When was earth thought to be round

C olumbus sailed off, with one part courage and two parts madness, to sail around a spherical world. Actually, his trip was based on a terrible miscalculation. He underestimated earth's diameter, and he overestimated the width of Asia. He thought Japan lay only miles west of the Canary Islands.

A correct calculation would've put it 10, miles away -- far beyond the reach of any 15th-century ship. And yet Columbus had access to a much better estimate of earth's size -- one that was years old. The ancients thought the earth was flat, but then so did you and I when we were too young to know about spheres.

Understanding a round earth came in stages. First the Pythagoreans argued by induction, years ago: The moon is round, they said. So is the sun. Surely the earth must also be round. Two centuries later, Aristotle argued from observation. And it measured about 7 degrees. Now, if the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow while a stick in Syene is not, it must mean that the Earth's surface is curved.

And Eratosthenes probably already knew that. The idea of a spherical Earth was floated around by Pythagoras around BC and validated by Aristotle a couple centuries later. If the Earth really was a sphere, Eratosthenes could use his observations to estimate the circumference of the entire planet. Since the difference in shadow length is 7 degrees in Alexandria and Syene, that means the two cities are 7 degrees apart on Earth's degrees surface.

Eratosthenes hired a man to pace the distance between the two cities and learned they were 5, stadia apart, which is about kilometres. He could then use simple proportions to find the Earth's circumference — 7. And just like that, a man years ago found the circumference of our entire planet with just a stick and his brain.

Read the original article on Business Insider UK. So Eratosthenes hired bematists, professional surveyors trained to walk with equal length steps. They found that Syene lies about stadia from Alexandria. Eratosthenes then used this to calculate the circumference of the Earth to be about , stadia. Modern scholars disagree about the length of the stadium used by Eratosthenes.

The Earth is now known to measure about 24, miles around the equator, slightly less around the poles. Eratosthenes had made the assumption that the sun was so far away that its rays were essentially parallel, that Alexandria is due north of Syene, and that Syene is exactly on the tropic of cancer.

His basic method is sound, and is even used by schoolchildren around the world today. Several decades after Eratosthenes measurement, Posidonius used the star Canopus as his light source and the cities of Rhodes and Alexandria as his baseline. Ptolemy included this smaller value in his treatise on geography in the second century A.

If Columbus had instead known Eratosthenes larger, and more accurate, value, perhaps he might never have set sail. APS News Archives. Librarians Authors Referees Media Students. Login Become a Member Contact Us. June, ca. Follow Us.