Is it really so hard to recognize when someone looks 'round'? Two or three things to know about the Earth's horizons

"Is the Earth round or not?" This question has been a popular topic from the past to the present.

Flat Earth? A round Earth?

Although the idea of a flat Earth is a shock to most people in Taiwan, there is a group of crazy people in the real world who believe that a round Earth is a NASA lie, and who have gone so far as to call out "Don't be fooled by the scientists" and to create the so-called Flat Earth Society, which organizes grand discussions of the idea.

Flat Earth proponents believe that the Earth actually looks like a CD. Pixabay

For these flat Earth proponents, what does the Earth look like? In fact, many proponents of the theory strongly argue that "the Earth is like a CD," with the North Pole at the center of the CD and the South Pole surrounding the outer rim of the CD.

In the 21st century, with its rapid advances in science and technology, the question of whether the Earth is round or flat is one that has long been known and answered by modern people who have had the benefit of compulsory education and a basic knowledge of the sciences, and so only a very small number of people are currently convinced by the geodesic argument.

In ancient times, however, it would have been difficult to imagine that the Earth we are on was a round ball!

"If the Earth is round, why don't we fall into the endless universe?"

If the Earth is round, why don't we fall into the endless universe?

"If the Earth is round, why don't we fall into the endless universe?

Ancient man was not stupid! All the signs of a round Earth

But in fact, as early as Ancient Greece, there were many different ways of proving that the Earth was round, and the idea became known as "geocircularity".

Scholars have also gradually come up with a lot of evidence for this theory, for example, by observing a ship approaching a harbor in the distance. If the Earth is flat, people should be able to see the whole ship slowly coming into the harbor, but when the ship is coming in, people see the stem of the ship first, and then see the ship's hull gradually.

There are a number of ancient scholars who supported and endorsed the idea of the Earth's roundness by observing various signs of life.Pixabay

When a lunar eclipse occurs, the shadow of the Earth blocking the Moon appears to be a circular arc, and it is assumed that the Earth should also be circular, and the caravans that traveled at various latitudes also found that the angle of elevation of Polaris is higher and higher as they traveled to the high latitudes in the north and the position of constellations changed from one latitude to the next. The position of the constellations changes from one latitude to another.

All of this evidence led ancient people to believe more and more in geodesy, and to accept that the Earth was in fact a round ball. The ancient Greek scholar, Eratosthenes, used the geodesic theory as a basis for successfully calculating the circumference of the Earth!

Two thousand years ago, they figured out the circumference of the Earth!

Eratosthenes was an ancient Greek scholar of mathematics, astronomy, and geography who lived in the city of Alexandria.

The ancient Greek scholar Eratosthenes. Wikipedia

There was a city of Saini, south of Alexandria, and at the summer solstice (when the sun was directly above the Tropic of Cancer), the sun would be just above Saini, and its rays would go straight into one of the city's wells, and when Eratosthenes heard about this, he came up with the idea of measuring the circumference of the Earth.

Relative positions of the city of Saini and Taiwan, China, on a map.

At noon on the day when the sun was directly above the Tropic of Cancer, Eratosthenes set up a vertical stick in Alexandria, and measured the angle of the sun on Alexandria according to the shadow of the stick. wooden stick in the middle by an angle of about 7.2 degrees.

Since the Sun is far, far, far away from the Earth, we can think of the Sun's rays as parallel rays, and when the Sun's rays hit the stick erected in Alexandria at an angle of 7.2 degrees, we can see by extrapolating that the angle from Alexandria to the center of the Earth and the angle of the city of Saini to the center of the Earth also make an angle of 7.2 degrees (the concept of the angle of an internal error in mathematics).

The extension from Alexandria to the center of the earth also makes an angle of 7.2 degrees with the extension from Cyrene to the center of the earth.

In other words, the length of the arc between Alexandria and Cyrene is 7.2 degrees, and the true circumference of the Earth can be calculated as a ratio by knowing the distance between Alexandria and Cyrene.

Eratosthenes asked the camel caravans that traveled between the two places, and simply estimated that the distance from Alexandria to Cyrene was about 800 kilometers, and since 800 kilometers corresponds to 7.2 degrees, and since the Earth is a sphere, it should be 360 degrees, Eratosthenes used the ratio to figure out that "the total length of the Earth should be about 40,000 kilometers".

From then on, through the camel and shadow, Eratosthenes became the first person to measure the circumference of the Earth, and the circumference of the Earth measured by Eratosthenes 2,000 years ago is very close to the circumference of the Earth that we measure today.

Two thousand years ago, Eratosthenes figured out the circumference of the Earth! The Chinese view of the universe as a "circle of heaven and a circle of earth"

The Western theory of the circle of the earth led Eratosthenes to figure out the circumference of the Earth, while in the East, two different doctrines were developed - Gaianism and Huntianism. .

Both the Gaiten theory and the Huntian theory believe that "the sky is round and the earth is square". The biggest difference between the two is that

although they have opposite concepts of the sky, they both believe that the ground on which we are standing is flat and not a sphere.

In the debate between the Gaitian theory and the Huntian theory, who won? It wasn't until the Tang Dynasty, when the astronomer Sang Sung conducted astronomical measurements to revise the calendar, that he succeeded in establishing the status of the Hun-Tien theory!

At that time, the calendar used by the Tang Dynasty showed that there should be a solar eclipse on a certain day in September, but the results of the observation were very different from the calendar, so Emperor Xuanzong of the Tang Dynasty decided to correct the errors in the calendar, and the work of correcting the calendar was given to Sang and his party.

The calendar has a lot to do with the rotation of the Earth around the sun, for example, the 24 solar terms, which are often heard today, are determined by the angle of elevation of the sun at noon, so the monk and his party decided to measure the changes in the length of the shadow of the pole when the sun shines on the ground at noon in various parts of China, and to use the angle of the sun's irradiation and the shadow changes to deduce the correct time of the calendar.

The monk and his party wanted to use the angle of the sun to calculate the correct calendar time. pixabay The first national geodesy in Chinese history!

First, the monks identified several observation points, from Mongolia in the north to Vietnam in the south, and began to measure the length of the sun's shadow at noon on different days at each point.

In ancient times, there was a saying that "one inch of sunlight makes a thousand miles of difference", so most Chinese people at that time believed that when the difference in the length of the sun's shadow between two places was one inch (about 2.5 centimeters today), the difference in the distance between the two places would be 1,000 kilometers (about 250 kilometers today).

However, as the monk and others continued to make measurements and calculations, he realized that the measurements were not consistent with the saying that "one inch of sunlight makes a difference of 1,000 kilometers", and that the difference in the length of the pole's shadow was not necessarily one inch when the distance between two places was 1,000 kilometers.

The error in the measurement of the sun's shadow is caused by the theory of the earth's circle and the theory of the earth's level. Courtesy of the author

Why did the ancient people circulate such a wrong argument that the sun's shadow is one inch, and the earth is a thousand miles away? The reason is that at that time, both the Gaiten theory and the Hundred Days theory believed that the earth was flat! In the round-earth view of the universe, ancient people believed that different locations would receive sunlight from different angles, and that the length of the shadow of different locations would become longer as they got further away (right side of the image above).

But in reality, the key to the difference is that the Earth itself is a tilted sphere, and it is the direct light from the distant Sun that makes the angle of the Sun's rays vary over time, and produces different lengths of shadow in different places!

Since "an inch of sunlight makes a thousand miles of difference" comes from the erroneous basis of the theory of the earth's horizon, it certainly does not match the results of the measurements made by the monks and others!

However, while Sang and others disproved the past, they did not know why.

He was not only a monk, but also an astronomer

In addition to disproving the past, Sang and the others also found other patterns!

At the time, scholars already had a foundation of knowledge that the starry night sky changes when seen from place to place, so he not only measured the length of the pole's shadow, he also began to measure the "elevation of the North Star" when seen from place to place.

Polaris elevation is the angle at which one looks up at the North Star from the ground, like the concept of latitude today, and the monk was curious about whether there was a correlation between changes in the elevation of the North Star in each place (i.e., the latitude of each place) and the distance between the two places.

Through calculations, Sang and others found that when two places are separated by about 130 kilometers on the same meridian, there is about a 1-degree difference in the height of the North Star, which means that when the latitude of two points on the same meridian differs by 1 degree, the distance is about 130 kilometers apartNote 1!

Sang and his team figured out the length of the meridian for a one-degree difference in latitude, i.e., the change in distance from north to south on the meridian for a one-degree change in latitude.Wikipedia Figured out the length of the meridian, but then what?

With the continued precision calculations and subsequent astronomical observations by Sheng Xing and others, they eventually succeeded in compiling the Dayan Calendar for the Tang Dynasty, which not only solved the calendar error, but also made the Dayan Calendar an important foundation for subsequent calendars.

Wait! That's it? They only measured 1 degree of the meridian?

And then, nothing more. Monk and the others only got one degree of the meridian, they didn't go on to figure out the circumference of the Earth. In the final analysis, the main reason why they stopped at this point was that the Orientals were practicing the "Hun-Tien theory" rather than the "Earth-circle theory".

Because of this, they were stuck with the ancient oriental cosmology and could not go any further than Eratosthenes, who thought that the ground was flat, and they could not get to the circumference of the earth, as they did.

Gradually escaping the grip of the geodesic theory

During the Ming Dynasty, although Chinese scientists finally came up with a concept that was "close" to the geodesic theory, it was still difficult to escape from the influence of the geodesic theory, and the concepts they came up with leaned towards the "terrestrial sphere" view: the Earth was barely recognized as round, but the oceans around it were still flat and surrounded the Earth.

In other words, although the Ming scientists recognized that the Earth's surface was a "round sphere", the core idea behind it was closer to the geodesic view, which barely recognized that the Earth was spherical, but considered the oceans to be planar, with the oceans occupying the lower half of the universe, and the land floating on top of the planar oceans.

The concept of a "round Earth" gradually began to enter the hearts of the Chinese in the late Ming Dynasty. Pixabay

It was not until the end of the Ming Dynasty that the concept of a "round Earth" was accepted in the East, after Western missionaries gradually brought Western geodesy and maps to the East, and people around the world began to believe that the Earth was a round sphere, through the evidence that scientists had presented!

Whether it was Eratosthenes who succeeded in calculating the circumference of the Earth, or Monk and his party who came up with the length of the meridian in one degree, in fact, science is an evolving process, in which scientists are constantly discovering, experimenting, making mistakes, and then discovering again, and jumping out of previously unquestioned concepts, and pulling out the threads and developing new knowledge.

From the surface of the earth to outer space, to the entire solar system, how much effort has mankind gone through?Pixabay

For us in the 21st century, "Earth" is just common sense, and the length of the Earth's circumference and the system of latitude and longitude coordinates are real numbers and basic concepts in textbooks, but now we know that behind all these seemingly taken-for-granted things, there are in fact hundreds or thousands of years of hard work and effort. But now we know that behind these seemingly "taken-for-granted" concepts are hundreds or thousands of years of hard work and trekking.

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