As follows:
The sky is empty; The vast space where the sun, moon and stars are listed. It is an important part of the earth. There is an ancient saying in China that the sky is round and the earth is round. People can observe meteorological or astronomical phenomena in the sky, so as to know the change of weather, the passage of time or their position. You can know the time of day at sunrise and sunset, and the moon's profit and loss for a month at night.
The big dipper can indicate the north. The thickness and shape of clouds can judge whether it will rain or not. You can enjoy many beautiful phenomena in the sky, such as rainbow, aurora and meteor shower. Birds can fly in the sky.
Due to the increasing use of fossil fuels such as oil, especially fuels such as coal that will release sulfur dioxide after burning, the visibility of the sky has gradually decreased since 1973, except in Europe.
The reason why the sky is blue is that sunlight is made up of seven kinds of light. Of the seven kinds of light, green, cyan, blue and purple have higher frequencies, and the light with higher frequencies is most easily scattered by air molecules and dust in the air, so when sunlight passes through the atmosphere, most of the four kinds of light are scattered.
The proportion of purple in the solar spectrum itself is relatively small, most of the purple light and ultraviolet light are absorbed by the ozone layer, and the rest is mainly cyan, so the sky will appear beautiful blue. The color of the sky (that is, the color of the atmosphere) is actually the composite color around the blue in the spectrum, which we call "blue".
If there is no atmosphere, the sun we see is a very dazzling fireball in the dark space background, and astronauts in the space station can see this scene. Because the sun is a star with a color temperature of about 5000K, its light tends to be yellow, so we often see the sun through the atmosphere.
The sky in rainy days is gray, because thick clouds mainly reflect sunlight, so the light is weaker than sunny days, and the sky looks darker than sunny days. When the sun passes through the thick clouds again, the rainy sky looks gray, because there is a lot of water and dust in the clouds.