Happy Summer Solstice! — Eratosthenes’ Inspiration

Seasonal changes are such a delight to the senses. The summer solstice marks longer days, if you live in the northern hemisphere, and ushers in fragrant blooming plants, delicious summer fruit, and adventures with friends and family. But the summer solstice also shares an anniversary of a scientific achievement. Here’s an excerpt from “Summer solstice: the perfect day to bask in a dazzling scientific feat” by Stuart Clark posted on The Guardian:

The summer solstice is the anniversary of one of the greatest achievements of the human mind: it marks the day we first calculated the size of the Earth.

This spectacular feat took place not in the modern technological age, but way back, over 2000 years ago, in the time of the classical Greeks.

Eratosthenes of Cyrene was the chief librarian at the great library of Alexandria in the third century BC. So the story goes, he read in one of the library’s many manuscripts an account of the sun being directly overhead on the summer solstice as seen from Syene (now Aswan, Egypt). This was known because the shadows disappeared at noon, when the sun was directly overhead. This sparked his curiosity and he set out to make the same observation in Alexandria. On the next solstice, he watched as the shadows grew small – but did not disappear, even at noon.

The length of the shadows in Alexandria indicated that the sun was seven degrees away from being directly overhead. Eratosthenes realised that the only way for the shadow to disappear at Syene but not at Alexandria was if the Earth’s surface was curved. Since a full circle contains 360 degrees, it meant that Syene and Alexandria were roughly one fiftieth of the Earth’s circumference away from each other.

Knowing that Syene is roughly 5000 stadia away from Alexandria, Eratosthenes calculated that the circumference of the Earth was about 250,000 stadia. In modern distance measurements, that’s about 44,000km – which is remarkably close to today’s measurement of 40,075km.

I first heard the story when it was told by Carl Sagan in his masterpiece TV series, Cosmos. I still marvel at Eratosthenes’s achievement – a stunning piece of deduction, based only on a few simple observations and an ocean of clear thinking.

So as you revel in the joys of the season, perhaps you can pause to marvel at the inspiration of Eratosthenes. 🌞


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