Why is the sky blue but space black?

The universe is full of phenomena that have seemed unexplainable to human beings, but only until science had an answer for it. One among those concepts is, “Why does the sky appear blue during the day but space looks pitch black?”

Doesn’t it really wreck the human mind? To truly get an answer to this, we need to understand the behavior of light and the way it behaves with Earth's atmosphere. The difference between how light behaves on Earth and how it travels in the emptiness of space, explains the beautiful blue skies and the vast darkness of space.

Why is the sky blue in colour?

The colour of the sky is largely due to a phenomenon called "Rayleigh scattering," named after the English scientist Lord Rayleigh, who studied it in the 1870s. When sunlight enters Earth's atmosphere, it’s made up of light waves of different colors, ranging from violet to red. Each colour has a different wavelength, whereas blue light has a shorter wavelength compared to red light, which has a longer wavelength. When sunlight passes through the atmosphere, it interacts with tiny air molecules, causing the light to scatter in all directions.

But what is important to note is that shorter wavelengths of light, like blue and violet, are scattered more effectively than longer wavelengths like red and orange. Rayleigh's findings showed that blue light is scattered about nine times more than red light due to its shorter wavelength. Even though violet light is scattered even more than blue, our eyes are more sensitive to blue light, which is why we see a blue sky during the day.

Why is Space black?

Space is a different medium as compared to the Earth and doesn’t have an atmosphere to scatter light. In the vast emptiness of space, sunlight travels in a straight line, without any interference from air molecules. Since there’s nothing to scatter the light, all the colours stay together and we see the sun as a bright, white light from a spacecraft or the Moon. When we look away from the sun, there is nothing but the dark expanse of space, which is why space appears black to us.

What happens at sunrise and sunset?

At sunrise and sunset, the light has to pass through more of the Earth's atmosphere compared to when the sun is overhead. As a result, most of the blue and violet light gets scattered out, leaving behind colours with longer wavelengths, like red, orange, and yellow. This is why we often see beautiful red and orange hues during the rising and setting sun.

Are there any other factors that affect the colour of the night sky?

Rayleigh scattering, however, is not the only factor responsible for the colours of the sky. Things like dust, water vapour, pollution, and even ozone in the atmosphere can also impact the colors we see. In fact, different weather conditions can make sunrises and sunsets look more vibrant or dull, depending on the amount of particles that are present in the air.