You say the earth is round, where is the evidence?

Everyone knows that the earth is round!

However, if we think about it carefully, we will find that this "common sense" seems to be taught to us by teachers and parents...

If I ask one more question: You say the earth is round, where is the evidence?

I believe that many friends must have to think for a long time before they can come up with one or two.

In fact, even now, we still often unconsciously regard the earth as a plane, and cannot use the common sense that "the earth is a sphere" very freely. For example, when facing direct flights between China and the United States, we often hear questions like "Why don't we go directly through the Pacific Ocean, but have to go around Alaska?"

Direct flights from China to the US via Alaska

So, today we will learn how humans, in the process of self-learning and continuous progress, overturned the intuition of a flat earth, understood the nature of the earth, and turned this knowledge into common sense!

The Earth's Roundness Theory Was Accepted

You may think that the concept of "Earth" appeared relatively late in history, at least after those great navigators sailed around the Earth. But in fact, the idea of ​​a spherical Earth has been around for more than 2,000 years and has been widely circulated. It is only in the past 300 years that people have correctly understood the origin of this sphere.

As early as the 6th to 4th centuries BC, Pythagoras and Plato successively pointed out that the earth is a sphere. Plato's disciple Aristotle clearly gave three observational evidences:

Evidence 1: Some stars in the southern sky that can be seen in Egypt are not visible in Greece. If the earth is a flat surface, then the celestial bodies above the surface should be visible throughout the surface.

Evidence 2: A sailboat coming from the sea always shows the top of the mast first and then the hull. If the earth is a plane, the mast and hull should always be above the plane and will not disappear below the horizon.

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Evidence 3: During a lunar eclipse, the shadow of the earth on the moon is always circular. We know that lunar eclipses can occur at any time, and even occur with or without an eclipse. Lunar eclipses at different times have different angles of sunlight shining on the earth. If the earth is a plane, then the shadow of the earth will only appear circular at a certain angle of illumination, and at other angles, the shadow of the earth should be an extremely flat ellipse or even almost a line.

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Of course, the first two pieces of evidence are open to debate. If the earth is not a completely flat surface, but has a spherical bulge in the middle, then the phenomenon seen by Aristotle will also occur. However, this model cannot explain why the partially curved earth does not make people feel uncomfortable, which is not as reasonable as the setting given by Aristotle below. In addition, if the whole world observes this spherical bulge continuously, then wouldn't the world still be rolled into a ball? So this argument is so exaggerated that it will end up with nothing.

Compared with the first two, the third piece of evidence is quite solid and powerful. In the 3rd century BC, another philosopher , Aristarchus, even estimated that the diameter of the earth was about three times that of the moon (the actual value was 3.6 times) by measuring the length of time the earth's shadow covered the moon during a lunar eclipse .

Quantitative calculations also include: In the 3rd century BC, Eratosthenes measured the difference in the solar elevation angle between Alexandria and Syene (now Aswan) (7°) and determined that the north-south distance between the two places was 7/360 of the earth's circumference. From this, he obtained the earth's circumference to be 39,060~40,320 kilometers, which is an amazing accuracy compared to the actual value (40,075 kilometers).

The philosophers' arguments were well-founded, and their social influence was great enough (Aristotle was the teacher of Alexander the Great), so Greece and even Europe naturally accepted the concept of a spherical earth long ago.

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However, the ancients had another question: since the earth is spherical, wouldn't the sea water on the other side of the earth flow into the sky? Wouldn't it be uncomfortable for people on the other side to walk backwards?

People did not question this very much, because Aristotle had an important assumption about the world: the earth is located at the center of the universe, and the earth and water elements that make up the world naturally tend to gather toward the center of the universe. Although this seemingly plausible theory of gravity is wrong (we now know it is not), it made sense at the time.

However, this tied the geocentric theory to the spherical theory. In the 2nd century AD, Ptolemy compiled a geocentric cosmological model to describe the movement of the sun, moon, planets, and even all the stars around the earth. In the Middle Ages, this model was regarded as orthodox by the Roman Catholic Church because it coincided with the special status of the earth in Christian theology. The geocentric theory, which supported the geocentric theory, was widely promoted in the Western world.

Evidence for the Theory of a Round Earth

Okay, so let’s believe that the earth is round, but can we travel around the world to experience it ourselves?

On September 6, 1522, a carrack named Victoria returned to Sanlúcar, Spain. Three years earlier, it had been part of a five-ship ocean-going fleet led by the flagship captain named Fernando Magellan.

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Magellan's fleet set out from Spain, crossed the Atlantic Ocean, explored the southeast coast of South America for a year, and finally entered the Pacific Ocean from the Strait of Magellan (two names, one in memory of Magellan, and the other given by Magellan himself), crossed the Pacific Ocean to the west and arrived at Guam. After several twists and turns, it set out from Timor to cross the Indian Ocean, then went around the Cape of Good Hope back to the Atlantic Ocean, and finally returned to its starting point, Spain. At this time, only the Victoria and 18 crew members were left in the fleet, and Magellan was also killed in Cebu, Philippines on the way.

To go off topic, everyone may feel sorry for Magellan's failure to complete the circumnavigation of the world himself. In fact, when he was young, he had visited Southeast Asia via the old route. So, when he revisited his old place from the Pacific Ocean, he had already circled the earth once in his lifetime.

The good friend that Di Yuan mentioned suddenly died

The feat of Magellan's fleet's circumnavigation of the world undoubtedly proved that the earth is a sphere. However, at this time, the geocentric theory, which had always been tied to the theory of the spherical earth, had problems:

In 1543, Copernicus's "On the Revolutions of the Celestial Spheres" was published.

In 1610, Galileo discovered four satellites of Jupiter , challenging the geocentric system that "all celestial bodies revolve around the earth."

In 1618, Kepler published his third and final law of planetary motion , which completely destroyed Ptolemy's geocentric model of the universe.

When we look back at this history, we will say: Well, Professor Aristotle said that "the earth element gathers towards the center of the universe", but the earth is not the center of the universe, so a new theory is needed to reexplain the shape of the earth.

Fortunately, new theories emerged quickly. In 1687, Newton published "Mathematical Principles of Natural Philosophy" and proposed the law of universal gravitation . It not only explains the shape of the earth, but also quantitatively explains the movement of all celestial bodies in the solar system. The shape of the earth finally got a better and more reassuring explanation than "the earth element gathers towards the center of the universe."

Modern Flat Earth Theory

Although the shape of the earth has become common knowledge, there are still some people who insist on believing that the earth is flat or nearly flat.

However, we should be careful to distinguish between this modern flat earth theory and the ancient flat earth theory, because the latter is due to the lack of knowledge and transportation, and has to rely on intuition. The former, based on our limited observations, seems to be due to being "too smart." Modern flat earth theorists often compare themselves to "The Truman Show", believing that the pictures, tools, and knowledge of the spherical earth are all a conspiracy agreed upon by the government, the scientific community, and the public around them to deceive them.

Because of the huge gap in basic ideas and terminology, it is almost impossible to communicate with the modern flat earth theory. However, we feel from observation that it is not that they do not read, but that they read very flexibly and can "extract" and "transform" fragments that can be used to support the flat earth theory from a very limited set of knowledge.

Let's take an example: if we say that the earth is flat, then gravity will point to the center of the world at the edge, and the flat land there is actually a cliff, and people have to lean sideways to walk on it. But the modern flat earth theory has two ways to crack our objections.

Method 1: The earth disk rotates, which dissipates the gravitational force pointing to the center. As for whether the gravitational force at all parts of this "earth disk" is equal, and whether the rotation of the edge will "throw people away", we have to pretend to be deaf and dumb and leave it to other weird theories to deal with it.

Method 2: Gravity does not exist. In fact, the entire earth disk is constantly moving upward with uniform acceleration, giving people the illusion of gravity.

Note that this explanation is somewhat "high-class" in that it is basically Einstein's thought experiment for deriving general relativity! In this thought experiment, Einstein assumes that there is an elevator that is accelerating upwards. The elevator is completely enclosed, and the people inside will not be able to distinguish between the gravitational field and acceleration. However, thought experiments are thought experiments, and when you ask them what is pulling the earth to produce acceleration, you will hear a lot of nonsense.

In addition, modern flat earthers will also list a lot of photos and so-called "experiments" to tell you that the earth is flat, NASA is a liar, and artificial satellites are fake. I will not go into details here. Friends who are interested and have time can search and take a look, and then compare the corresponding scientific explanations. You will find that those photo evidences are often misunderstandings caused by photography, optics and naked eye errors, and those so-called experiments often have many flaws in accuracy or experimental design.

Misreading the Shape of the Earth

While criticizing the flat-Earth theory, we must also be wary of misinterpretations of the shape of the Earth , because specious "knowledge" is more likely to pollute cognition than obvious fallacies.

A common misreading may come from our own elementary school geography teacher: the earth is an irregular ellipse. Everyone has heard this, right? The teacher's words are not wrong in themselves, but if the teacher does not say a few more words, we are likely to have a peach in our minds and use this peach to judge the authenticity of satellite photos.

In fact, this "ellipse" (strictly speaking, an ellipsoid) is very close to a perfect circle, with a diameter difference of only 0.3% between north and south and east and west. That is, a ball with a diameter of one meter is three millimeters shorter, and the "irregularity" is also a similar difference, which is actually invisible to the naked eye. So when we see a photo of the earth that shows a beautiful sphere, don't doubt its authenticity from this perspective.

Another type of misunderstanding is to interpret the expressions in scientific research as geometric shapes. I believe many people have seen the picture below and were impressed by the caption "This is what the Earth actually looks like! It turns out we have been deceived for so many years." In fact, this is the distribution map of the Earth's gravity field, and the same base number has been subtracted to magnify the difference.

Although "the earth is round" has become common knowledge among primary school students, when we look at the process of our understanding of the earth from the perspective of scientific history, we will find that it is not simple at all, let alone taken for granted. Looking back on the whole process, we can see how humans constantly put forward hypotheses to explain our world, and how they constantly overturn old hypotheses and put forward new hypotheses based on observations and experimental results, and finally keep getting closer to the facts.

Planning and production

Author: Qu Jiong, popular science creator

Review丨Han Wenbiao, Researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Planning丨Ding Zong

Editor: Ding Zheng and Cui Yinghao