Greek scientist

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External Websites

- APSNews - This Month in Physics History - June, ca. 240 B.C. Eratosthenes Measures the Earth
- Khan Academy - Eratosthenes of Cyrene
- World History Encyclopedia - Eratosthenes
- CORE - Eratosthenes on the “Measurement” of the Earth
- Planet Facts - Biography of Eratosthenes
- Famous Scientists - Biography of Eratosthenes
- Academia - Eratosthenes of Cyrene: Biography & Work as a Mathematician

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Articles from Britannica Encyclopedias for elementary and high school students.

- Eratosthenes - Student Encyclopedia (Ages 11 and up)

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External Websites

- APSNews - This Month in Physics History - June, ca. 240 B.C. Eratosthenes Measures the Earth
- Khan Academy - Eratosthenes of Cyrene
- World History Encyclopedia - Eratosthenes
- CORE - Eratosthenes on the “Measurement” of the Earth
- Planet Facts - Biography of Eratosthenes
- Famous Scientists - Biography of Eratosthenes
- Academia - Eratosthenes of Cyrene: Biography & Work as a Mathematician

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

- Eratosthenes - Student Encyclopedia (Ages 11 and up)

Also known as: Eratosthenes of Cyrene

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Article History

Eratosthenes' method of measuring Earth's circumference

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- In full:
- Eratosthenes of Cyrene

- Born:
- c. 276 bce, Cyrene, Libya

- Died:
- c. 194 bce, Alexandria, Egypt

- Subjects Of Study:
- Earth
- circumference
- sieve of Eratosthenes

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Top Questions

What were Eratosthenes’ major achievements?

What were Eratosthenes’ major achievements?

In addition to calculating Earth’s circumference, Eratosthenes created the Sieve of Eratosthenes (a procedure for finding prime numbers), tried to fix the dates of literary and political events since the siege of Troy, and is thought to have created the armillary sphere (an early astronomical device for representing the great circles of the heavens).

What is Eratosthenes famous for?

What is Eratosthenes famous for?

Eratosthenes measured Earth’s circumference mathematically using two surface points to make the calculation. He noted that the Sun’s rays fell vertically at noon in Syene (now Aswān), Egypt, at the summer solstice. In Alexandria, also in Egypt, at the same date and time, sunlight fell at an angle of about 7.2° from the vertical.

How did Eratosthenes die?

How did Eratosthenes die?

Eratosthenes died in his 80s in Alexandria, Egypt. He had become blind in his old age and could no longer work by 195 BCE.He reportedly fell into despair, and he is said to have committed suicide by voluntary starvation in 194 as a result.

**Eratosthenes** (born c. 276 bce, Cyrene, Libya—died c. 194 bce, Alexandria, Egypt) was a Greek scientific writer, astronomer, and poet, who made the first measurement of the size of Earth for which any details are known.

At Syene (now Aswān), some 800 km (500 miles) southeast of Alexandria in Egypt, the Sun’s rays fall vertically at noon at the summer solstice. Eratosthenes noted that at Alexandria, at the same date and time, sunlight fell at an angle of about 7.2° from the vertical. (Writing before the Greeks adopted the degree, a Babylonian unit of measure, he actually said “a fiftieth of a circle.”) He correctly assumed the Sun’s distance to be very great; its rays therefore are practically parallel when they reach Earth. Given an estimate of the distance between the two cities, he was able to calculate the circumference of Earth, obtaining 250,000 stadia. Earlier estimates of the circumference of Earth had been made (for example, Aristotle says that “some mathematicians” had obtained a value of 400,000 stadia), but no details of their methods have survived. An account of Eratosthenes’ method is preserved in the Greek astronomer Cleomedes’ *Meteora*. The exact length of the units (stadia) he used is doubtful, and the accuracy of his result is therefore uncertain. His measurement of Earth’s circumference may have varied by 0.5 to 17 percent from the value accepted by modern astronomers, but it was certainly in the right range. He also measured the degree of obliquity of the ecliptic (in effect, the tilt of Earth’s axis) and wrote a treatise on the octaëteris, an eight-year lunar-solar cycle. He is credited with devising an algorithm for finding prime numbers called the sieve of Eratosthenes, in which one arranges the natural numbers in numerical order and strikes out one, every second number following two, every third number following three, and so on, which just leaves the prime numbers.

Eratosthenes’ only surviving work is *Catasterisms*, a book about the constellations, which gives a description and story for each constellation, as well as a count of the number of stars contained in it, but the attribution of this work has been doubted by some scholars. His mathematical work is known principally from the writings of the Greek geometer Pappus of Alexandria, and his geographical work from the first two books of the *Geography* of the Greek geographer Strabo.

After study in Alexandria and Athens, Eratosthenes settled in Alexandria about 255 bce and became director of the great library there. He tried to fix the dates of literary and political events since the siege of Troy. His writings included a poem inspired by astronomy, as well as works on the theatre and on ethics. Eratosthenes was afflicted by blindness in his old age, and he is said to have committed suicide by voluntary starvation.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Encyclopaedia Britannica.