# We all know that the earth is round, but why is the ground flat?

2020-01-21 | Scientific Beauty Boy original |

The earth is not round, but an irregular circle-an ellipse. We feel that the earth is flat, a large part of the reason is that the curvature radius of the earth is too large, and the eyes of humans living on the earth's surface are too short.So everything is flat. If we see all the mountains and mountains on the high mountains, we can't tell whether the ground is flat.

short-sightedness

Human eyes are too small compared to the size of the earth.

For example :

No.1, No.2, No.3 balls, each of which is larger than one. It is assumed that the range of human sight is a red rectangle. The larger the ball, the closer the surface of the ball is to the plane, and the No.1 ballA semicircle is in the red range, a little arc can be felt in the red range of the No. 2 ball, and the eyes in the No. 3 ball can no longer tell that it is curved.

Compared with the earth, the circle of this proportion is the little witch seeing the big witch, we will use Shanghai as an example below.

The accuracy is not high

Human "accuracy" is not high yet. Think of us usually hanging a picture on the wall, one person hanging, and one person under the command. "The right side is higher", "the left side is lower", even soIf we take a professional tool to measure, it will definitely be crooked. The error we command through the eyeball is also an acceptable error, so it is not necessary to be too real.

Let's take a look at the figure below and ask whether it is horizontal or inclined.

The answer is 0.2 degrees oblique. Our eyes cannot discern a slight angle, so we usually need tools when we are looking for a level.

the tip of the "iceberg"

Some people say that we can use tools to measure the height difference within 5 kilometers.

First of all, measuring and seeing are two different things. Secondly, even with the tools, the surface of the earth is uneven. Even plains have uphills and downhills. Plains are not “leveling” against the surface of the earth as we imagine.Even if we measure 5 kilometers, it is only the tip of the iceberg for the earth. We cannot judge how big this iceberg is, and what shape the iceberg is overall.

Only by pulling away and seeing the whole can you see the shape. Just like a table tennis is flat to bacteria, a basketball is flat to ants. Although we all know that these two balls areIt is round, but the surface of the earth is flat to human beings, because the earth is to humans is like table tennis to bacteria. If you keep ants away from basketball and see the whole of basketball, you can also see that basketball is round, likeHumans see the earth in a distant space.

The ground approaches the level

The above said that the human field of vision is too narrow, and the distance of the line of sight that the field of vision can encompass is too short, which results in the curvature of the field of view being too small to allow the eyes to tell us through the brain that this is "bent."When our perspective is limited to a point on the earth, the part of our field of view is approaching a straight line. Just as you can't see the straight line in the above picture tilted by 0.2 degrees, so you can't judge that our field of viewThe bending amplitude in the range. Just like the following figure, as the radius of curvature increases, the range we see becomes more and more "straight".

Another example

Assuming you have a clairvoyant eye, you can see Shanghai's westernmost side by standing on the easternmost side of Shanghai.

The perimeter of the earth is about 40075km, the width of Shanghai is about 90.25km, the perimeter of the earth is about 40075km, the circle is 360 °, and the fan arc in Shanghai 90.25 / 40075 * 360 ° ≈0.81 °

We zoom in on the Shanghai area and we can see the red color.

Zoom in again, and you can see that the red Shanghai area is approaching the black straight line.

If red represents the width of Shanghai, think of humans again. The size in the picture above cannot be seen with a microscope. Even if a person has a clairvoyant eye, from the east of Shanghai to the west of Shanghai, the eyes cannotRecognize the "error" of this slight angle. Not to mention that the surface of the earth is irregular and there is no perfectly horizontal reference, so the brain will tell us that the ground is also flat.

from circle to flat

Enlarge the earth a little bit, and you can more objectively experience the process from circle to straight. Distant space :

Near-Earth orbit, the observed Earth is no longer a complete circle.

Zoom in again, the angle of the aerial view is still radian. But people are too small, and the surroundings are rough and meandering. If you place them in the mountains and forests below, they are flat and round., We can't judge at all.

But if we fly at a low altitude, driving on a straight highway, running in the fields, we will find how straight the ground is!

Because of the error of vision, the limitation of vision, the curvature of the earth, the smallness of human beings and we live on an uneven surface, we feel that the ground is flat.

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