The current knowledge we have of the universe is small in many ways. Yet it is amazingly large considering we have been able to observe it mostly from only one small floating rock in space, during individual lifetimes that are but a moment in the life of the universe. Our knowledge and understanding today is based on the accumulation of information gathered and deducted by mankind over thousands of years. Like bricks that form a house, each piece of knowledge has been stacked on top of previous pieces to form our as yet uncompleted "house of understanding". Human history includes some amazing stories of individual ingenuity, cleverness and courage, that are part of this accumulated effort to understand. None of these stories is more remarkable than that of an ancient Greek scientist and author named Eratosthenes.Without any of today's modern astronomical instruments, Eratosthenes, who lived in the ancient Egyptian city of Alexandria in the third century BC, was able to determine that the Earth was round. He did so at a time when this was contrary to common or expert belief. Even more remarkable, he was able to determine to a high degree of accuracy the actual size of the Earth - its circumference. And he did it with a stick!
Eratosthenes had read that in the southern Egyptian town of Syene (now Aswan), at noon on the day of the summer solstice each year (currently June 21st) , the sun was at such an angle that the temple columns cast no shadows. He didn't know it then, but the reason for this was that Syene was almost directly on the Earth's latitude circle of 23.5 degrees north (called the Tropic of Cancer), where the sun does in fact pass directly overhead at noon on the day of the summer solstice, briefly causing shadows of vertical objects to disappear. He became curious about whether this was also true where he lived, in Alexandria - if not, why?
He discovered that on the same day and time, sticks and other vertical objects in Alexandria did in fact cast shadows. How could it be that at the same instant there was no shadow in Syene and a substantial shadow in Alexandria? The only possible answer he saw was that the surface of the Earth was curved, resulting in the angle of the sun's rays being different at different locations. Furthermore, he conjectured, the greater the curvature, the greater the shadow.
The difference in shadow lengths between Syene and Alexandria indicated that the difference in angle of the sun's rays in the two towns was about 7.2 degrees. Eratosthenes knew that the distance between Alexandria and Syene was about 800 kilometers. Concluding that the Earth was round (quite a conclusion at that time) he used some relatively simple mathematics to calculate that if 800 kilometers of distance resulted in approximately 7.2 degrees of curvature, then it would take approximately 40,000 kilometers of distance to result in 360 degrees of curvature, or to go all the way around. Therefore, Eratosthenes estimated the circumference of the Earth to be 40,000 kilometers.
This is the right answer, with an error of only a few percent! Eratosthenes was the first person to measure the size of a planet!
Motivated by curiosity and the human drive to understand, Eratosthenes determined the size of the Earth with a stick, and with the sharpest astronomical instrument of all - his brain.
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