Satterfield

The sun looks a bit yellow here, but that is because it's closer to the horizon and there was quite a bit of dust in the air over London. Dan's pic March 2006.
Ask a meteorologist “why is the sky blue?” and they will (at least they should) quickly give you the proper answer. The basics are this: shorter wavelengths of blue light are scattered by the air molecules much more than longer wavelengths of red light. The blue light gets scattered across the sky and makes the sky look blue. The technical term is Rayleigh scattering.
This is actually a question that any weather person gets rather frequently. Often, it’s from parents who have been stumped by their three year old! Kids really do ask the best questions, and probably because they aren’t afraid to ask something that sounds silly. I mean really good questions, like why is water wet and how come it’s colder on mountains.
Here is another one that’s related to the blue sky question.
Why does the sun appear yellow?

The sun looks white in this picture. It was taken at around 3,000 meters elevation in the cleanest air on Earth. It's the sun over the South Pole. Dan's pic.
Ask any weather nerd and they will likely tell you that when the blue light is scattered from the sun’s white light, what’s left is yellow. I’ve given that same answer countless times.
Apparently it’s wrong.
Astronomer Phil Plait writes the Bad Astronomy blog, and has a great book about common astronomical myths. It’s also called BAD ASTRONOMY, and well worth a read.
I was skimming through the chapter on the blue sky when I spotted something very surprising. It seems that not enough blue light is scattered away from the incoming white sunlight to turn the sun yellow. Furthermore, no one really seems to know why it’s yellow!
I’ve been thinking about this, and I think the first thing to ask here is this: Does the sun really look yellow? Yes, it does at sunset and sunrise, when dust and a thick atmosphere scatter all but the red and yellow light away, but what about at high noon? It’s hard to look at it then, so do we just assume it’s yellow?
I looked through some snaps I took in Antarctica and guess what, the sun looks pretty white to me. Maybe it’s an illusion caused by our eyes when looking at the sun surrounded by a blue sky. Phil Plait says this is not likely the case, although that would be most peoples first guess. It was what I first thought of as well.
Here’s my take. The sun is white in the sky except when near the horizon. It can be yellow when there is enough dust/pollution to increase the Rayleigh scattering but most of the time this is not the case.
The link at the beginning of this post on Rayleigh scattering goes to the excellent article on the subject on Wikipedia, except that it too says the sun is yellow because the blue light get scattered away!
Feel free to disagree! If you want more details on it, pick up Bad Astronomy.
PS: The sun can be dangerous to look at when high in the sky, you can permanently damage your vision! So look a little away from the sun if you are curious about it and be careful!
Completely O/T, but a while back you posted about e-books and e-readers. I was skeptical of them, but you gave them high praise. I received one for Christmas. They are fantastic–I have several climate and paleoclimate books, and other science books too, on my Kindle now. It was your post that got me seriously looking into them so thanks!
I have 8 books going on my iPad right now. Glad you are finding them interesting. They are indeed amazingly good. You’ll find that real science is absolutely fascinating. I write this blog to share it with people so thanks! You made my day and that post worthwhile.
Dan
I’d always blithely assumed that the yellow colour was due to sodium emission lines? Is this completely off track?
When he was at UAH, Gordon Emslie used to give a fascinating presentation on how you can tell the sun is actually … green. Green light is a combination of the blue light that is scattered by air molecules and the yellow light that comes streaming through. The tale goes on from there, but the bottom line is that if you get above the atmosphere and use scientific instruments, you will see that the sun is (somewhat) green. Unexpected, but apparently true.
Very cool! Would love to get a guest post from him on the subject!