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Mostly Mozart


Blue Sky
Posted February 18, 2001

A few months ago, someone asked me whether I was going to run out of music to play on my show. I figured that wouldn't really be a problem, partly because I have lots of CDs and records that qualify as Mostly Mozart, but also because I figure I can play any given piece two or three times before anybody will actually notice.

Then, just the other day, I started to wonder whether I would ever run out of things to say, in the science segment of the show. That would be a much bigger problem because I couldn't just run out and buy some more things to say, like I could if I wanted more Mozart music. If you ever read a book called Mouse Tales, you know about the mouse who walked until his feet wore out, and then bought a new pair of feet. But I'm not that kind of mouse.

I'm not worried, though. I don't think I will run out of things to say about science any time soon. Especially since I've expanded my definition of science to include math, technology, and perhaps a little philosophy. Did you know that science is a branch of philosophy? It used to be called natural philosophy. See? Obscure facts about science.

After a brief musical interlude, which I admit may not seem so brief if you've been listening to 4-minute pop songs all day, I'll respond to Victoria's question from a few weeks ago, and I'll try to convince you that the sky is not, in fact, blue. It's actually red. Depending on what time of day you decide to measure it. So you can expect to hear words like: cones; stratosphere; ultraviolet; diffraction grating polarized wavelength; All this and more.

First, I'll shut up for a while so you can hear Symphony # 29, in what would be the patriotic key of A, if Mozart had been Canadian.

Mostly Mozart is sponsored by Comfort and Joy, a unique children's store. This symphony was performed in 1993 by the Orchestra of the 18th century, using instruments that are about as old as the music itself.


Why is the sky blue? It's not blue, remember? It's red. Or... orange. Sometimes it's white. It all depends on when you look at it. First of all, it's usually orange or red at sunrise and sunset, and it's blue during broad daylight. Why?

The answer, of course, has to do with optics.

You've all heard of the atmosphere. There's a 50 km thick layer of gases, mostly hydrogen, oxygen, and nitrogen, surrounding the earth. Before we see any light from the sun, it has to get through the atmosphere. Getting through the huge expanse of empty space between the sun and the earth is no problem for your average photon, but things are very different in that last 50 km stretch.

Before they enter the atmosphere, these photons collectively look like white light because some of them are blue, some are red, some are yellow, and so on. But the atmosphere has a different effect on photons of different colours. Blue photons are easily scattered, while red photons tend to maintain a straight path all the way to the surface of the Earth.

If you were in space, the sun would look extremely bright, and the space around it would look black. Photos taken from space tend to agree with me on this assertion. The only reason that the rest of the sky is any colour at all when viewed from earth, and not just black like it is in space, is that a lot of sunlight gets reflected in random directions, or scattered, by particles in the atmosphere. The light that looks like it's coming from the clear blue sky is, of course, actually coming from the sun. But it has to be scattered in order to get to us at all. So if the atmosphere is better at scattering blue light than it is at scattering red light, then that explains why the sky is blue, even though the sun isn't.

So, if that's why the sky is blue, then why is the sky red? Well, first of all, when the sun is at a low angle, those photons have a much longer distance to travel between when they enter the atmosphere and when they reach the surface. It's just like slicing carrots; if you slice straight across, you get a 1-inch circle, but if you slice at an angle, you can get a 2-inch by 1-inch oval. Your knife has to do more cutting, because it has to get through a bigger cross section.

As the photons make their way through the atmosphere, which seems to them so much thicker than it was during the day, more and more blue photons get scattered. Some of them reflect into space, perhaps one day to show up in a photograph of the earth's atmosphere. Some of them reflect toward the Earth, perhaps making the sky look blue somewhere else. But very few of the blue photons maintain a straight trajectory to reach us at sunset. Red photons, on the other hand, aren't as suscpetible to scattering, so most of them arrive intact.

So, in short, red photons tend to arrive from the direction of the sun, while blue photons tend to arrive by an indirect path, if they arrive at all, so they appear to be coming from some random point in the sky. So the sun looks yellow or orange, and the rest of the sky looks blue. If the atmosphere is thick enough from the point of view of the sun, like it is at sunset, then we don't get much blue at all.

Fine, then. The sky is blue. But where does it end? How high do you have to go in order to escape the blue sky and see some black outer space?

Earth's atmosphere consists of a stratosphere sitting on top of troposphere. The ozone layer, which is part of the stratosphere, is responsible for filtering out almost all of the ultraviolet light that comes from the sun. The troposphere is the part that we breathe, and also the part that contains weather. It absorbs lots of infrared light, both from the Sun and from radiant heat from the surface of the Earth.

Between the two of them, the stratosphere and the troposphere contain about 99% of the gases that surround the earth, all in a tiny 50 km skin. Which means that if you go 50 km straight up, the sky will still be blue, but it will be below you. If you look up, you won't be looking at the sky, you'll be looking at outer space, so it will be black. By the time you notice any of this, you might have a very bad sunburn, but you won't be worrying about it because you'll be feeling quite cold and having a lot of trouble breathing.

And on that note, I'd like to remind you that Mostly Mozart is sponsored by Comfort and Joy, a unique children's store. Next up, I'm pleased to introduce Mozart's concerto for 2 pianos, which started the whole Mozart Makes You Smart scam. If you heard my first, or third, or fifth show on November 7, you'll remember that there's a small chance that listening to this piece will make you smarter, for the next 10 or 15 minutes.

While you get smarter, it can't hurt to give me a call in the studio at 352-3706, and suggest a topic for a future show. I could make up the topics myself, but it might be more fun for you if you get to hear something you're actually interested in. The number again is 352-3706.

The piece we're about to hear is a sonata for two pianos, K.448, played by Murray Perahia and Radu Lupu in 1985. Well before 1993, which is when Mozart first started making people smart.