Eratosthenes

Eratosthenes was a Greek astronomer, geographer, and mathematician who lived from 276-194 BC. He is most famous for making the first accurate measurement of Earth's circumference.
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Eratosthenes was an ancient Greek astronomer, geographer, and mathematician. He lived from 276 to 194 B.C. Eratosthenes is most famous for making the first accurate measurement of the circumference of the Earth. He lived and worked for most of his life in the city of Alexandria in Egypt.

Eratosthenes was born in 276 B.C. in the city of Cyrene which is in the modern-day country of Libya. He apparently studied for a few years in Athens. He studied and spent most of his adult life in Alexandria, where he died in 194 B.C. at the age of 82 years. Some sources claim that Eratosthenes intentionally starved himself to death after going blind. He never married.

Eratosthenes is most famous for making the first accurate measurement of Earth's circumference sometime around 240 B.C. He knew that at noon on the summer solstice the Sun was directly overhead in the Egyptian town of Syene, for there was no shadow at the bottom of a well in Syene. By measuring the length of the shadow of a tall tower, on the solstice, in his home city of Alexandria, and by determining the distance between Syene and Alexandria (about 800 km), he was able to calculate the circumference of the Earth. There is some historical uncertainty regarding the actual size of the distance units Eratosthenes used (called "stadia"), so we are not absolutely certain how accurate his results were. He may have been correct to within 1%, or he may have been off by about 16%. Either way, Eratosthenes was the first to make a reasonably accurate determination of the size of our home planet!

Eratosthenes became the second head librarian of the famous Library of Alexandria in 236 B.C. Besides his determination of Earth's circumference, Eratosthenes also made several other important contributions to the fields of mathematics, geography, and astronomy. He devised a mathematical technique, now referred to as the Sieve of Eratosthenes, that provides a straightforward way to find all prime numbers up to a specified integer. He compiled one of the best maps of his day of what he thought was the whole world, although it actually only shows the area around the Mediterranean and parts of Asia. Eratosthenes devised a system of latitude and longitude, and a calendar that included leap years. He invented the armillary sphere, a mechanical device used by early astronomers to demonstrate and predict the apparent motions of the stars in the sky. He may have measured the distances from Earth to both the Moon and to the Sun, but the historical accounts of both deeds are rather cryptic. Eratosthenes also compiled a star catalog that included 675 stars.

A crater on Earth's Moon is named after Eratosthenes, as is a seamount in the eastern Mediterranean Sea.


How did Eratosthenes measure the circumference of Earth?

Eratosthenes

Eratosthenes was a Greek scientist who lived from 276-194 BC. He studied astronomy, geography, and math. He is famous for making the first good measurement of the size of the Earth.
Click on image for full size (83 Kb)
Public domain image.

Eratosthenes was a Greek scientist who lived from 276 to 194 B.C. He studied astronomy, geography, and math. Eratosthenes is famous for making the first good measurement of the size of the Earth. He spent most of his life in the city of Alexandria in Egypt.

Eratosthenes was born in 276 B.C. in the city of Cyrene which is in the modern-day country of Libya. He studied for a few years in Athens. Later, he studied and spent most of his adult life in Alexandria. He died in Alexandria in 194 B.C. at the age of 82 years. Some people believe that Eratosthenes starved himself to death on purpose after going blind. He never married.

Eratosthenes is most famous for making the first good measurement of the size of Earth. He did that sometime around 240 B.C. He knew that there was no shadow at the bottom of a well in the town of Syene on the summer solstice. That meant that the Sun must be straight overhead in Syene on that day. He measured the length of the shadow of a tall tower in Alexandria on the same day. He also measured the distance between Syene and Alexandria. With this information, he was able to calculate the circumference of the Earth. We aren't quite sure about his exact answer, because we don't know the exact length the distance units he used (called "stadia"). He may have gotten it right to within 1%, or he may have been off by about 16%. Either way, his answer was pretty good... and he was the first person to make a good measurement of the size of Earth!

The Library of Alexandria was the most famous library in the world in ancient times. Eratosthenes was the second person to be head librarian there. Besides figuring out the size of the Earth, Eratosthenes made several other important inventions and discoveries. He invented an easy method for finding out prime numbers. That mathematical method is now called the Sieve of Eratosthenes. He made a map of the "whole world". Actually, the map only showed the parts of the world that the Greeks at the time knew about; but it was one of the best maps of its time. Eratosthenes also invented a system of latitude and longitude, and a calendar that had leap years. He also invented a mechanical device that astronomers used for many years to figure out where the stars were in the sky how they appeared to move (as the Earth turns). That device was called the armillary sphere. Eratosthenes also made a catalog of stars that had 675 stars listed in it.

A crater on Earth's Moon is named after Eratosthenes.


How did Eratosthenes measure the size of Earth?

Eratosthenes

Eratosthenes was a Greek scientist. He lived from 276 to 194 BC. He studied astronomy, geography, and math. He is famous for making the first good measurement of the size of the Earth.
Click on image for full size (83 Kb)
Public domain image.

Eratosthenes was a Greek scientist. He lived from 276 to 194 B.C. He studied astronomy, geography, and math. Eratosthenes is famous for making the first good measurement of the size of the Earth. He lived, for most of his life, in the city of Alexandria in Egypt.

Eratosthenes was born in 276 B.C. in the city of Cyrene. He studied for a few years in Athens. Later, he studied and spent most of his adult life in Alexandria. He died in Alexandria in 194 B.C. He was 82 years. He never married.

Eratosthenes is most famous for making the first good measurement of the size of Earth. He did that sometime around 240 B.C. He knew that there was no shadow at the bottom of a well in the town of Syene on the summer solstice. That meant that the Sun must be straight overhead in Syene on that day. He measured the length of the shadow of a tall tower in Alexandria on the same day. He also measured the distance between Syene and Alexandria. He used that information to calculate the circumference of the Earth. We aren't quite sure what his answer really was. That's because we don't know the exact length the distance units he used (called "stadia"). He may have gotten it right to within 1%, or he may have been off by about 16%. Either way, his answer was pretty good... and he was the first person to make a good measurement of the size of Earth!

The Library of Alexandria was very famous library in ancient times. Eratosthenes became the head librarian there in 236 B.C. Besides figuring out the size of the Earth, Eratosthenes made several other important inventions and discoveries. He invented an easy way for finding out prime numbers. That math method is now called the Sieve of Eratosthenes. He made a map of the "whole world". Actually, the map only showed the parts of the world that the Greeks at the time knew about; but it was one of the best maps of its time. Eratosthenes also invented a mechanical device that astronomers used for many years to figure out where the stars were in the sky how they appeared to move (as the Earth turns). That device was called the armillary sphere. Eratosthenes also made a catalog of stars that had 675 stars listed in it.


How did Eratosthenes measure the size of Earth?


Last modified July 31, 2007 by Randy Russell.
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