How far away from the earth can you know that the earth is round, and how far can you see the complete earth?

Although sometimes "seeing is not necessarily believing", most people still believe what their eyes see. In the process of growing up, we learned through various channels that the earth is round, but not many people can really see the outline of the earth. So can the outline of the earth be seen on the earth? How high do you have to stand to see that the earth is round?

There is an ancient poem called "I don't know the true face of Mount Lu, only because I am in this mountain", it is usually used to describe the authorities who are obsessed with bystanders, and human beings have experienced similar feelings in the process of knowing and understanding the earth. Due to the confinement of the earth, our ancestors believed that the earth was square and the sky was round. Later, with the development of science and technology, human beings gradually broadened their horizons and realized that the earth is actually a sphere. Now humans can use artificial satellites to photograph the earth in space to prove that the earth is round, so what position do people on earth need to stand in to see the outline of the earth?

For this problem, the ancient Greeks already had experience from a very early time. By looking at the sea level, they found that if there is a ship approaching from the horizon to the shore, the first thing people see is the sail of the ship, and the whole ship is revealed as the distance continues to approach. It is through this method that the ancient Greeks began to think that the earth is not square, but may be round. When the time has come to the age of great navigation, many countries have sent fleets to sail around the world, and through many voyages, they have found that their routes can be closed in the end, proving from the side that the earth is round.

However, these are always indirect methods. If you want to intuitively feel the outline of the earth, you have to observe it visually, so what is the appropriate position to stand in? Some people may say that standing on the top of Mount Everest, the highest peak in the world, can see the contour of the earth with a curvature, but experts say that this is far from enough, after all, the height of Mount Everest has not exceeded 10,000 meters. So if you sit on a plane flying at an altitude of 10,000 meters and look into the distance, can you see the outline of the earth? This is also impossible. In addition to the fact that there will be clouds at an altitude of 10,000 meters to block the line of sight, the lack of altitude is the main problem.

To know that the radius of the earth is about 6,400 kilometers, it is very difficult to see the outline of the earth at about 10,000 meters. It is understood that an extreme sports challenger took a space shuttle to an altitude of about 39,000 meters above the ground in 2012, and then jumped. According to the description of the challenger, when he was preparing to skydive, he was able to clearly see the outline of the earth and photographed the phenomenon he saw. However, after the photo was published on the Internet, it was "fake". Many people believed that the arc of the earth was captured by a wide-angle lens and could not be seen by the naked eye.

Scientists from NASA believe that when a person stands up to 100 kilometers above the ground and looks down, he can see the arc of the earth, because by definition this is the boundary between the earth and space. When a manned spacecraft enters a place up to 400 kilometers from the surface of the earth, the shape of the earth can be completely seen. From this point of view, it is still impossible to visually see the outline of the earth on the earth.