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Astronomy# Eratosthenes

## Who was Eratosthenes?

## Biography of Eratosthenes

## Contributions and inventions

## Eratosthenes screen

## Measurement of the earth

## Importance

## Eratosthenes works

## Phrases

**Eratosthenes** is considered to be the **first scientist** in world history who managed to measure with excellent precision the **circumference** of our **earth** at a time when people thought that the world instead of being round was flat.

**Personal information**

**Born**: 276 B.C.**Birthplace**: Cyrene, Libya**When he died:**194 B.C.**Where he died**: Alexandria, Egypt

Eratosthenes was an important Greek **mathematician** and **astronomer**, who for many decades was director of the **Alexandria** library and was recognized as one of the most important minds of his time.

Cyrene Eratosthenes was born in 276 B.C. in **Cyrene**, North Africa, and died in 194 B.C. in **Alexandria**, Egypt. He was a disciple of the Greek poet Callimachus of Cyrene, and the grammarian Lysanias, director of the **Library of Alexandria** and friend of **Archimedes**. His enemies called him the “**Beta**“, because according to what they thought, he was the second best in everything. Those who knew and appreciated him, however, called him the “**Alpha**,” because they knew he was first in almost everything.

Eratosthenes ended his days dying of **voluntary starvation** at the age of 80, because he was extremely **depressed** because he had lost his sight.

One of the main contributions of Eratosthenes to **science** and **astronomy** was his important work regarding the **measurement** of the **earth**. Eratosthenes, studying the papyri of the Alexandria library, found a report in which it was said that when the sun’s rays fall on a rod during the midday of the **summer solstice** no **shade** was produced.

He correctly assumed that, if the Sun was located at a great distance, when its rays reached the earth, they should reach it in parallel, supposing that the earth was flat as it was considered at that time and therefore, there should be differences between the shadows projected by objects at the same time of the same day, regardless of the place. However, by proving that this did happen, he deduced that the earth **was not flat** and using the known **distance** between the two cities and the measured angle of the shadows, he calculated the **circumference** of the **earth** in approximately 250 stadiums, about 40,000 kilometers, an estimate that was almost close to reality.

Other of his contributions was that he was able to calculate the **distance to the Sun** in 804. 000. 000 **stadiums** and the distance to the **Moon** in 780. 000 stadiums. He was able to measure the inclination of the ecliptic quite well and with great precision at 23º 51′ 15″.

Another of his important astronomical works was a **catalogue** of about 675 **stars**. He also created one of the most advanced **calendars** for his time and a **chronological history** of the world since the beginning of the Trojan War.

He researched **geography** by drawing **maps** of the known world, large stretches of the **Nile River** and described the region of **Eudaemony**, which we now know as Yemen in Arabia. He also invented the **Eratosthenes sieve**.

Screening is a method that allows us to find all **prime numbers** smaller than a given natural “**N**” number. This algorithm could be summarized as follows:

- You should start with the
**number 2,**highlight the number 2 as prime, but cross out all**multiples**of 2 (i.e. cross out 4, 6, 8, etc.). - Then we continue with the next number that is not crossed out in the table, the
**number 3**, we highlight the number 3 as**prime**and we**cross out**all multiples of 3 (i.e. we crossed out 6, 9, 12, etc.). - The next number that is not crossed out in the table is the
**number 5,**so we must highlight it as**prime**and cross out all**multiples**of 5 (i.e. we crossed out 10, 15, 20, etc.). - This process must be
**repeated**until we reach the number N, having previously crossed out all the multiples of the prime numbers found.

In order to take **measurements** of the **earth**, he invented and used a **trigonometric** method, different notions of **latitude** and **longitude** that had already been introduced in the area of science, apparently by **Dicearco**, so he deserves the title of father of **geodesy**.

His importance in the modern world is not only based on the fact that he made important progress in **geometry**; he was responsible for the “**Eratosthenes sieve**“, a system for determining prime numbers. He was one of the most important **geographers** and **mapped** the **Nile River** to Khartoum, in which he showed its two Ethiopian tributaries, and gave the first satisfactory explanation as to why the Nile River was flooded. In addition, he elaborated a **calendar** with **leap years**, elaborated a small **stellar catalogue**, was a **poet** and wrote treatises on **Ethics**.

Among his most important works we can mention the following:

**Works by Erygone**: was the best known and was an astronomical and geographical poem that deals with the Earth’s shape, its temperature, different climates and constellations.**Treatises on moral philosophy**and other philosophical works. All his works were closely linked to mathematics.**Chronography**: in this work, he manages to compile the dates of the most important**literary**and**political**events to date; it is believed that the Olympics, cited by Diogenes Laertius and Athenaeum, were part of the Chronography.**Treatise on the ancient comedy attica**: of it are fragments**Arjitectonic**and**Skenographic**in which he spoke of the decoration, the wardrobe, the declamation and the plot of works by**Aristophanes**and**Cratino**among others.- He studied
**Homer’s work**and wrote a**biography**of the poet’s life that has not reached our days.

There are no recognized Eratosthenes phrases, but there are two phrases written by Carl Sagan that reflect his essence:

*“Eratosthenes had no***tools**other than sticks, eyes, feet and head and a great desire to experiment with these tools correctly deduced the**circumference**of the**earth**with enormous precision and a minimum percentage of error”.*“Eratosthenes criticized***Aristotle**for his blind**chauvinism**, believing that in all countries, there were good and bad things”.

Written by Gabriela Briceño V.

Briceño V., Gabriela. (2019). *Eratosthenes*. Recovered on 8 November, 2022, de Euston96: https://www.euston96.com/en/eratosthenes/