When you look at the sky on a clear day, it seems as if the earth is covered by a blue dome①. The sky is blue because of what happens to sunlight as it enters the earth＇s atmosphere. Sunlight is made up of light rays of all lengths. The longer rays make up red light and the shorter rays make up blue light. As sunlight enters the earth＇s atmosphere, it strikes the tiny molecules ② of air and is scattered③. The scattered light is made up mostly of violet④, blue, a little green, and a little yellow and red. The combination of these colors is sky blue.
Sometimes at sunset the sky looks red. The sunlight is coming toward you from a lower angle in the sky. The lower the sun, the longer is the path that the light rays must travel through the air. The shorter blue rays are scattered immediately when striking the molecules of air. The sunlight that you see lacks the blue and violet rays that the air has scattered. As the sun gets lower in the sky, the light rays must pass through more air,and so the longer orange and red rays also become scattered. At sunset, the shorter blue rays never get through the atmosphere, so you do not see them.
If you stand at a place where the surface of the earth is flat, the sky seems to meet the earth in any direction you look. The point at which the sky and the earth seem to“meet”is called the horizon. The horizon is at the same distance from you, no matter in what direction you look. If you walk toward the horizon, you do not get closer to it. No matter where you are, the surface of the earth curves ⑤ downward and away from you. The distance at which you can no longer see the earth＇s surface----because it has curved down too far----is the horizon.
①dome [d+um] n. 圆盖；穹窿
②molecule [＇m&likju:l] n. 分子
③scatter [＇sk$t+] v. 使消散；使分散
④violet [＇vai+lit] n. 紫色；紫罗蓝色
⑤curve [k+:v] v. 成曲形；弯曲