## Why doesn’t the sky fall on us all?

The blue dome of skies,
or an exercise in explaining things slowly

(Huge giant 2000 word monster post warning!)

Sky is up there, blue. We’re down here, down on the ground. If you were to go north, and keep going, you would eventually come back to the place where you started, but from the south; and if you were to start west, you would walk (and sail, or fly) days and days, but finally come back to where you left, though from the east. This tells us the world we live on, this Earth, is not infinite.

If you were to stand on a beach, and watch a ship coming in from the deep oceans, you would not see it first as a speck, then larger, but always visible in the same way. No, you would, if you had good enough eyesight, first see the ship’s masts rising above the horizon, then the decks, then finally the whole ship. This tells us that the shape of oceans (and thus of the Earth) is not flat, and neither does it curve upwards — for then you would see alien lands arching overhead — no, if you walk, very slowly the ground (or sea) curves downwards, curving until it has gone a full circle, and comes to rest where you started. This explains how our Earth can be finite, and how starting northwards you eventually return to where you started, though from a southernly direction: we live on a sphere.

But how can this be? How can the ground curve down without it seeming sloped? And why don’t people (and oceans) slide down, fall down this slope?

But then: what is down? Everywhere you go, “down” is right under your feet. “Down” is just another word for “the direction where a rock would drop should I let go of it”. That direction doesn’t need to be invariant, and indeed it isn’t. It seems all things drop towards a point, not in some fixed direction; and the point is that which is in the center of this sphere called Earth.

The science of downs and dropping things is called gravity, and another of its effects is this: seas are high when the moon is high in the sky. If you look closely at the moon (say with a telescope), you will see it is a pockmarked, uneven landscape of craters and valleys: indeed, the moon is just another piece of rock, just like the Earth, but smaller. And — though the details are complicated, both those of tides and those of Moon’s movements — the essence can be grasped here: the direction “down” seems to be determined by the direction where there is most mass, most stuff, which is, in this case, most rock. The Earth is a giant sphere of stone, and thus our feet are drawn towards the center of that sphere, and to us that is down. The moon is a smaller sphere of stone, too: maybe one could walk on that rocky surface. And though the Moon is far away (it must be either very small or very far since we can see all of it), its center, its Moon-down, seems to pull things to it just a little, though not as much as our Earth-center, our Earth-down, and thus the tides: the moon’s pull is just strong enough to shift the waters up towards it a bit, and thus a high tide is formed.

Now, this Earth is a sphere one cannot walk out of, but what is the sky?

First, what is air? You can take a bottle and seal it — and it seems empty. However, you can make it emptier still, suck the air out of it: and then it truly is empty, because air is a thing. For this reason a man can drown: it is not the presence of water which harms him, but the absence of air. (Actually, the absence of oxygen, which is a part of air, which is a mixture of many things, but that is not essential here.) If you walk up a mountain, you will get tired, but that is not all: if you go up far enough, you will find that breathing gets more difficult even if you rest; truly, air gets thinner the farther up you go. If there was a mountain high enough, one could “drown” up that mountain, go so high that there wouldn’t be enough air, and one would suffocate. Air is a thing, and it is thickest near the bottom, where we are.

Now, by balloon and plane and rocket it has been found that if you go far enough up — away from the ground, away from the sphere of stone that is this Earth — air will get thinner, and then altogether end. There will be emptiness, vacuum, like the emptiness of a bottle from which even air has been sucked out. The moon has no air, and it travels through no air as it turns around the Earth. (The reasons of that rotation we bypass, as they are complicated.)

But why do we have air? That is simple, given what has been already seen. We do not fall up to the sky because the Earth pulls us down; and it is not such a leap to think that we have weight, and that weight draws up closer to the immeasurably greater weight of the Earth. Gravity is just a different word for the attraction of mass towards mass. So, if we jump up, the Earth will pull us down. The same is true of air: it too has a weight, and cannot fly up and away towards the moon because the Earth pulls it down, too. (And why the Moon does not fall on us? Well, that is complicated and so tangential to the blueness of the sky that we will not think about that right now.)

You, down here on the ground, can observe oceans of water, but there is also a subtler ocean, one of air, and you live on the bottom of it. There are a few islands in it: Mount Everest the highest of mountains is one, punching up to heights where the currents of air don’t quite flow anymore: not an island from water to air, but from air to emptiness. The reason why you do not notice this vast ocean is simple: it has always been there for you. A fish doesn’t notice the water it swims in unless that water disappears; similarly a man doesn’t notice the air he lives in except should that air disappear, or should philosophical leg-stretching like this make him aware of it.

But where in all this is the blue sky? The ground is below us, and as we go up, there is air, thinning air, and then even that fades away, and then there is only the emptiness of space, and the moon and the sun and the stars rolling along in it. Where is the sky?

Now, you remember, I hope, that “down” is an illusion: not one direction, but a direction towards: towards the center of this sphere of stone we call the Earth. You have also noticed that “empty air” is not empty at all, but just another thing, a swirling mixture of gases we need to breathe and thus live, and whose absence would quickly kill us. What of the blue dome of the skies? If you go up by balloon or plane or by rocket, you will not hit that dome, so what is it?

And the answer is, of course: it is just another illusion.

And to perceive this, note first that the sky is blue only during days. The nightly sky is black, the black emptiness of space visible through the transparency of air. If during a day you were to go up by balloon or plane or by rocket, up to thin air, you would see the blueness fade to black. Why, then the blueness must be because of two things: the sun, and the air. It is sunlight which makes a day, and it is sunlight and air which makes the sky blue. It is not that a night’s darkness obscures some blue dome; no, the blueness is an illusion of sunlight.

If you take a prism and hold it up to sunlight, you will see that sunlight is broken, separated into its component pieces: red, green, blue, different colors that all beam out of the sun together, and together hitting the Earth light our days. We do not see sunlight as a tangle of red, green and blue, but neither do we see in a painter’s dye its component parts, and still accept that it has been mixed of simpler primary colors. The same is true of sunlight, and while a dye is almost impossible to re-separate, a simple prism will do to break sunlight apart.

Now, when a beam of sunlight comes to us, it first comes through air, through our atmosphere (our “ocean of air”), and only after having passed through that hits my eye or yours. The secret of blue skies is that the parts — red, green and blue — of this seemingly unified sunlight don’t react to air quite the same way.

To make a crude comparison: if you throw rocks at a hedge, you will soon note that the smaller rocks will hit the leaves and be deflected, fail to fly through, while bigger rocks sail through the hedge with a crash. Something similar is at work with sunlight and air.

The different parts of sunlight, of which we need here only contemplate red, green and blue, have one property that makes them different from each other. The exact nature of that property (wavelength) is complicated and not relevant here. It is enough to note that (as with a prism) red light corresponds to the heavier stones flung at the hedge: red light doesn’t much mind air, and sails relatively unimpeded through it. Green light is like middleweight stones: somewhat less likely to go straight. And blue light is the most erratic of all: as only some small stones fly through the hedge, so only some pieces of blue light pass through air like an arrow. The rest are deflected, like the rock that fails to go through the hedge: they take a new direction and go on, more like stunned swallows than deflected stones, then hit more air and bounce again and again, around and around. (Another deep but here unessential point: It is weird, but in a sense quite correct, to talk of pieces of light and pieces of air.)

Thus, when a beam of sunlight rides the emptiness of space from the sun to us, and then goes through air to our eyes, small parts of it are broken away: a few pieces of red light, some pieces of green, and much more pieces of blue. The light isn’t diminished enough to make it seem very reddish, but enough is broken away: the pieces of blue light bounce and collide with air, going this way and that, and eventually they hit the ground: but not from the direction of the sun. They have hit air and changed direction so many times that they shower us from all over the sky: and since that blue light seems to us just a blueness up there, this is why the sky is blue.

And since the blueness of skies is just an illusion of sunlight, there is no blue dome up in the skies, and thus nothing is needed to keep it from falling.

Oh, and one final thing: Sunrises and sunrises can be red. The reason is, subtly, already explained above: When the sun is near horizon, the light has to come a longer way to us across air: it cannot plunge straight down, but needs to slant down, going through a much, much larger amount of pieces of air. And thus even more of blue light is reflected and turned away, and even a visible portion of green light is scattered: and since after blue and green are diminished only red remains, a sunset or a sunrise will thus seem red.

* * *

A note: Yes, some things were simplified a bit for the sake of making this small story more readable. Some of the details avoided are deep, and especially so deep (“Why doesn’t the moon fall down on us?”; “What about the other high tide?”; “Seriously, pieces of light?”) that explaining them here would have made this thing fall flat.

Another note: Yes, I occasionally play this little game of “Explain it to a medieval!” with myself. The results are something like this.

A third note: Those that want to know more of sky-related things should read Phil Plait, since he has a threefold superiority on me: He’s an official world-explainer (I’m just a dumb mathematician), he does it for a living, and he is a native speaker of English. (He also has a blog called Bad Astronomy. And a book about the ends of world… er, the end of worlds. Ends of worlds? No. About, um, world-ending-scenarios. See, I’m not a native speaker.)