Having acknowledged by now without a shadow of a doubt that our planet is not flat, it is normal to have the curiosity and the need to know when measuring the **Circumference of the Earth**. It is not a question that has come up recently, it has been for many centuries that men have wanted to know the exact size of our planet and year after year they are looking for the best method to make an estimate.

Our predecessors made a lot of progress in this regard, but it must be said that from the very beginning, the first scholars who dealt with it had brilliant intuitions. In light of the measurements made today, with** modern and sophisticated tools,** we can say that they did a great job. Their estimate is very close to that made today, millennia later.

The figure that, in this calculation of the **Circumference of the Earth**, gave a real turning point, is the well-known Eratosthenes. We will get to know him personally, discovering how he did his best to give answers to his questions.

## Circumference of the Earth: modern measure

With the modern "powerful means" we are endowed with, today it is possible for us to measure our planet much more easily than then, in the days of **Eratosthenes**. With the advanced tools at our disposal we were able to get a number, for the **Circumference of the Earth**, very similar to what scholars of yore left us. It is surprising to see how brilliant they were to make such a precise measurement even before the year zero and with the devices of the time.

But let's get to the point, since we have not yet said how much the Earth measures. Its circumference is 40,009 Km. Just to get an idea, **Eratosthenes** he had calculated in his own way a number that deviates from this by a little, less than 700 kilometers.

## Circumference of the Earth: Eratosthenes

Eratosthenes, **Eratosthenes of Cyrene**, is a personality whose name is closely and inextricably linked to the measurement of the circumference of the Earth. He lived a long time ago, **between the III and the II century BC**. dedicating much of his time to scientific and geography studies. He is in fact a geographer.

Let's take a closer look at what brilliant method this is **geographer** it was invented to be able to get where we, centuries later, have arrived, with much more advanced means. It all begins with a well, a well located in the city of Sirene, in Egypt, where the sun was reflected on June 21 of each year, at noon. Eratosthenes did four maths and understood that this fact meant that at that exact moment the Sun was on the exact vertical of that place.

In Alexandria, on the same day and at the same time, the sun was not vertical but inclined. With a geometry manual in hand, ours is insightful **Eratosthenes** what did he do? He measured the angular distance between the sun and the zenith, i.e. the amplitude of the angle between the vertical and the direction of the sun, obtaining the measurement corresponding to just over 7 ° or the angular distance between Alexandria and Sirene.

With these figures he was able to carry out the **calculation of the circumference of the Earth** using nothing more than a proportion between the angular values and those of the linear distance between the two cities. This is the "magic" formula:

360 °: 7 ° = circumference Earth: linear distance (known) between Alexandria and Sirene. Exactly in this way, and without the aid of any computer, Eratosthenes came to say that the earth's circumference measured about 257 stages, or **39,375 kilometers.**

## Circumference of the Earth and radius of the Earth

By definition, the** radius of the Earth** is the distance of the center of the Earth from its surface to mean sea level. This size can vary depending on where the measurement takes place because it is now known that the shape of our planet is not perfectly spherical. We can say that we live on the surface of a **ellipsoid of rotation** which is flattened at the North and South Pole, in the jargon we also call it oblate spheroid or geoid. This has led us to have various versions of "Earth's radius", depending on the method used to measure this magnitude.

There are the equatorial radius, the polar radius, the mean square and the mean. The **equatorial radius** is in fact the radius of the imaginary circumference such as the equator and measures approximately equal to 6378.388 kilometers, that **polar** it is nothing more than the distance of the center of the Earth from one of the two poles and measures approximately 6356.988 kilometers.

When we go to evaluate the **root mean square** of an ellipsoid we use a more accurate method and formulas, obtaining the value of 6373.044737 kilometers for the Earth. There remains the **medium radius,** that is, the one calculated by averaging the center-surface distances of all points on the globe. With this method we get the figure of 6371.005076123 kilometers.

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