You might have heard that the Earth was earlier believed to be flat. You might have even laughed at the thought, but if you try and empathise with those who lived thousands of years ago, it would not seem strange after all.
If the Romans had a predecessor to the modern Swiss Army knife , it was the Greeks who were among the first to come up with the idea that the Earth is round and not flat. Pythagoras became the first person (in 500 BC) to voice his opinion about a spherical Earth. His proposal, however, was more to do with aesthetics than any real evidence.
The credit for coming up with an evidence goes to Aristotle, who noticed that ships disappeared slowly over the horizon and that the Earth casts a circular shadow on the moon during lunar eclipses, directing him to the concept that the Earth is spherical. But it was not until 240 BC, that the size of the Earth was determined. And the person who achieved this was Eratosthenes of Cyrene (an ancient Greek and Roman city near present-day Shahhat, Libya).
Second best
Born around 276 BC, Eratosthenes studied at the Lyceum in Athens. He possessed extraordinary skills and knowledge, making him a natural fit for King Ptolemy III of Alexandria as the librarian for the finest repository of knowledge during those times, the library of Alexandria.
Eratosthenes earned the nickname “Beta”, for he was second only to the best in almost every field that he worked in. His works touched upon a wide array of subjects, including mathematics, astronomy, geography, poetry and philosophy.
While his method – the sieve of Eratosthenes – is still used in a modified form to identify prime numbers, his greatest achievement has been the determination of the circumference of the Earth. And to do that he picked the summer solstice, which happens around June 21 every year, in the year 240 BC.
Eratosthenes had heard from travellers about a well in Syene (now Aswan, Egypt) where the sun was directly overhead during summer solstice and hence cast light till the bottom of the well, without any shadows. And yet, at Alexandria, even on midday at solstice a sundial cast a shadow as the sun was not directly overhead. Eratosthenes found the angle that it made to be about 1/50th of a complete circle.
By assuming that the sun was sufficiently far away so as to cast parallel rays at Syene and Alexandria, Eratosthenes realised that he could determine the circumference of Earth by finding out the distance between these two places and multiplying it by 50.
No easy task
Computing accurate distances, however, was no easy task in those days. By hiring bematists – specialists in ancient Greece trained to calculate distances by counting their steps – Eratosthenes arrived at a figure of 5,000 stadia as the distance between Syene and Alexandria. Multiplying by 50, he ended up with the first measurement of the size of the Earth – 2,50,000 stadia – for which details are still known.
For someone without modern measurement tools, Eratosthenes’ estimate is remarkably close to the actual figure. Based on the value of stadia that modern scholars use, Eratosthenes’ circumference is calculated to be somewhere between 38,500 km and 46,500 km, which includes the now accepted figure of 40,030 km.
Stood the test of time
Eratosthenes not only assumed that sun’s rays could be taken as parallel, but also that Alexandria is to the north of Syene and that Syene is on the tropic of cancer. Though these assumptions aren’t absolutely right, they were good enough for his method to obtain an accurate value of Earth’s size. What’s more, his method has stood the test of time and is employed by teachers around the world to help students understand how they can measure the Earth’s circumference. Would you like to try?
