Who was Eratosthenes?
Eratosthenes was a Greek mathematician, geographer and astronomer. He was born in Cyrene (now Libya) in 276 BCE but lived most of his life in Alexandria, Egypt.
Eratosthenes made many significant scientific contributions to mathematics, geography, astronomy and meteorology. He is best known for being the first person to measure the circumference of the Earth with reasonable accuracy (he calculated it at 24,662 miles).
How did Eratosthenes measure the size of the Earth?
Eratosthenes’ method for measuring the circumference of the Earth was simple: he observed the shadows cast by sticks on the ground for two days in midsummer and two days in midwinter and drew a line from one end of each shade to the other end. Then, he measured how long it took for these lines to connect. The distance between them represents an angle (35 degrees) that determines how far apart they are on opposite sides of the line through Earth’s centre. By multiplying this figure by 360 degrees (one full rotation), you can calculate your entire circumference!
To make sure that Eratosthenes didn’t make any mistakes when making his calculations, he compared the lengths of shadows in Athens with those at Alexandria—where there would be no difference due to altitude—and found that they agreed perfectly
. This was a very clever method because it considered the difference in angle between the sun’s rays during midsummer and midwinter.
The process was simple: he observed the shadows cast by sticks on the ground for two days in midsummer and two days in midwinter and drew a line from one end of each shade to its other end. Then, he measured how long it took for these lines to connect. The distance between them represents an angle (35 degrees) that determines how far apart they are on opposite sides of the line through Earth’s centre. By multiplying this figure by 360 degrees (one full rotation), you can calculate your entire circumference!
How does the Eratosthenes method work?
The Eratosthenes method is based on two assumptions.
The first is that the sun’s rays are parallel to each other, which means that if you hold up a stick and point it at the sun, they will all strike the ground at a single point. This makes sense because, as we learned earlier, sunlight travels in straight lines.
The second assumption is that when you measure how far your shadow extends from noon (which happens to be the longest day of summer for places near where Eratosthenes lived), your shadow will be exactly 1/50th of your height (or about one meter). This also makes sense because we know that shadows get longer as the angle between a light source and an object increases—but since there aren’t any mountains around where he was measuring shadows, this was probably an approximation!
Final thoughts
Eratosthenes is famous for his calculation of the size of the Earth.
In this article you have learned how to measure the size of earth like Eratosthenes!
