Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)
Part II. From the Earth to the Moon
The Earth is a big place. There are 511,209,977 square kilometers of it, give or take a kilometer or two, which might seem like room enough for everything. But even that much surface area isn't enough to contain all the bad astronomy out there. Not by a long shot. I wish it were at least true that it could be restricted to nearEarth space, but even then we run out of room pretty quickly. Still, there's a lot to be seen in our extended neighborhood. You need not even wait for nightfall. Most people might associate astronomy with nighttime, but we can scrounge up some during the day, too. As I write this the sky is a deep, rich blue, and the warm sunshine is blanketing my backyard. Just a few steps outside my house I can feel the warm embrace of an environment fraught with myths, misconceptions, judgment errors.
That cerulean-blue sky is a good place to start. True to the cliche, one day my five-year-old daughter asked me why the sky was blue, and I had to figure out how to answer her. I explained to her about molecules and sunlight, and the cosmic pachinko game played as the light from the sun makes its way to our eyes. When I was done, she thought about it for a second, and said, "All that stuff you just said doesn't make any sense."
I hope I've done better writing it all down in the next chapter.
But why stop with our air? We can move out of the atmosphere and peer back down on the Earth, seeing our frigid poles and tropical equator. Why are those two locales different, and why does everything in between change from season to season? That's a fair question, too, and the cause is rooted in astronomy.
Moving a bit farther out, we encounter the Moon, our closest neighbor in the universe. I cannot think of any other object so loaded down with grossly inaccurate theories. The Moon only shows one face to us, but it does spin; it goes through phases that look like minature eclipses, but they are nothing of the sort; it looks unchanging and unchangeable, but that, too, is an illlusion. In the past, in the future, and even right now as you read these words, the Moon is being sculpted by unseen forces, just as it is profoundly changing the Earth. These same forces are at work throughout the universe, shaking mighty volcanoes, tearing apart stars, devouring entire galaxies.
If we can put a man on the Moon, you'd think we could stamp out most of the bad astronomy floating in the Earth's immediate vicinity.
Chapter 4. Blue Skies Smiling at Me: Why the Sky Is Blue
n the course of every parent's life there comes the inevitable question from their child: "Why is the sky blue?" As we grow to adulthood we sometimes learn not to ask such questions, or we just forget how. The vast majority of adults in the world have seen a clear-blue sky tens of thousands of times, yet only a few know just why it's blue. If you don't know, don't fret: the question baffled scientists for hundreds of years. Nowadays we are pretty confident that we know the real reasons, but I've never heard of them being taught in schools. Even worse, a lot of web sites I've seen give an incorrect answer to the question. College textbooks on optics and atmospheric physics cover the topic correctly, but who wants those lying around the house?
Well, I do, but then I'm a huge geek. I'm operating on the principle that you are a normal human. And, lucky for you, the reason behind the blue sky isn't all that complicated, and it can be easily explained, even to a five-year-old. Let's start with some of the incorrect reasons given for the sky's cerulean hue.
Probably the most common idea is that the sky is blue because it reflects the blue color of the ocean. However, a moment's reflection (ha-ha) reveals that this can't be right: if it were true, the sky would look bluer when you are sailing on the ocean than when you are on land. But that's not the way it happens. It still looks just as blue from say, Kansas-a healthy hike from the nearest significant body of water-as it would from an ocean liner steaming its way from the United States to England.
Another commonly given incorrect answer is that blue light from the Sun scatters off dust in the air. As we'll see, this answer is close, and certainly better than the one about reflections off water, but dust is not the cause.
The correct answer, if you want details, is a little more involved. In the end we can simplify it for our hypothetical five-yearold, but first let's look at the whole problem.
When you examine most problems in astronomy, or for that matter in any other field of science, you'll commonly find that to get to the solution you need two separate lines of attack. The color of the sky is no exception. To understand the blueness we actually have to understand three things: just what sunlight is, how it travels through our atmosphere, and how our eyes work.
You may be surprised to learn that when it leaves the Sun's surface, sunlight is white. By this scientists mean it is actually a balanced combination of many colors. The individual colors like red, green, and blue are all produced by the complex physics near the sun's surface. The roiling, writhing gas making up the Sun's outermost layers produces light of all different colors. But when this light gets mixed together, it produces what looks to our eyes like white light. You can prove this for yourself: Hold a glass prism up to a beam of sunlight. When the sunbeam passes through the prism, the light gets "broken up" into its constituent colors. This pattern of colors is called a spectrum.
This same thing happens after a rainstorm. The raindrops suspended in the air act like little prisms, breaking up the white sunlight into a spectrum. That's how we get rainbows. The order of the colors in a rainbow is the same every time: red on the outside, then orange, yellow, green, blue, indigo, and finally violet, which makes up the innermost curve of the arc. This pattern may be tough to remember, so it's usually taught to students using the acronym ROY G BIV, like that's a common name or something. Still, that's how I remember it, so it must work.
Those colors are coming from the Sun all at the same time, but a funny thing happens on the way to the ground. Molecules of nitrogen and oxygen (N2 and Oz) in the air can intercept that light. Almost like little billiard balls, photons-the fancy name for particles of light-bounce off these molecules and head off in a different direction every time they hit one. In other words, nitrogen and oxygen molecules scatter the incoming sunlight like bumpers in a pinball machine.
In the mid-1800s the brilliant British physicist Lord Rayleigh found out a curious thing: this scattering of light by molecules depends on the color of the light. In other words, a red photon is a lot less likely to scatter than a blue photon. If you track a red photon and a blue photon from the Sun as they pass through the air, the blue photon will bounce off its original course pretty quickly, while a red one can go merrily on its way all the way down to the ground. Since Lord Rayleigh discovered and quantified this effect, we call it Rayleigh scattering.
So, what does this have to do with the sky being blue? Let's pretend you are a nitrogen molecule floating off in the atmosphere somewhere. Nearby is another molecule just like you. Now let's say that a red photon from the Sun comes at you. As Lord Rayleigh found, you don't affect the red photon much. It pretty much ignores you and your friend and keeps heading straight down to the ground. In the case of this red light, the Sun is like a flashlight, a shining source of red light in one small part of the sky. All the red photons the Sun emits come straight from it to some observer on the ground.
Now let's imagine a blue photon coming in from the Sun. It smacks into your friend, rebounds off him, and obligingly happens to head toward you. From your point of view, that photon comes from the direction of that molecule and not the Sun. Your molecule friend saw it come from the direction of the Sun, but you didn't because it changed course after it hit him. Of course, after it hits you that photon can rebound off you and go off in another direction. A third nitrogen molecule would see that photon as coming from you, not the Sun or the first molecule.
Now you're a person again, standing on the ground. When a blue photon from the Sun gets scattered around, at some point it will hit some final air molecule near you, go through a final scattering, and head into your eye. To you that photon appears to come from that last molecule and not from the direction of the Sun. These molecules are all over the sky, while the Sun is in one little part of the sky. Since blue photons can come from any and all of these molecules, the effect is that it looks like blue photons are coming from every direction in the sky and not just the Sun.
That's why the sky looks blue. Those blue photons are converging down on you from all directions so that it looks to you like the sky itself is giving off that blue light. The yellow, green, orange, and red photons from the Sun get scattered much less than do blue ones, and so they come straight at you from the Sun without having suffered all those scatterings.
At this point, you might reasonably ask why the sky isn't violet. After all, violet light is bent even more, and actually does scat ter more, than blue light. There are two reasons why the sky is blue and not violet. One is that the Sun doesn't give off nearly as much violet light as it does blue, so there's a natural drop-off at that color, making the sky more blue than violet. The other reason is that your eye is more sensitive to blue light than it is to violet. So, not only is there less violet light coming from the Sun but you're also less prone to notice it.
Red photons travel through the Earth's atmosphere relatively unimpeded, because of their relatively long wavelengths. Blue photons, however, with their considerably shorter wavelengths, bump and careen around as they are scattered by molecules in the air. By the time they reach your eye, they appear to be coming from everywhere in the sky, making it look blue.
You can actually test this scattering idea for yourself in the safety of your own home. Get a glass of water and put a few drops of milk in it. Mix in the milk, then shine a bright white flashlight through the mixture. If you stand on the side of the glass opposite the flashlight, you'll see that the beam looks a bit redder. Go to the side and you will see the milk is bluer. Some of the blue photons from the flashlight are scattered away from the direction of the beam and go out through the sides of the glass, making the light look bluer. The light that passes all the way through is depleted in blue photons, so it looks redder.
This also explains the very common effect of red sunsets. One of the lesser known aspects of living on a big curved ball like the Earth is that as the Sun sets, the light travels through thicker and thicker air. The atmosphere follows the curve of the Earth's surface, so the light from an object that is straight overhead travels through far less air than the light from something near the horizon.
When the Sun is on the horizon, the sunlight travels through a lot more air than when it is up high during the day. That means there are more molecules, more scatterers, along its path, increasing the amount of scattering you'll see. Although blue light gets scattered a lot more than, say, yellow light, the yellow photons do scatter a little. When the Sun is on the horizon, the number of scatterers increases enough so that even green and yellow light can be pretty well bounced away into the rest of the sky by the time the sunlight reaches your eye. Since now the direct sunlight is robbed of blue, green, and yellow, only the red photons (which have longer wavelengths) make it through. That's why the Sun can be those magnificent orange or red colors when it sets, and also why the sky itself changes color near the horizon at the same time. It can look like that when it rises, too, but I think more people are awake at sunset than sunrise, so we see it more often in the evening. The Moon glows from reflected sunlight so it can change color, too, when it's on the horizon. Under unusually good conditions it can take on a startlingly eerie blood-red appearance.
This effect is amplified when there's more stuff in the air. Sometimes, when there are big volcanic eruptions, the sunsets are spectacular for quite some time afterwards. There's not much good to be said of explosive volcanic events, but they do put on quite an evening sky show for years.
There's another aspect of the curved atmosphere you've almost certainly seen as well. Have you ever noticed the Sun looking squashed when it sits on the horizon? The atmosphere, like a drop of water, can bend light. The amount that the light gets bent depends on the thickness of the air through which it travels. The more air, the more it's bent. When the Sun is on the horizon, the light from the bottom part of the Sun is traveling through more air than the top part. That bends the light more from the bottom part of the Sun. The air bends the light up, toward the top half, making the Sun look squashed. It doesn't get compressed left-to-right because the light from the left half of the Sun is moving through the same amount of air as the right half. As it sets the Sun looks normal horizontally, but it becomes more vertically challenged. The squashed, glowing, magenta Sun on a flat horizon is a sight not soon forgotten.
When you look straight up, you are looking through less air than when you look toward the horizon. Even green and yellow photons scatter away through the longer path they travel from the horizon, making the Sun look red or orange when it sets or rises.
And now we have the three reasons the sky appears blue. First, the Sun sends out light of all colors. Second, the air scatters the blue and violet light from the Sun the most. And third, the Sun emits more blue than violet light, and our eyes are more sensitive to the blue light, anyway.
Now that we've established the color of the sky, we can tackle a related question that seems to cause a lot of anguish, and that is the color of the Sun.
If asked, I would say that the Sun is yellow. I think most people would, too. Yet we just went through a lot to show that sunlight is actually white. If the Sun is white, why do we think it looks yellow?
The key to the sentence above is the word "looks." Here's a sanity check: if the Sun were really yellow, clouds would look yellow, too. They reflect all the colors that hit them equally, so if they look white the Sun must be white. Don't believe me? Try this simple test: go outside and hold up a piece of white paper. What color is it? Okay, duh, it looks white. It looks white for the same reason clouds do. It reflects sunlight, which is white.
This brings us back to the original question: why does the Sun look yellow?
I have to cop out here. It's not really well-known why. Some people think the blue sky is to blame. If blue light is being scattered out of the direct sunlight hitting our eyes, the resulting color should look yellowish. While it's true that some blue light is scattered away, not enough of it is scattered to make the Sun very yellow. Even though a lot of blue photons are scattered away from the Sun to make the sky look blue, it's only a fraction of the total blue photons from the Sun. Most of them come straight to your eye, unimpeded by air molecules. So the relatively small number of photons making the sky blue doesn't really affect the intrinsic color of the Sun enough to notice.
Another common idea is that the Sun looks yellow because we are comparing it to the blue sky. Studies have shown that we perceive color not just because of the intrinsic properties of the light but also by comparing that color to some other color we see at the same time. In other words, a yellow light may look even yellower if seen against a background of blue. However, if this is why we see the Sun as yellow, clouds would look yellow, too, so this can't be right either.
There is another possibility. When the Sun is up high, you can never look directly at it. It's too bright. Your eyes automatically flinch and water up, making it hard to see straight. You can only see the Sun from the corner of your eye. Under those conditions it's not surprising that the colors may get a little distorted.
As was mentioned before, at sunrise and sunset the Sun can look remarkably red, orange, or yellow, depending on the amount of junk in the air. Also, the light is heavily filtered by the air, making the Sun look dim enough to be bearable to look at. So the only time of day we can clearly see the Sun is when it's low in the sky, which, not so coincidentally, is also when it looks yellowish or red. This may also play a part in the perceived color of the Sun. Since it looks yellowish at the only time we can really see it, we remember it that way. This is an interesting claim, although I have my doubts. I remember it most when the Sun is a glowing magenta or red ember on the horizon, and not yellow, so why don't I think the Sun is red?
I have heard some people claim the Sun does look white to them, but I wonder if they know that sunlight is supposed to be white, and have fooled themselves into thinking it is white to them. It still looks yellow to me, and I know better.
Clearly, there's more to the Sun than meets the eye.
So, after all this, I'll ask one more trick question: of all the colors of the rainbow, which color does the Sun produce the most? We know it produces less violet than blue; literally, fewer violet photons come from the Sun than blue. But which color is strongest?
The answer is: green. Surprise! So why doesn't the Sun look green? Because it isn't producing only green but a whole spectrum of colors. It just produces more green than any other color. When they are all combined, our eye still perceives the light as white.
Or yellow. Take your pick.
Okay, I lied a minute ago; I still have one more question. If the sky isn't blue because it reflects the color of the oceans, why are the oceans blue? Do they reflect the sky's color? No. Of course, they do reflect it a little; they look more steely on overcast days and bluer on sunny days. But the real reason is a bit subtler. It turns out that water can absorb red light very efficiently. When you shine a white light through deep water, all the red light gets sucked out by the water, letting only the bluer light through. When sunlight goes into water, some of it goes deep into the water and some of it reflects back to our eyes. That reflected light has the red absorbed out of it, making it look blue. So the sky is blue because it scatters blue light from the Sun, and the oceans look blue because that's the only light they let pass through.
At the start of this section, I promised you'd understand all this well enough to explain it to a five-year-old. If a little kid ever asks you just why the sky is blue, you look him or her right in the eye and say, "It's because of quantum effects involving Rayleigh scattering combined with a lack of violet photon receptors in our retinae."
Okay, that might not work. In reality, explain to them that the light coming from the Sun is like stuff falling from a tree. Lighter things like leaves get blown all around and fall everywhere, while heavier things like nuts fall straight down without getting scattered around. Blue light is like the leaves and gets spread out all over the sky. Red light is like the heavier material, falling straight down from the Sun to our eyes.
Even if they still don't get it, that's okay. Tell them that once upon a time, not too long ago, nobody knew why the sky was blue. Some folks were brave enough to admit they didn't understand and went on to figure it out for themselves.
Never stop asking why! Great discoveries about the simplest things are often made that way.