Sure, we're all familiar with sunsets, and how they appear to turn the entire sky close to the horizon red at night.
But it turns out it isn't just the Sun, and it isn't just the sky. If you look at the Moon at either Moonrise or Moonset, guess what color it appears to be? (Even in urban settings!)
Too difficult to tell? Let's find you a better picture done with time-lapse photography.
What's going on to cause this? Why do things which aren't normally red appear red when you look at them on the horizon, from the Sun to the Moon to the sky itself?
This is all the atmosphere's fault. After all, things look exactly the same on the Moon's horizon as they do directly overhead:
So why so different here? What exactly does our atmosphere do? Well, the simple answer is that it scatters light. Not all light equally, though. The atmosphere is better at scattering blue light away, which means that blue light gets dispersed all throughout the sky pretty easily. But red light is more likely to pass directly through, which is why things appear redder on the horizon: more of the bluer light gets scattered away, while the red light comes (mostly) through to you. See this dramatic photo of the sky just after sunset from an altitude of 500 meters:
But there's another huge effect, as you've probably guessed. After all, this doesn't work when the Sun is directly overhead, or even close to it, but it works very well on the horizon. The Earth is pretty big, having a diameter just under 13,000 kilometers. But the Earth's atmosphere is very thin. In fact, the troposphere (where air is breathable), which contains nearly 80% of the Earth's air, only extends 17 km up on average. Like I said, thin, at least compared to the Earth.
Going through 17 km of air doesn't have such a dramatic effect on light; about 84% of it still gets through. But when you look at the horizon, the light has a lot farther to go through that atmosphere. Think about it!
When the Moon is overhead, it goes through 17 km of tropospheric air. But when the Moon is on the horizon, it goes through 465 km of troposphere! That's not only enough to scatter away most of the blue light, it scatters away about 99% of the total light that comes from the Moon!
So, pretty much, only the red light gets through. And this very same phenomenon not only explains why the sky looks red at sunset, it also explain why the Moon looks red during a lunar eclipse! Because the only light that gets through is the little bit of red light that makes it all the way through the Earth's atmosphere and onto the Moon:
So if you've got a message to send hundreds of kilometers through the air, send it in red, please. The atmosphere is just too strong for blue (or even green) signals to get through very far. Thanks a lot, Nitrogen gas.
Comments
Excellent as usual Ethan! Now I can just tell people to read your blog rather than nag me for details. :)
For a future article, can you explain the illusion of the blue oceans as seen from space? If I hear one more "aren't the oceans a lovely blue?" I might tear out what little remains of my hair.
Posted by: MadScientist | September 8, 2009 6:16 PM
Would an observer standing on the Moon during a lunar eclipse see the whole surrounding landscape as tinted red? As if a red light bulb were the light source? That would make an incredible sight to see.
Posted by: Navin | September 8, 2009 6:27 PM
Actually, its thank you very much, nitrogen gas for being transparent to allow me to see at all (oxygen gas too).
Posted by: NewEnglandBob
| September 8, 2009 7:14 PM
> blue oceans
Isn't that the same lovely blue color of scattering in our sky, except we're seeing through it from above? You can see it along the horizon.
I had a friend who used to take long time exposure photographs by moonlight; moonlight is blue -- the very same sky blue. Daytime shadows are also blue -- sky blue, with no added yellow from direct sun punching through
Posted by: Hank Roberts | September 8, 2009 7:51 PM
Just Raleigh? Not ozone absorption at all?
Are you sure?
Posted by: Robert | September 8, 2009 8:14 PM
Robert, check it out: http://www.globalwarmingart.com/wiki/File:Atmospheric_Absorption_Bands_png
Posted by: Ethan Siegel | September 8, 2009 8:39 PM
I don't accept that as a valid reference.
I have text books and journal articles that say it's Raleigh plus ozone. They work together. Raleigh removes the blue and ozone removes the middle of the visible, leaving red as the result.
Ozone by itself does not make the sun red. But it deepens the red in conjunction with scattering.
Posted by: Robert | September 8, 2009 9:37 PM
The third pic also explains why the Moon seems bigger when it is close to the horizon. It is closer to us there. You can see it by the shadows the foreground Moons cast on the background Moons.
Now working on a theory that explains the yellow phase that leads to the B&W phase...
Posted by: Lassi Hippeläinen | September 9, 2009 4:35 AM
@ Lassi H...
The moon is not closer to us when it is near the horizon.. Look at that pic again.. Observer B is on the Surface of the earth and not at the center of the earth.. From the point B to the Moon the length is actually shorter than from A to the Moon...
The moon I guess appears larger because we tend to compare it's size with the other stuff that are present in our view near the horizon, the buildings, trees, mountains etc. as a result of which the moon appears bigger. It is an optical illusion. Just look through a gap formed by your fingers blocking away everything else on the horizon and you'll be amazed to see the Moon no bigger than it is directly overhead.
Posted by: Sriram | September 9, 2009 6:23 AM
@Hank Roberts: That's it. For folks who don't believe that the oceans aren't blue, they're welcome to go to the beach at night and shine as many lights as they please to see what color the ocean is.
@Robert: So what books and journal articles would those be? Rayleigh scattering is simply elastic scattering by particles roughly the size, or smaller, of the wavelength being scattered. Ozone and oxygen (and all the trace gases in the atmosphere) also act as Rayleigh scatterers.
Ozone also has a few visible absorption lines; however those lines are very narrow and will not effectively block the "middle of the visible" as you put it. The red tints are enhanced by Mie scattering off aerosol particles; that has been well known for a very long time. More dust or ice (or other aerosols) in the air = more red. The relation of Rayleigh and Mie scattering with wavelength are different; Rayleigh scattering dominates at shorter wavelengths (blue, violet, ultraviolet) but at some point at longer wavelengths, and depending on aerosols, Mie scattering will dominate. If you want an easy to understand discussion of atmospheric effects, have a look at Marcel Minnaert's "Light and color in the open air".
Posted by: MadScientist | September 9, 2009 8:10 AM
Lassi: never make jokes on the internet. You will only become depressed.
Posted by: Vagueofgodalming | September 9, 2009 9:20 AM
and i thought delighted sailors make the sky red.
Posted by: rob | September 9, 2009 10:10 AM
OK, I admit, I was trying to be sarcastic.
Anyway, while photo #2 is time lapse (i.e. several exposures on the same frame), #3 is a collage of several separate photos. It looks like a Photoshop Disaster (overlapping moons, anyone?).
Posted by: Lassi Hippeläinen | September 9, 2009 10:42 AM
@MadScientist
Reference 1: Palle, E. et. al, Earth's transmission spectrum from lunar eclipse observations, Nature, 2009, Vol 459, p. 814-816.
Quote: "The rising nature of the transmission spectrum continuum towards longer wavelengths ... is caused by the Raleigh scattering of air, which, in addition to the ozone Chappuis band absorption between 0.375 and 0.650 [micrometer], is rather efficient in scavenging short wavelength radiation through a long atmospheric path."
Reference 2: "Fundamentals of atmospheric radiation" by Craig Bohren and Eugene Edmund Clothiaux
Quote: "Absorption by ozone takes a big bite out of the middle of the radiance spectrum." [Note: in their figure they show a broad band for ozone over the entire visible spectrum with maximum absorption between 550 and 650 nm and there is another quote where they mention absorption in the middle of the visible.]
Posted by: Robert | September 9, 2009 12:58 PM
Ethan's explanation is great, but I thought I'd add the following (obvious?) observation:
Why is the moon/sun red at the horizon? The same reason the sky is blue during the day.
It's always nice to link different observations that originate from the same mechanism.
Posted by: Anonymous Coward | September 9, 2009 1:52 PM
#15: Not exactly. In the early moring and late afternoon the sky is blue because of ozone. Ozone creates the blue color that gets scattered back to our eyes. Without ozone, the sky color would change at different times of day.
I understand why that is hard to believe. But it's in the Bohren and Clothiaux text book.
Posted by: Robert | September 9, 2009 2:52 PM
@Robert: Thanks for the references; I should have no trouble looking up the one in Nature. The Chappuis bands are so weak though; I'll have to do a calculation for the case with the sun just at the horizon to convince myself that the red color is significantly affected by the Chappuis bands. Either that or someone can send me a spectrum showing that the Chappuis bands are obvious and responsible for the redness. Unfortunately I don't have a portable spectrometer that I can point at the horizon.
Posted by: MadScientist | September 10, 2009 5:21 AM
OMG!
I just realized that I have been misspelling "Rayleigh" every single time, included the quote where I was copying directly from the Nature article and I unthinkingly changed the correct spelling that was in that article.
Sorry!
Posted by: Robert | September 10, 2009 1:45 PM
#16 Thanks! I just learned something! You are, of course, correct: my statement about the "color of the sky during the day" is inaccurate near sunset. But, as you know from Bohren and Clothiaux, "...over much of the day ozone plays no essential role in the blue of the sky."
Would it be less misleading to say "The preferential scattering of blue light vs. red light contributes to both the blue color of the sky during the day and the reddening of the sun at sunset."?
This comment would be of little interest to an atmospheric physicist interested in a complete model (which would need ozone absorption to quantitatively explain how very red the sun gets near sunset) but for an audience that just barely understands how color works it might add an appreciation of what's going on.
Posted by: Anonymous Coward | September 10, 2009 2:55 PM
#19: I found an article that says that a scattering model is sufficent for an undergraduate physics student to explain the color of the sun.
So you can, if you wish, ignore ozone. You will not be able to predict the exact color of the sun with your scattering model, you might say it's orange or orange-red when it's really red, but as long as you're happy with that, so am I. :D
Posted by: Robert | September 10, 2009 5:00 PM
Oh, I have always wondered what caused the moons to be bloody red sometimes. I didn't connect it to the horizon, didn't think of it. ^_^ Sometimes, it has a cheesy color to it, which I imagine has the same cause? Whatever it is, I made myself some great pictures, albeit of low quality.
Posted by: IBY | September 11, 2009 8:34 PM
The moon is white and we merely see the red moon in Tokyo, Japan.
If the atmosphere is polluted, is there any effect to the color of the moon?
Posted by: Akiko & Mizue | September 18, 2009 2:06 PM
thanks professor, it is valuable information and the language of the article is very clear for us to read as beginner students.
we have two questions:
first, you mentioned in the last paragraph that "So if you've got a message to send hundreds of kilometers through the air, send it in red, please." As you said, We can infer that the red light travel more than other colors.
So, is it the reason why red flashing light is used on the high towers? so people can see it from long distance.
If yes, why the color of the light in the light house is not red?
Posted by: Youngsu & Bandar | September 18, 2009 2:17 PM
Thanks for your explanation.
I didn't realize this phenomenon before. I just thought the red sky is caused by the brilliant of the setting sun.
And I'm thinking if both blue sky and blue ocean are related to wavelength of blue light and red light as well as the red sky on the horizon? I think someone who doesn't know the knowledge about the reflection of light is not able to understand your explanation.
Posted by: Xialu Zhao | September 18, 2009 2:18 PM
Thanks, that blog make it easier for us to understand and visualize when and why we see the red light. But we wonder if the same process happens when the sun eclipse?
As explained in the blog, the Earth is big but the Earth’s atmosphere is very thin. So, is there a relation between the red light density and the blue light? Is that why “not all light equally”?
We don’t have much knowledge about this topic, but maybe that could be another reason for the scattering the blue light in the horizon
Posted by: Eman & Sabreen | September 18, 2009 2:21 PM
Such an Interesting information to share!! Thanks.
You were talking about blue and red light that scatter differently. In biology we know the wavelengths of both colors are different. So does the wavelength influence the phenomena of red moon?
Do you know how long is the time span for moon to turn red?And does the time span have any dependent factors?
Have a nice weekend.. >_
Posted by: Fumiyo&Falorna | September 18, 2009 2:21 PM
Does anyone have an explanation as to why there are a cone of white light that appeared around a cloud for a number of seconds last night then slowly disappeared in NYC? It was a clear night and 8 of us witnessed this light that we could only rationalize as a UFO. I later read that there was a rocket launched at 7:55PM from VA but I can't find another source to confirm this.
Posted by: samoa | September 20, 2009 9:51 AM
what is it when u see a red glow in the sky at 8:30 at night it move from the north to northwest
Posted by: marie | October 24, 2011 9:55 PM
Wanting to know the same thing as marie??
Posted by: gina | October 24, 2011 10:33 PM
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Posted by: james | February 5, 2012 7:36 AM