Big question
Why is the sky blue but space black?
The sky looks blue because Earth's atmosphere scatters blue light from the Sun far more than other colours, spreading it across the whole sky. Space looks black because it has almost no air to scatter light at all, so apart from stars and other light sources, there is nothing to light up.
Scattering Blue Across the Sky
Sunlight looks white but actually contains every colour of the rainbow, each travelling as a wave of a different length. As sunlight enters Earth's atmosphere, it collides with tiny molecules of nitrogen and oxygen gas, and this collision bounces some of the light off in random directions, an effect called scattering. Shorter wavelengths of light, especially blue and violet, scatter far more easily off these small molecules than longer wavelengths like red and orange, a pattern first explained properly by the physicist Lord Rayleigh, so it is often called Rayleigh scattering. Our eyes are more sensitive to blue than violet, so the scattered light that reaches us from every direction in the sky appears as the familiar blue colour, rather than violet.
No Air, No Scattering
Space is close to a total vacuum, meaning it contains almost no gas molecules at all, especially once you get beyond Earth's thin atmosphere. Without molecules for sunlight to bounce off, there is nothing to scatter light in random directions the way Earth's atmosphere does, so light from the Sun simply travels in a straight line unless it happens to hit something solid, like a planet, moon or spacecraft. This is exactly why astronauts, even in bright sunlight, see a completely black sky around them once they are outside the atmosphere, with only the Sun, the Moon, planets and stars appearing as small, sharp points of light against the darkness.
Why Sunsets Turn Red and Orange
The same scattering effect that makes the daytime sky blue also explains why sunsets and sunrises often glow red and orange. When the Sun is low near the horizon, its light has to travel through a much longer, thicker stretch of atmosphere to reach your eyes than it does at midday, when the Sun is overhead. Along that longer path, almost all of the blue light gets scattered away in other directions before it can reach you directly, leaving mostly the longer wavelength colours, reds, oranges and pinks, to continue straight through to your eyes. This is also why the sky can look especially fiery after a volcanic eruption or a dusty day, when extra tiny particles in the air scatter even more light.
Try it yourself
Add a few drops of milk to a glass of water and shine a torch through it from the side in a dark room. The milky water should look slightly blue from the side but the torch beam looks more orange or reddish when you look straight through it, just like sky and sunset colours.
Questions children ask
Why does the sky look black from space instead of blue?
The sky looks black from space because there is almost no air up there to scatter sunlight and spread it across the sky the way Earth's atmosphere does. Without air molecules to bounce light off, space stays dark except for stars, planets and other direct light sources.
What is Rayleigh scattering?
Rayleigh scattering is the effect where tiny gas molecules in the atmosphere scatter shorter wavelengths of light, like blue, much more than longer wavelengths, like red. It is the main reason the daytime sky appears blue rather than white or colourless.
Why is the sky orange or red at sunset?
At sunset, sunlight travels through a much longer stretch of atmosphere to reach your eyes, so almost all the blue light gets scattered away before arriving. This leaves mostly red, orange and pink light to reach your eyes directly.