Why is it dark?

The earth is an unlit and opaque celestial body, which keeps going around the sun. At the same time, it also rotates around its own axis of rotation. At a certain moment, the sun shines on the earth, and the side illuminated by the sun is the hemisphere, which is the day; On the other side is the night hemisphere, which is night; Because the earth keeps spinning, day and night will keep changing, and the sky will turn black.

The earth is an unlit and opaque sphere. At the same time, sunlight can only illuminate half the earth. The hemisphere irradiated by sunlight is daytime, and the hemisphere not irradiated by sunlight is the dividing line (circle) between night, day and night hemisphere, which is called the end line (circle).

At any moment, the day and night state of the earth can be expressed by the height of the sun. The height of the sun is the abbreviation of the solar altitude angle, which indicates the inclination of the sun's rays to the local horizon. In the daytime hemisphere, the height of the sun is always greater than 0 degrees, that is, the sun is above the horizon. In the whole termination line, the height of the sun is equal to 2 degrees, that is, the sun is just on the horizon; In all parts of the night hemisphere, the height of the sun is always less than 0 degrees, that is, the sun is below the horizon. Because the earth keeps running and alternating day and night.