①What is the sky? How high is it? What color is it? What birds can live above it? These questions are difficult to answer, aren't they? In fact, the sky has no color. Then why does the sky look blue?
②We 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's because the air gets thinner when they go higher. The sane will happen to us if we go high enough. If we go up about 1,200 kilometers 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.
③Looking up above us, you'll see the blue sky with white clouds. It is usually light blue. The color would be bluer when you see on the top of a high mountain. And you'll see fewer clouds in the sky between you and the sun. Sometimes the sky doesn't look blue. When the sun rises and fails, you will see red, orange and other colors in the sky. They are the most beautiful moments of the day.
④Who can see the colors of the sky high above the earth? The astronauts! They can see those beautiful colors of the sky from the outer space(外层空间).However, before flying into the outer space, they sit in a spaceship high above the earth and there is no air at all. The sky would be dark and its color would be black.
⑤Now you may understand that we see colors because the air scatters(使散射)the sunlight. Light travels through the air. Without air there would be no colors.
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