Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

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.

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.

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 scatter 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.

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.

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.

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.

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.