If you follow one bird instead of the flock, it turns out to be doing something almost boringly simple. It watches a handful of neighbours — the nearest few, not the crowd — and keeps roughly the same distance from them, roughly the same speed, roughly the same heading. That is the whole instruction. Nothing in it mentions the flock at all.

And yet the flock is there. It has a shape, it has edges, it swerves away from a hawk as one thing. Order at the large scale is being produced entirely by rules that operate at the small scale, and no bird anywhere in that sky has the faintest idea what the shape looks like from the ground.

This is worth sitting with, because our first instinct is almost always wrong in the same direction. When we see coordination we go looking for a coordinator. When we see a pattern we assume a planner. Usually there is neither. The impression that the flock knows something is not in the birds at all. It is assembled in the head of whoever is watching.

Now look lower down, at the reeds.

The wind comes through and something crosses the reed bed — a dark line travelling from one side to the other, faster than you could walk after it. It looks exactly as though an object is moving across the field.

But nothing is crossing the field. Each reed leans over and comes back. No reed goes anywhere; every one of them finishes the afternoon exactly where it started. What moves is not stalks. It is the timing of their leaning. The reed beside it bends a moment later, and the next one a moment after that, and the eye collects those delays and calls the result a wave.

This is the cleanest demonstration of a wave I know, and the reason it is clean is that reeds are countable. You can watch the individual parts and confirm for yourself that none of them has travelled.

The sea is doing precisely the same thing and hiding it. A wave carries no water; it carries a disturbance through water. But the surface of the sea does not show you its parts. There is nothing to follow, no single one to keep your eye on, so it looks entirely convincing that a piece of ocean has moved from over there to here. It has not. The sea is simply very good at concealing the trick the reeds are performing out in the open.

Then there is the wind itself, which changes its mind every day.

In the morning it comes in off the water. In the evening it goes back out. This is not weather in the sense of something arriving from elsewhere. It is the land and the sea disagreeing about temperature.

Land is easy to heat. The sun warms only the top of it and warms it fast. Water is difficult: it is transparent, so the heat is spread down through a depth rather than sitting on the surface, it moves and mixes, and it takes a great deal of energy to raise its temperature at all. So by the middle of the day the land is hot and the sea is not. Warm air over the land rises, the pressure there falls, and air comes in off the water to fill the gap.

At night the whole thing runs backwards, for the same reasons. The land gives up its heat quickly, while the sea, which was slow to warm, is now slow to cool. The sea becomes the warm one, and the wind turns around.

Which means the wind is not really telling you about the wind. It is telling you which of the two surfaces is currently ahead in a very slow race, and that depends on nothing except how long the sun has been up.

So if you watch one view for long enough, you can begin to read the time off it without looking at a clock. Not from the light — from the direction the leaves are moving.

None of these three things is complicated. A bird watching its neighbours. A stalk leaning over and coming back. A rock warming faster than a bay. What is interesting is that in each case the thing we notice — the flock, the wave, the wind — is not a thing at all. It is a pattern produced by parts that know nothing about it.

We spend a great deal of time looking for the organiser. Most of the world is running without one.

The air above us is not an empty, invisible box. It is more like an ocean with no clear surface. Near the ground, the air is compressed by the weight of all the air above it. Higher up, it is thinner. Light travelling toward us near the horizon must therefore pass through many layers of air, each with a slightly different density.

When light moves through changing densities, its path bends.

We can see the same general effect when we place a spoon in a glass of water. The spoon appears to break at the surface, even though nothing has happened to it. The light has changed direction on its way to our eyes, and our brains have reconstructed the object in the wrong place.

The atmosphere performs a gentler version of the same trick.

There is no sharp boundary between air and more air, so the sunlight does not make one sudden turn. It curves gradually. Our eyes, however, assume that light has travelled in a straight line. We follow the incoming rays backward and place the sun above its true position.

Near the horizon, the difference is roughly the width of the sun itself.

That is a surprisingly large error. When we see the bottom of the sun touching the horizon, part of the sun is already geometrically below it. When we see the final edge disappear, the real sun has already gone.

The atmosphere is showing us an image that has been lifted into view.

This also explains why a setting sun often looks squashed.

The lower edge of the sun is seen through denser air than the upper edge, so the lower part is bent more strongly. We might imagine printing a photograph of the sun on a sheet of rubber and pushing its bottom edge upward. The image becomes flattened, although the object represented by it has not changed at all.

The sun remains round. The air edits the picture.

Now we can ask a second question. Why does the sun become orange?

It is tempting to say that the sun itself changes colour, but the sun is sending out approximately the same mixture of visible light as before. What changes is the route that light takes through the atmosphere.

When the sun is overhead, its light takes a relatively short path through the air. Near the horizon, it travels sideways through a much longer stretch of atmosphere.

We can picture shining a flashlight through a glass of slightly cloudy water. Through a thin layer, the beam changes very little. Through the entire length of the container, much more light is scattered away.

The atmosphere does something similar, but it does not scatter every colour equally.

Blue and violet light are much easier for small air molecules to redirect than red and orange light. This is why the daytime sky is blue: blue light from the sun is scattered across the sky and reaches us from every direction.

At sunset, the sunlight has travelled through so much air that much of its blue light has already been scattered out of the direct beam. What continues toward our eyes is the light that was harder to divert: mainly the longer orange and red wavelengths.

So an orange sunset is not the atmosphere painting red onto sunlight. It is the atmosphere removing much of everything else.

The exact result depends on what is suspended in the air. Dust, smoke, salt, water droplets and pollution all scatter light in different ways. A small amount of aerosol can make a sunset more vivid. Too much can make it dull and grey. A sunset is therefore also a crude experiment on the atmosphere, performed every evening with the sun as the lamp.

Now consider a photograph in which the sun is not visible at all. There may be orange light, distant blue mountains and a flock of birds crossing the sky.

The sun is absent from the frame, but almost everything in the photograph is evidence of it.

The orange sky tells us that sunlight has travelled through a long stretch of air. The pale blue mountains tell us that a great deal of atmosphere lies between the mountains and the camera. The birds appear dark because they interrupt the bright background.

We do not need to see the source to study what it has done.

Physics often works this way. We cannot watch an electron move around an atom as we might watch a planet orbit the sun. A distant planet may be too faint to photograph directly. A particle may exist for only a fraction of a second. Instead, we look at tracks, shadows, deflections and missing light.

We infer the thing from the disturbance it leaves behind.

A sunset is a simple version of the same method.

The sun provides the light. The atmosphere bends it, sorts it, scatters it and delivers a modified version to our eyes.

So what are we watching when we watch a sunset?

Not merely the sun crossing the horizon.

We are watching the atmosphere reveal its structure by interfering with the light passing through it.