The Sun Is Red Because The Sky Is Blue

SteelyKid missed the bus this morning-- she was dressed and ready, but I was talking to Kate, and if there isn't a person at the end of the driveway when the bus comes around the corner, they won't stop. So I drove her over to school myself (which is faster, anyway). The GE research lab complex is behind her school, so there's a nice view from the parking lot to the eastern horizon, where the sun was just poking over a big band of clouds.

"Hey, look at that cool sunset!" she said as we were walking from the car to the building.

"That's not a sunset, honey, it's a sunrise. It's morning."

"Oh, right. I don't usually see the sunrise, because I'm usually on the bus on the opposite side."

"Mm-hmm."

"It looks pretty orange. The sun's usually yellow."

"Right, that's because it's sunrise. When the sun's near the horizon, it looks more red."

"Yeah, at sunset it's red, too."

"Do you know why that is?"

"Why?"

"Because the sky is blue."

"What?"

"You see the blue sky above us? That looks blue because light from the sun that's headed that way" (big westward hand gestures) "hits stuff in the air and gets bounced down toward us. And that works better for blue light than for red light."

"So the sky looks blue."

"Right. And when the sun is low on the horizon, its light passes through a lot of air to get to us, and hits a lot of stuff. So all the blue light that ought to be there gets bounced down, to make blue skies for people who live over that way," (big eastward hand gestures). "That leaves red light, so the sun looks red to us because the sky looks blue to them."

"Oh. That's pretty cool."

"Isn't it?"

"How do you know that?"

"Well, it's physics. The kind of science I do looks at how light interacts with stuff, and that lets us understand blue sky and red sun. It's Rayleigh scattering."

"Oh. I was wondering, because I didn't think you, like, went up into the sky to look."

"No, but that would be kind of cool... Anyway, here we are at the door. Have a good day, honey."

"Thanks, Daddy."

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By Brian Postow (not verified) on 15 Jan 2015 #permalink

If you did go "up into the sky to look", the sky would look more and more black, like at night, and the sun would look more and more white. You knew that, but I wondered what color exactly? Apparently, "peach pinkish", according to http://casa.colorado.edu/~ajsh/colour/Tspectrum.html (which has its background color set to an appropriate RGB value, but my monitor is far from calibrated).

By Peter Morgan (not verified) on 15 Jan 2015 #permalink

I've always wondered: the sun looks yellow in the sky because of the Raleigh scattering you mention. Why does the Moon and Venus look white? It seems that their photons should also be scattered.

By Randy Potter (not verified) on 17 Jan 2015 #permalink

Randy, the Moon and Venus aren't generating light across a wide spectrum like the Sun is. They're reflecting some of the Sun's light, and absorbing the rest. So I'm going to guess that the light incident on our atmosphere is different in spectrum from sunlight. Maybe there's just not a lot of yellow light there to begin with. But there is a noticeable reddening of the moon when it's low in a dark sky, which would be a result of Rayleigh scattering.

There are a whole bunch of things going to to produce the color we see for the Sun and the sky-- the most intense single wavelength of light from the sun is greenish, but there's a good deal of light through the whole visible spectrum. And there are issues about the color response of our eyes, absorption by stuff in the atmosphere, etc.

As Tom says, the Moon and Venus further complicate this by reflecting light, probably not entirely uniformly across the visible spectrum. But the biggest issue is that moonlight just isn't very bright relative to sunlight, making it harder to perceive color at all, and Venus is both less bright and much smaller, so we add trouble resolving it to the list of issues.