Shop Windows to the Universe

Earth Science Rocks! Select one of our four cool NESTA t-shirts from our online store, and express your love of Earth and space science!
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.
Click on image for full size
Public domain image.


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.

Last modified July 31, 2007 by Randy Russell.

Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

Windows to the Universe Community



You might also be interested in:

Science, Evolution, and Creationism

How did life evolve on Earth? The answer to this question can help us understand our past and prepare for our future. Although evolution provides credible and reliable answers, polls show that many people turn away from science, seeking other explanations with which they are more comfortable....more

The Summer Solstice

Let's pretend, for the moment, that you're the person standing on the Earth in the picture to the left, living in Topeka, Kansas, around 40 N latitude. The picture on the left shows the view from the...more

The Earth's Moon

The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter), making the Earth-Moon system virtually a double-planet. Because of its smaller size,...more


Archimedes was a Greek mathematician and engineer who lived between 287-212 B.C. His greatest contributions are in the field of geometry, where he identified the relationship of a sphere and cylinder's...more


Aristotle was a Greek philosopher who lived between 384-322 B.C. He was one of the greatest thinkers of the world and his written works encompassed all major areas of thought: logic, science, metaphysics,...more


Democritus was a Greek philosopher who lived between 470-380 B.C. He developed the concept of the 'atom', Greek for 'indivisible'. Democritus believed that everything in the universe was composed of atoms,...more


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...more


Plato was a Greek philosopher who lived between 427-327 B.C. In 387 B.C., he founded the Academy in Athens, a school which flourished for about 1,000 years and made many important scientific and mathematic...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA