This is a question that many netizens are confused about. Some even think that Seen from the ground, how come the earth is flat? Even if the earth is round, water should be flat, right? Because measuring the so-called "level" of an object requires it to be flat?

First of all, let me state that the earth is round

In fact, the earth is round. Not only the land is round, but the water is also round. This is all the result of gravity. Because the mass of the planet is large enough to a certain extent, the gravity is enough to overcome the rigid force of the object and achieve hydrostatic equilibrium. What is hydrostatic equilibrium? In layman's terms, it means making an object become a sphere.

It has long been common knowledge that the earth is round, and it seems undisputed. But there are some people who insist on saying that the earth is flat, and a group of people in a certain country have established an organization to prove their "truth." To this end, they are collecting donations everywhere and want to build a rocket to fly into the sky. They must go up and see for themselves. They just don't believe what modern science has proven long ago, and think that scientists all over the world are liars.

However, they have been raising funds for more than ten years, but they have not raised enough money for a plane ticket to circle the earth. Because almost everyone looked at this kind of person strangely, as if they were seeing a group of monsters.

But the world is so strange. There are many things that have been scientifically proven for a long time, but some people don't believe it, and they think that scientists have the same brains as them. What about those unexpected high technologies?

If you don’t believe it, we can see it in the comment areas of various popular sciences, and there are not a few such people. Although these people already believe that the earth is round, their knowledge is just a step ahead of those who do not believe that the earth is round. They often struggle with some scientific common sense that has long been a consensus, and they argue endlessly. , and wantonly derogatory.

Now let’s get down to business, let’s talk about how far you need to know that the earth is round, and how far you need to see the entire earth.

Evidence that the earth is round is everywhere around us

If I say, You don’t need to go very far to know that the earth is round when you stand on it. Do you believe it?

None of us can feel that the earth is round without leaving the earth. You are standing on a plain and have an unobstructed view of the wilderness or grassland in front of you, but how far can you see? If the earth was flat, you could theoretically see the end of the world, assuming of course that the world has an end.

Even if distant objects cannot be seen clearly, they can be seen with a large telescope. Has anyone seen this? No.

This is because the earth is a ball. The ground under our feet is not flat, but an arc. The ground in the distance curves down and falls below our line of sight, so All we see is the horizon.

What is the horizon? It is the range of the curved surface that you can see on a sphere at a certain height. The radius of the Earth is about 6371km (kilometers or kilometers). Therefore, if we assume that the Earth is a perfect sphere (in fact it is not, the equatorial radius is slightly larger than the polar radius), the curvature of the Earth is about 1/6370.

This is the degree of curvature of the earth. This is especially important for aviation. When an aircraft flies parallel to the ground, it must consider the change in curvature, especially when flying at low altitudes. Without considering the curvature, it will Will hit the ground.

It is precisely because the earth is round that we can stand high and see far

Anyone who has climbed mountains or flown on an airplane has this experience: the higher you go, the further you can see. When sailing on the sea, when we reach the high seas and far away from the coast, the waves on the sea surface are endless. When we look around, we feel like we are in the center of a large disk. The sea and sky are the same color as far as we can see. Where the sky and the sea meet, it is the sea level, which is where we stand. At that altitude the furthest point of the Earth's curvature can be seen.

In the sea, when we see a ship approaching us in the distance, we will always see the smoke coming from the mast or chimney first, and then we can gradually see the hull of the ship. This is because the earth is round and the water surface is also round, and the ships in the distance slowly rise from the horizon of people's vision.

This is how you can know that the earth is round without leaving the surface of the earth. But if you want to see the arc shape of the earth more intuitively and see that our world is a beautiful blue planet, you have to go to a higher place.

The relationship between altitude and visual distance

A perfect sphere can see much The distance is directly proportional to the observation height. So on Earth, how high can you see and how far can you see? Based on the principle of the above figure combined with geometric knowledge, we can easily derive such a formula:

d^2=（R+h)^2-h^2

Here d represents the distance that can be seen; r is the radius of the earth, with a value of 6371km; h is the height of the observer. Based on this formula, we can easily calculate how far away the Earth's curved surface is seen at different altitudes. For example, from a mountaintop of 1,000 meters, we can see a range of about 113km; if we take a plane at an altitude of 10,000 meters, we can see a range of about 341km.

In the International Space Station, at an altitude of 400km from the surface, you can see a range of approximately 2292km. This is only the range that the human eye can see. Human eye observation is different from that of a camera. The viewing angle is wider than that of a lens, and it can scan in all directions like a video recorder. The human eye has a 120-degree viewing angle, so people at an altitude of 1,000km can completely see the entire earth.

Cameras are different. At an altitude of 1,000 kilometers, it is impossible to put the entire earth into a viewfinder except by splicing multiple photos. You have to go to an altitude of 12,000 kilometers to take a picture. Earth in its entirety. Of course, whether it is direct human observation or camera photos, the so-called "full picture" is only half of the earth facing the observer, and it is always only close to half.

Because the earth is spherical, and objects viewed from the front are at angles, to see 50% of the earth's hemisphere is theoretically infinite. But humans can never reach infinity, so the half-sphere of the Earth photographed can only be infinitely close to 50% of the entire sphere, and will never reach the true 50%.

Just like a satellite or spacecraft photographing the Earth, at an altitude of 36,000 kilometers, the entire shape of the Earth can be photographed, but only about 45% of the total area of ​​the Earth's surface can be seen; at a distance of 384,000 kilometers from the Earth On the moon, only 49.5% of the total area of ​​the earth was photographed. The farther away you are from the earth, the closer the circular surface you see is 50% of the total area.

Okay, this is the answer to how far away can you see that the earth is round? What do you think? Discussions welcome, thanks for reading.

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