Why is the sky blue by day and black at night?

Light has to reach the eye

Imagine a torch shining across a dark room. You can see the bright spot on the wall. If there is dust in the air, you may also see the beam from the side: some light changes direction at the dust particles and enters your eye. This change of direction is called scattering.

The example explains the path of the light, but not the sky’s colour: large dust particles and air molecules interact with light differently. Even clean air is enough to produce a blue daytime sky, mainly through its nitrogen and oxygen molecules. The Sun lights them up, and they scatter some of that light towards us. Light therefore reaches us from parts of the sky far from the Sun, too. NASA: how light scatters.

In the nearly empty space between illuminated objects, there is no such bright atmospheric background. A beam passing by your eye does not make itself visible. You can see brightly lit ground and a black sky above it at the same time.

The Moon gives us precisely this comparison. It has an extremely thin surrounding layer of gas, an exosphere, but lacks the dense atmosphere that creates Earth’s blue dome. NASA’s overview of conditions on the Moon describes this difference.

Why air adds a blue tint

Sunlight contains a whole range of visible colours. A prism can separate them into a coloured band, a spectrum. Light has wave properties: its wavelength is the distance between neighbouring crests of a light wave. Within the visible spectrum, red corresponds to longer wavelengths, while blue and violet correspond to shorter ones. NASA: visible light.

Air molecules are much smaller than the wavelength of visible light. Under these conditions, short wavelengths scatter more effectively than long ones. This is Rayleigh scattering. When we look at a clear sky away from the Sun, we receive light with an increased share of short wavelengths, which we perceive as blue.

Red light scatters too, just less strongly. Air does not sort rays into ones that “always go straight” and ones that “always turn”. It gradually changes the balance of colours in the direct and scattered light. NASA: how scattering depends on wavelength.

In the diagram, compare two conditions separately: whether there are molecules to redirect light towards the eye, and whether the Sun illuminates them. An ideal vacuum lets us imagine removing the first condition; night on Earth removes the second.

A longer path makes a redder sunset

Now let’s change the geometry. When the Sun is high, its light travels a relatively short path through the atmosphere to the surface. Near the horizon, the beam crosses the atmosphere at a shallow angle and passes through much more air. The atmosphere has not suddenly grown thicker in the evening; the beam’s direction has changed.

Along this longer path, scattering removes more of the short wavelengths from the direct beam. Longer wavelengths make up a greater share of the light that continues from the Sun. This is why the low Sun and the clouds it illuminates can look orange or red. It is the same mechanism that gives us a blue daytime sky, but now we are following the light left in the direct beam. NASA Earth Observatory: the longer path of sunset light.

A red sunset does not mean the entire sky must turn red at once. In different directions we see light that has followed different paths; clouds, dust and haze change the picture. This brief explanation does not predict the exact shade of a particular evening.

Clouds also show the limits of the molecular rule. Their droplets are much larger than molecules and scatter visible colours less selectively. A cloud lit by white light may look white; its thick lower part may look grey because less light reaches it. NOAA: cloud colours.

At night the illumination fades; the air remains

When an observing location is on Earth’s night side, the planet blocks direct sunlight. The molecules above us remain, but the source of the bright daytime background no longer illuminates them as it did during the day. Scattering has not “switched off”: the lighting has changed. NASA StarChild: day, night and dark space.

It is still light immediately after sunset. The Sun is no longer visible from the surface, but some of the atmosphere higher up remains illuminated and scatters sunlight towards us. This is twilight. As the Sun sinks farther below the horizon, the daytime illumination weakens. NWS: what twilight means.

The night sky is not completely black, either. Air scatters moonlight and light from city lamps. The upper atmosphere also emits a faint light of its own, airglow: atoms and molecules release energy they received from solar radiation. NASA on the atmosphere’s natural glow. “Black” in the title is our everyday description of a dark background, not a claim that no light is present at all.

Why don't all the stars replace daylight?

Earth’s shadow explains where the sunlight background went. But the stars keep shining. Why are there dark gaps between them?

“They are far away” seems a sufficient answer until we ask another question: how many stars are there at ever greater distances? Imagine a simplified universe: infinite, unchanging, uniformly filled with stars that have always shone. Distant stars are fainter, but each more distant shell of space contains more of them. In that model, distance alone does not save us from a uniformly bright sky. The conflict between this expectation and the dark night is called Olbers’ paradox. Penn State: the stellar model explained.

The real universe does not have an infinite, unchanging history of starlight. Stars began to form at a particular stage, and their light takes time to reach us. We therefore cannot add up the contributions of an endless succession of stars supposedly shining forever. The limited time available for light to travel in an evolving universe removes that expectation. It does not mean we have seen a spatial edge of the universe. Penn State: resolving Olbers’ paradox.

The universe is also expanding: the wavelengths of distant light grow longer, weakening its contribution to the visible background. Some radiation moves beyond the range our eyes can detect. NASA StarChild on lengthening wavelengths. A dark night is consistent with this picture; by itself, it neither measures the universe’s age nor proves every detail of its history.

Three conditions to keep distinct

A blue daytime sky needs sunlight, air molecules and a path for scattered light to reach the eye. Along the long path near sunset, the direct light loses more short wavelengths and becomes redder. After twilight, Earth shades the air above its night side, so the bright daytime background fades. The stars do not create a uniformly bright background because their light has a finite history in a changing universe.

Try making a prediction: if we imagined removing the air but kept the sunlight, would day turn into night? The sky’s background would darken, but the surface and objects would remain illuminated. We have removed the scatterer, not the light source: this is what distinguishes daytime on Earth from daytime on the Moon.

To make your own observation, wait until the Sun has completely disappeared below the horizon and watch the sky overhead fade. The prediction is that darkness arrives gradually, because the illuminated upper atmosphere still provides twilight. Compare evenings with different cloud cover; do not point your eyes or optical instruments at the Sun.

You can revisit the structure of our planet’s air envelope in “What is space, and where does it begin?”. The next question in the course is what comes into view when the bright daytime background disappears.