What is the sky? How high is it? What colour is it? These questions are difficult to answer, aren't they? In fact, the sky has no colour. Then why does the sky look blue?
We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can't fly very high. That is because the air gets thinner when they go higher. The same will happen to us if we go high enough. If we go up about 1,200 kilometres from the earth, we'll find no air there. Some birds can fly into the sky as high as a plane, but they cannot fly or live without air.
Looking up above us, you will see the blue sky with white clouds. It is usually light blue. The colour would be bluer when you see on the top of a high mountain. And you will see fewer clouds in the sky between you and the Sun. Sometimes the sky doesn't look blue. When the Sun rises and falls, you will see red, orange and other colours in the sky. Such are the most beautiful moments of the day.
Who can see the colours of the sky high above the earth? The astronauts(宇航员)! They can see those beautiful colours of the sky from the outer space. However, before flying into outer space, they sit in a spaceship high above the earth and there is no air at all. The sky would be that dark and its colour would be black.
Now you may understand that we see colours because the air scatters(使散射) the sunlight. Light travels through the air. Without air there would be no colours.
