"Far above the Yellow River, among the white clouds, there is an isolated city called Wanren Mountain. Why should the Qiang flute blame the willows? The spring breeze does not reach Yumen Pass." This is the famous poem "Liangzhou Ci" written by the poet Wang Zhihuan.
The first sentence "The Yellow River is far up among the white clouds" expresses the special feeling of looking at the Yellow River from near to far. The Yellow River is rushing in. Because the direction you look at is opposite to the flow of the river, you feel that it is the Yellow River. He ran towards the white clouds.
The Yellow River is on the ground and the white clouds are in the sky. Why do they meet together in the eyes of the poet? Is it the poet's illusion? no. This is a natural phenomenon. When a person stands on the earth and looks around, he will feel that he is in the center of a circle as far as his eyes can see. The edge of this circle is the horizon. The ancients called the horizon Tianya, which is the edge of the sky as far as the eye can see. In other words, when people look in the horizontal direction, the place where the sky and the earth intersect is the horizon. In many places, the true horizon is obscured by trees, buildings, mountains, etc., and is replaced by a visible horizon. If you are on a boat in the ocean, you can easily see the true horizon. During ancient wars, the farthest distance you could see the beacon smoke was the distance to the horizon that your eyesight could reach. The distance between two beacon towers must not exceed this distance, otherwise the beacon tower will lose its role. Even today, when aircraft are flying under visual conditions, pilots orient themselves spatially based on the horizon and use the relationship between the aircraft and the horizon to control the aircraft.
The horizon is inaccessible, it recedes as you move towards it. When you step back, it comes towards you. As far as we can see, the sky and the earth meet at the horizon. Near the Yellow River, as long as you look around, the Yellow River in the distance seems to be connected with the white clouds on the horizon, and the Yellow River is far up between the white clouds.
How far is the horizon from us? It is related to the height of the observer above the Earth's surface and the Earth's radius. According to calculations, if there are no obstacles, the visible distance of a medium-sized person from the earth will not be farther than 4.8 kilometers. In other words, the horizon will not be farther than 4.8 kilometers from him. If you stand higher, the horizon will be further away. But no matter how high you stand, people's field of view will be limited to a certain range. This is because the field of view is not only related to the height of the observer, but also related to the fact that the earth is a sphere. If the earth is flat and there is nothing No matter how high you stand, you can see the same distance. In fact, no matter where you are on the earth, you can climb up and look far because the earth is a sphere.
The horizon is different on different planets. The diameter of the moon is 3,476 kilometers, which is about a quarter of the Earth's diameter of 12,756 kilometers. An average-sized observer on the moon would be only 2.3 kilometers away from the horizon. The diameter of Jupiter is 11 times that of the Earth, and the distance to the horizon for an average-sized observer on Jupiter is 15 kilometers.
The place that people can see is limited, but the place beyond the horizon is infinite, where people's imagination can fully expand. The Yellow River reaches the horizon and disappears. The poet can think that it is "far above the white clouds". When people stand in a vast place and look around, there are so many dreams hidden at the end of the world. Those who run towards the horizon must be those with dreams. How vast the sky is, how open the earth is, how long the horizon is...
Wang Zhihuan didn't know that the earth was a sphere, nor did he understand what a horizon was. He carefully observed the natural scenery and truly reflected it The scene of natural changes uses the poet's eyes to describe the scene of the Yellow River disappearing into the horizon, so there is the poem "The Yellow River is far above the white clouds".