Eratosthenes | Biography, Discoveries, Sieve, & Facts (2024)

Greek scientist

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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?

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?

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.

Britannica QuizAll About Astronomy

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.

Eratosthenes | Biography, Discoveries, Sieve, & Facts (2024)

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What discoveries did Eratosthenes determine? ›

Eratosthenes was also the first to calculate the tilt of the Earth's axis, which he figured with remarkable accuracy; the finding was reported by Ptolemy (85-165 CE). Eratosthenes also calculated the distance from the Earth to the Moon and to the Sun, but with less accuracy. He made a catalog of 675 stars.

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What was the Sieve of Eratosthenes observation? ›

Given a number n, print all primes smaller than or equal to n. It is also given that n is a small number. The sieve of Eratosthenes is one of the most efficient ways to find all primes smaller than n when n is smaller than 10 million or so.

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Who was Eratosthenes for kids? ›

The Greek scientist Eratosthenes was the first person to calculate Earth's circumference. He worked as chief librarian of the Alexandrian Library in Egypt and was also known as a writer, an astronomer, a mathematician, and a poet.

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What is Eratosthenes' full name? ›

Eratosthenes of Cyrene (/ɛrəˈtɒsθəniːz/; Greek: Ἐρατοσθένης [eratostʰénɛːs]; c. 276 BC – c. 195/194 BC) was a Greek polymath: a mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria.

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What is the Sieve of Eratosthenes history? ›

The 3rd-century bc Greek scientist Eratosthenes of Cyrene developed a systematic procedure for finding prime numbers that is known as the sieve of Eratosthenes. Prime numbers are not divisible by an integer greater than 1, except themselves. The procedure results in all the natural numbers (1, 2, 3…)

Who invented the sieve? ›

CE book which attributes it to Eratosthenes of Cyrene, a 3rd cent. BCE Greek mathematician, though describing the sieving by odd numbers instead of by primes. One of a number of prime number sieves, it is one of the most efficient ways to find all of the smaller primes.

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How did Eratosthenes measure the Earth? ›

Eratosthenes method was very simple; he measured the length of a shadow from a vertical stick of a known height in two cities on the same day. The ratio between the north-south distance between the two cities and the angles measured gave a ratio which allowed Eratosthenes to calculate the size of the Earth.

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Was Eratosthenes the first person to use the word geography? ›

The first recorded use of the word geography was by Eratosthenes, a Greek scholar who lived from 276–to 194 BC and is credited with creating the discipline of geography. The word 'geography' originates from two Greek words.

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Who is Eratosthenes father of geometry? ›

Eratosthenes of Cyrene, the ancient Greek mathematician, geographer, astronomer, poet and one of the most educated scholars, who lived from 276 to 194 BC, is known as the Father of Geography particularly for his most renowned achievement is the accurate calculation of the Earth's circumference.

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What are some interesting facts about Eratosthenes? ›

Eratosthenes was an ancient Greek mathematician, geographer, and astronomer. He existed from 276 B.C. until 194 B.C. and is most notable for having the first accurate calculation of the Earth's circumference. He lived and worked in the Egyptian city of Alexandria for much of his life.

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Was Eratosthenes Greek or Egyptian? ›

Eratosthenes was a Greek mathematician who is famous for his work on prime numbers and for measuring the diameter of the earth.

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How old is Eratosthenes crater? ›

The crater is believed to have been formed about 3.2 billion years ago. At low Sun-angles, this crater is prominent due to the shadow cast by the rim. When the Sun is directly overhead, however, Eratosthenes visually blends into the surroundings, and it becomes more difficult for an observer to locate it.

What did Eratosthenes accurately measure? ›

The Earth's circumference was first accurately measured more than 2,200 years ago by a Greek astronomer named Eratosthenes. Eratosthenes method was very simple; he measured the length of a shadow from a vertical stick of a known height in two cities on the same day.

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How did Eratosthenes define geography? ›

While serving as the head librarian and scholar at the Library of Alexandria, Eratosthenes wrote a comprehensive treatise about the world, which he called "Geography." This was the first use of the word, which in Greek means "writing about the world." Eratosthenes' work introduced the concepts of torrid, temperate, and ...

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Who is the father of mathematics? ›

Archimedes is widely regarded as one of the greatest mathematicians in history, earning him the title of the "Father of Mathematics." Born in Syracuse, Sicily, in 287 BC, Archimedes was a polymath who made significant contributions to a wide range of fields, including mathematics, physics, engineering, and astronomy.

How did Eratosthenes know what time it was? ›

He watched the shadow of the gnomon on the day in question, and measured the length of the shadow at noon. The height of the stick and the length of the shadow allowed him to calculate the angle between the vertical at Alexandria (as indicated by the gnomon) and the direction of the Sun's rays at noon.

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