Eratosthenes - Measuring the Circumference of the Earth in 240 BC


Eratosthenes was a Greek scientific writer, astronomer, and poet, who is credited with making the first approximation of the size of the Earth for which any details are know. He was born in Cyrene, Libya c. 276 BC. After studying in Alexandria and Athens, he settled in Alexandria around 255 BC and served as the director of The Great Library for over 40 years.

The ecliptic and celestial equator intersect at the spring and autumn equinox points. The ecliptic and celestial equator intersect at the Spring and Autumn equinox points.

In the town of Syene (now Aswan) in Egypt, it was well known that at noon on the day of the Summer solstice, light from the sun would shine directly down a local well. One could look down the well and see his or her own shadow at the bottom, but no shadow from the sides of the well. Eratosthenes found it curious that this never happened at any day in Alexandria. And so he was acutely aware that there was something to be learned from this phenomenon.

He began to observe the distance of a shadow caused by a very tall tower in Alexandra. He noted that in Alexandria at the same time, during the same day, sunlight fell at an angle of about 7.2 degrees from the vertical. Eratosthenes records this angle to be "a fiftieth of a circle", as measuring angles in degrees had not yet been adopted from the Babylonians.

He correctly assumed the Sun's distance from the Earth to be very great, and therefore rays of light falling towards the Earth to be parallel. If he could determine the distance from Syene to Alexandria, all he would have to do to determine the circumference of the Earth was multiply this distance by 50. And so he enlisted the talents of a professional pacer to determine the exact distance from the well in Syene to the tower in Alexandria. He determined that Alexandria is about 5,000 stadia (489 miles) North of Syene.

Eratosthenes parallel sun rays used determine the circumference of the Earth

Given these estimations, Eratosthenes was able to calculate the circumference of the Earth to be 250,000 stadia. The length of one stadia is not exactly known, but it is thought to be between 515 and 686 feet. This corresponds to a circumference of between 24,384 and 32,481 miles, but it is likely that he calculated a value of about 25,000 miles. Compared to today's accepted measurement of the Earth's circumference of 24,901 miles, Eratosthenes' calculation is off by less than 1%.

Calculating the circumference of the Earth to within 100 miles is remarkably accurate given that it was done around 240 BC. Had Christopher Columbus used these measurements for the Earth's size, he might have realized that he had discovered a new continent in 1492, instead of believing he had arrived in India. Eratosthenes also measured tilt of the Earth's axis, and wrote at length of the eight year lunar-solar cycle. He is remembered by history as a great contributor to the field of astronomy. He would die in Alexandria in c. 194 BC at the age of 82.

Originally published
Updated March 2023
Researched and Written by: Thomas Acreman

  Meteora by Cleomedes
  The Britannica Guide to Geometry
  A history of ancient geography among the Greeks and Romans from the earliest ages till the fall of the Roman empire by
  Die geographischen Fragmente des Eratosthenes by

connect Twitter Follow Classic History on Facebook Instagram youtube

Sign up for email updates.

Welcome new readers!

In a hope to share any interesting historical stories I come across in the future I will be writing and posting articles whenever I can. Hopefully quite often.

I'll also be keeping you up to date on any good reads I come across in the Recommended section.

Looking for something in particular? Find it more quickly on the Search page.

And here is a complete list of all articles since the beginning.

H. G. Wells Portrate

"It is possible to believe that all the past is but the beginning of a beginning, and that all that is and has been is but the twilight of the dawn."


The Discovery of the Future

 How far is far?
Link - Commissions Earned


Eratosthenes - Measuring the Circumference of the Earth in 240 BC

  • Bruce McClure
    I'd like to use the above graphic as a sidebar to an upcoming equinox post at EarthSky. My article informs the reader of the intriguing fact that the tip of a shadow stick (gnomon) follows a straight (west-to-east) path on the day of an equinox. If given permission, I plan to credit the graphic to Classic History and to provide a link to this Eratosthenes page. Thank you for your consideration!
  • Wake
    Yes please feel free to use anything you want so long as you reference this website as a source. Here is a slightly larger resolution image. Thanks for reading!

  • If you would like to leave a comment or a reply, please answer this security question: