# How far away can the complete Earth be seen?

The surface area of the earth is about 510.1 million square kilometers, and we alone are just a drop in the ocean to the earth.

So the whole range we see when we stand on the earth is flat. When we stand in a very open location, such as a desert, the Gobi, or the sea, we can see the sky and the earth gradually connect into a line in the distance. It looks as if the sky and the earth are connected as one.

In fact, because the earth is a sphere, because the range of vision can not see the location are because the surface of the earth curved downward. The curved surface of the earth we can not see, and the sky and the sight of the surface of the picture overlap. That's why we have the illusion that "the surface is flat".

Since we ordinary people cannot know whether the earth is round or not by calculation like mathematicians, nor can we know whether the earth is round or not by sailing like adventurers. Then can we directly see whether the earth is round or not through our own two eyes? The answer is yes, and many phenomena in life can prove that the earth is round.

For example, if we look at the sea when we find a giant ship driving across. Then we must see the top of the giant ship first, and as it gets closer and closer to us, we can see the hull and the full view of the ship. If the earth is flat, then no matter what direction the ship is coming toward us, it should appear in our view as a complete hull at the same time.

We can't observe that the Earth is round because humans are too small. So we are far away from the earth is not able to see its full picture? For example, we all know that "near is big, far is small", if we stand under a tall building and can not see the top floor, then we will take a few steps back to expand the field of vision. Similarly, if we maintain a certain distance from the earth, we can directly see through the naked eye "the earth is round". So how far away does this distance need to be?

On a very regular sphere, the higher the height of the observation, the greater the range that can be seen. This observational variation can be expressed by the equation d^2 = (R + h)^2 - h^2. Where d represents the range distance that can be reached, r is the radius of the Earth, and h represents the observation height.

If we consider the Earth as a very round sphere, and its radius as 6371 km, then different observation values will calculate different observation ranges. When the observation range reaches a circular boundary, the observation height can be considered as the distance we need to see that "the Earth is round".

According to this formula, we can calculate. If we stand at the top of a 1 km high mountain, we can observe a range of 113 km. If we are at a height of 10 km, we can see a range of 341 km. At this point, we see only 1.7% of the Earth's surface area.

According to a photo taken by an Austrian extreme skydiver, we can see that at an altitude of 21.8 kilometers above the ground, the picture we take of the Earth is still not a complete circle. However, at this point, the edge of the Earth can still be seen in a circular shape when looking down at the edge of the Earth.

At present, the satellites launched by humans to revolve around the Earth are 400 kilometers away from the Earth, and photographs of the Earth from satellites can show an area of 2,292 kilometers. That is 11% of the Earth's surface area. It is worth mentioning. The size of the picture that a camera can hold depends on whether it is a full-frame camera or a half-frame camera, while the human eye view can directly cover a range of 188 degrees. In other words, the human naked eye viewing range than the camera to capture a wider range.

China's Fengyun-4 satellite had taken a photo of the Earth in 2017, at an altitude of 36,000 kilometers. The photo clearly shows that the Earth is a circle. In the picture, the Earth is covered by blue oceans, the dappled yellow and green are the land where human beings live, and the floating and spiritual snow-white are the clouds and the atmosphere. But at this point, we can only see 45% of the Earth's surface.

In 1968, Apollo 8 successfully carried out the first mission around the Moon in the history of mankind, and at that time, during the execution of the program, "the appearance of the Earth in the eyes of the Moon" was photographed.

When looking at the Earth from the Moon, 380,000 kilometers away from the Earth, the Earth looks like a small and delicate glass bead in the dark universe, the blue and white color contains endless vitality and vigor. At this time, the area we can see only accounts for 49.5% of the Earth's area.

However, we do not need to pursue in how far away. According to the formula, if we want to observe the round boundary of the Earth by pulling away a certain distance, we need to be at an altitude of 1100 km from the ground in order to see the "Earth is round" in its entirety.

# How did humans first discover that the Earth is round?

A great mathematician was born in the sixth century B.C. The name of this ancient Greek mathematician was Pythagoras. He believed that the earth's body was the most perfect geometry, so the earth was spherical. Later he proposed that the sun, the moon and the stars were all in circular motion around and around. He believed that the Earth was in a circular trajectory, and on the opposite side of the trajectory was the "opposite star", which had a similar mass to the Earth. Therefore, the Earth and the "opposite star" are like the two ends of a scale, maintaining a smooth running state.

Later, the ancient Greek philosopher Plato put forward another view. He believed that the whole universe is a sphere, and the whole universe is in constant motion. In the universe, all celestial matter is in circular motion. He speculated that because circular motion does not require external propulsion and can be repeated from week to week.

Plato's ideas clearly influenced his disciple Aristotle, who more than 2,000 years ago wrote in his book "On the Heavens" that by observing the changing stars in the sky, it was possible to conclude that the earth was round. And this "circle" for all things in the universe is not large, because a slight shift in the position of our observation of the stars, we can find that our field of vision has changed dramatically.

Later, the ancient Greek mathematician Eratosthenes, in the 3rd century BC, measured the circumference of the earth. Eratosthenes came to the city of Aswan at the summer solstice to observe the reflection of the sun in the water of the well. With the data of the length of the shadow in the port of Alexandria north of Aswan at the same moment, and the distance between Aswan and Alexandria, the circumference of the earth was measured to be between 39,690 and 46,620 kilometers.

Modern scientific research has measured the circumference of the Earth to be about 40,008 kilometers, a figure that is well within Eratosthenes' range of measurement.

However, whether it was Pythagoras, Plato, Aristotle or Eratosthenes, they all came to the conclusion that "the earth is round" only through mental deduction or mathematical calculation. The person who really proved that "the earth is round" through practical action was Magellan.

The Portuguese explorer Fernando de Magellan was convinced that the earth was round and at the age of 33 asked the king to organize a fleet of ships to sail around the world. In 1518, the King of Spain agreed to support Magellan's voyage around the world. In the following year, Magellan led 270 men in five huge ships from the port of Seville to start the voyage around the world.

Magellan first led a fleet of ships across the Atlantic Ocean, and on the way to escape the Portuguese pursuers who prevented him from sailing away. After sailing for a year, he finally arrived in South America.

As the weather was very cold at this time, food was also in short supply, so the sailing party appeared to rebel. But Magellan through his wisdom and courage to solve the rebellion of people. After that, Magellan continued the voyage with two South American aborigines, "Bigfoot", whom he had tricked. From August 1520, Magellan led a fleet of ships across America and into Oceania.

It was not until March 1521 that the fleet led by Magellan arrived at the island of Homonhon in the Philippines. The crew had slaves of Malaysian origin who heard that the people of this island spoke the Malaysian language. So everyone found that they could also reach the vicinity of Malaysia all the way west from the port of Seville. Magellan then died in a conflict with the aborigines of an island during the voyage, and the rest of the crew continued to sail as planned, finally completing the circumnavigation in 1522. Magellan's voyage was a direct proof that the earth is round.

# The Earth is flat?

Although Magellan proved that the Earth is round through navigation so long ago, there are still people who insist that the Earth is flat. People who believe that "the earth is flat" even have an organization, which is called the Institute of Horizons, also called the International Institute of Horizons.

The Ground Level Sayings Society was first established in 1956 by Samuel Shenton in England. Charles K. Johnson then succeeded as leader and continued to preach the ground level theory. Under his preaching, the number of the organization reached more than 3,000 at one time. Later, Charles K. Johnson died, and the new leader, Daniel Shenton, set up discussion groups on the Internet to once again keep the organization Ground Level Saying active in the public eye.

The organization believes that the shape of the Earth is not a sphere, but a disk. The center of the Earth is the North Pole, and the Antarctic glaciers are the edge of the disk. Because of the Antarctic glaciers, the water from the ocean does not flow out. The moon and the sun, on the other hand, are like lighting lamps that would provide illumination for the earth by flickering.

Given our modern popular scientific knowledge, the idea of a geodesic doctrine is clearly absurd. Their misconception is actually a learned bias. Hopefully, in the future, human beings can overcome various technical difficulties so that everyone can have the opportunity to travel in space. At that time, we do not need to use all kinds of theories to convince people who believe in the doctrine of the earth's horizon, we just need to take a space ticket and tell him that if he wants to know whether the earth is round, he can go to space and see for himself.