Ask Ethan: How do we know that the Earth is not flat?

Periodically, a hoax or statement is announced on the Internet, which, despite the surprising absurdity, begins to spread at the speed of the virus, and even the most cautious people may be drawn into this process. But science does not cease to be true, even if someone does not want to accept it. Any scientific question is beautiful as follows: if you have reached a sufficient expert level in it, you can find the answer yourself. This opportunity to convince oneself of the truth, despite the fact that any authority can claim, separates science from all other activities. It is on this occasion that our reader asks a question with a friend rejecting the round Earth:

I have a friend who has been believing and telling everyone about the theory of “flat earth” for several years. I do not believe that our or any other planet is flat, but what do you think about such a theory?

The great thing is that this is not my faith or belief - this is a logical conclusion.



Of course, it will be easiest for us to go into space and photograph the Earth. Since the 1940s, our rockets have been able to climb high enough to directly photograph the curvature of the Earth. Moving farther, from suborbital heights to low Earth orbits and further, to enormous distances from the Earth, we could obtain its images of ever-increasing quality, and as a result we could see it in its entirety. We even visited other worlds of the solar system, using the gravitational influence of the Earth and other worlds to get on them. The NASA Messenger probe used this technique several years ago, and took several series of photographs flying past Earth, capturing our circular and spinning planet.



But until the 1940s, it was impossible to determine the shape of the Earth by simply leaving it and looking at it from the side. It may seem that it is impossible to answer this question without the help of modern technologies, just as it is impossible to find out the color of your eye without seeing reflection, photographs, or without other sources of information. But we have some evidence of a certain form of our planet, which does not require leaving it and taking a photo. Consider, for example, a lunar eclipse - when the Earth passes between the Sun and the Moon.


During incomplete phases of a lunar eclipse, the shadow of the Earth can be seen on the surface of the Moon, and it is quite clear that it casts a round shadow.

Looking at the Moon during an ideal or almost perfect location, we see a shadow from the Earth on the Moon. And it doesn’t matter when the eclipse occurs, at what time of year, in which part of the orbit around the Sun the Earth is located, or from what places of the Earth you can see the eclipse - the shadow always says one thing: the Earth’s shadow is round, and its physical size is much larger than the moon. You might argue that the Earth may be flat or round, rather than spherical, but what we observe as the Moon and stars rise at different times in different places (at different longitudes), and that around the globe you can Traveling makes the flat disk option impossible.



But suppose you don’t want to rely on the technology of the 16th century, you don’t know anything about heliocentrism, the existence of the moon, or even something about geography outside the tiny region where you live. If you want - believe it or not - but it was in such a situation that Eratosthenes of Kirensky appeared about 2300 years ago, when he became the first person to not only conclude that the Earth is round, but also to measure its size! Living in Alexandria, Eratosthenes received amazing letters from the city of Siena (now Aswan) in southern Egypt. In particular, the letters stated that the time of the summer solstice

the shadow of a man who looked into a deep well at noon overlaps the reflection of the Sun.

In other words, at this time, the Sun is directly above your head, not a degree to the south, north, east or west. And if you take an absolutely vertical object, it will not cast any shadow. In the world there really are such places.




Noon lahine in Hawaii, where twice a year objects cast a vertical shadow.

But Eratosthenes knew that in Alexandria, where he lived, this did not happen. The sun, of course, comes closer to the zenith at noon of the summer solstice than at any other time of the year, but vertical objects still cast some shadows. And, like any good scientist, Eratosthenes conducted an experiment. By measuring the length of the shadow cast by a vertical stick during the summer solstice (when the shadow was minimal), he was able to find out at what angle the sun's rays fall on Alexandria relative to the vertical.



He got 1/50 of a circle, or 7.2 degrees. But at this time in Siena, the Sun made an angle of zero degrees with a vertical wand! What could have caused this? During a brilliant insight Eratosthenes realized that the rays of the Sun can be parallel, and the surface of the Earth - curved!

If the Earth were perfectly flat, then the rays of the Sun would cast identical shadows at noon on the day of the summer solstice throughout the Earth, regardless of your location. But if the Earth’s surface is curved, the Sun in different places will cast different shadows on the same day, depending on the angle at which the rays fall on the object. By measuring the difference in the angle of the shadow between two points on the surface of the Earth, for the first time it became possible to calculate the size of the Earth.

If Eratosthenes could know the distance between Alexandria and Siena, knowing the angular difference between the two cities, one could calculate the circumference of the Earth! If he had his own graduate student, he would send him on a journey to measure distance! But he had to rely on other people's messages about the distance between cities. And the most accurate method of measuring that time was ...



Camel Travel Therefore, criticism of its accuracy can be understood. However, he got that the distance between Siena and Alexandria is 5,000 stadia. The question is the length of the stage. The answer depends on whether Eratosthenes, the Greek living in Egypt, used the Attic or Egyptian stages - and historians still argue about this. The Attic stage, more frequently used, was 185 meters. Using it, you can get the Earth's circumference of 46,620 km, which is only 16% more than real. But the Egyptian stages are equal to 157.7 m, and it is quite possible that it was precisely what Eratosthenes had in mind. And in this case, we get a value of 39375 km, which is only 2% different from the current value of 40,041 km!

Regardless of specific values, Eratosthenes became the first geographer in the world, invented the concepts of latitude and longitude that we still use today, and built the first models and maps based on spherical Earth. And although much has been lost over the past one and a half millennia, ideas about the sphericity of the Earth and the approximate size of its circumference have not disappeared. Today, anyone can repeat the same experiment with two places located on the same longitude - just moving north or south from their place - and using the same measurements of shadow lengths measure the circumference of the Earth. So, Michael, you do not need to take my word for it, invite your friend and do the experiment yourself. If you are ready to listen to what the Universe tells you about yourself, and you will spend time on the exact measurement and you will not be mistaken in geometry,