The sun usually appears yellow to our eyes when it’s high in a clear blue sky. Photo: Bob King
Think of the sun. What color is it in your mind’s eye? Most of us would say yellow without hesitation. That’s how appears in a clear blue sky. I looked at it this morning and thought pale gold was also a good fit. We’ve also seen red and orange suns around sunrise and sunset, but we know that’s not the intrinsic color of our star but soupy air and dust at work.
White light or sunlight is composed of every color in the rainbow from violet to green to yellow to red. When the sun is high in the sky, all these colors reach our eye with equal intensity and the sun appears pure white. But wait – the sun doesn’t look white. It’s yellow, right?
In truth, colors aren’t created equal when it comes to Earth’s atmosphere. Each is affected differently by everything ranging from air molecules to suspended dust to volcanic aerosols.
White sunlight composed of all the rainbow colors streams through
space. When it hits Earth’s atmosphere, the oxygen and nitrogen
molecules scatter the blue and violet part of sunlight across the sky to color it blue. Credit: www.pingry.org
Air molecules scatter blue and violet rays away from the hidden rainbow of colors in white light and send them bouncing around the sky. That’s why the sky is blue. When you look up to admire a blue sky, you’re seeing the blue part of sunlight set free. Come to think of it, we’re literally inhaling blue sky every time we take a breath. And to go one step further, we spend most of our time with our heads in the sky, since our feet are the only part of us touching the ground.
This loss of blue and violet to the sky causes the sun to look look yellow or “warmer” than it should. If we could peer out the cupola windows of the space station at a sun unfiltered by the atmosphere it would appear its natural color – glaring white.
Around noon, sunlight takes a short, direct path through the atmosphere and appears pale yellow. When near the horizon around sunset and sunrise, it passes through the lower, ”dirtier” part of our atmosphere the entire distance. All that extra dust, smoke, etc. effectively scatter much of the sun’s light, leaving only oranges and reds. Illustration: Bob King
The size and concentration of particles in the atmosphere like smoke, dust, pollen and pollution affect what colors we see from the sun. When the sun’s high overhead, its light takes the short path through mostly rarefied air and then through the bottom 10 miles of atmosphere where the air is thickest. No great loss of light. But at sunset and sunrise, the sun is near the horizon and has to shine horizontally through hundreds of miles of the lowest, thickest layer of Earth’s atmosphere.
A red sunrise over Lake Superior. Dust, pollen, smoke, salt and
other particulates, collectively called “aerosols”, scatter violet,
blue, green and even yellow from sunlight leaving only orange and red. Credit: Lyle Anderson
Air molecules and aerosols across that great distance scatter away not only the shorter wavelength violet and blue parts of sunlight but also the longer wavelength yellows, leaving the familiar rich oranges and reds of a beautiful sunset.
This is a weird analogy but imagine tossing a handful of rocks, some very tiny pebbles and others the size of golf balls, at a beaded curtain. The teeny-tiny rocks will be scattered back by the curtain but the big ones will sail right through. Blue light (tiny pebbles) consists of very short wavelengths easily scattered by air molecules; red light rays (big rocks) have longer wavelengths and move through the air unimpeded.
When particles are few, sunsets are a bright yellow, but when the air is laden with dust or salt (near the oceans) the sun looks like a big red ball of fire. What color will it be tonight?
A white sun in airless, black outer space seen from the International Space Station. Credit: NASA
The British Astronomical Association has Comet Panstaars at Magnitude 9.5 now. It will get 11 times closer to the Sun. If this holds we could be looking at a 15-16 magnitude brightening by Perihelion.
Edward,
Sounds like a good prospect, but again I’d like to caution, that unlike planets, comets often don’t follow projections to the letter.
I meant to say a 14-15 magnitude rise. It is performing better than expected. And it is more than 2 magnitudes brighter than originally predicted.
Edward,
That would put the comet at -5. Wow! We can only hope. Hale-Bopp, one of the brightest comets in years, only reached -1 magnitude. The brightest comet I’ve ever seen was C/2006 P1 McNaught in 2006 when it was visible through my scope in a blue sky not far from the sun. That one reached about -5. Here’s a list of some of the brightest recent comets: http://www.icq.eps.harvard.edu/brightest.html
Diggin’ the pic of the sun in space.
Likewise, diggin’ the perpendicular variance @ color between overhead and sidelighting.
Can you expand on how the “short path” scatters more blue than the “long path” does? I was taught that the angle of entry relative to the wavelengths that passed through was responsible for the color that was allowed to come through the atmosphere?
Can you compare/contrast this?
Thanks.
Hi Kent,
The sideways (sunrise / sunset) path scatters the most light because sunlight travels through hundreds of miles of the lowest, dustiest, most water-saturated part of the atmosphere. Light from an overhead sun passes through a few hundred miles of very rarefied air and then through only about 10 miles of the low, dense, dusty layer. That angle makes a big difference in how much dense air vs. thin air sunlight passes through.
The answer that the “true” color of the sun is white makes no sense given that its that its spectral class is G2.I say that any star as powerful as the sun and as close to earth would appear white!
Take
rl,
You’re certainly correct about that. Add in classes O,B and A too. The blog’s intent was simply to make a distinction between how the sun appears when viewed through our atmosphere (which scatters away blue light) and from airless space.
The sun is BLUE! That’s why the sky is blue!
If you look at the sun a short wild and look on a white paper the daze-spot will appear yellow. And as many of you discovered, the eye compensates for one color with it’s opposite on the colordiagram if you stare at something. (ex – photo negative)
Therefor, if you later close your eyes after have looked at the sun, the daze-spot will appear blue.
Thinker,
The sky is blue because air molecules scatter the shorter-wavelength blue light from the sun more than the other colors, not because the sun itself is blue. You’re right about the after-image of the sun appearing yellow, but it’s not an indicator of the sun’s actual color. Because air removes some of the blue light, the sun really appears slightly yellow.