C8

Todd Arsenault
SCIENCE EAST

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This story provides a good example of the surprising truth that all you really need to do science is your brain, a vivid imagination, and a careful approach to making experiments and recording your observations.

These days we can sit at our computers and view satellite photos of the entire surface of the earth, in many cases at a resolution high enough that we can pick out our own car in the driveway.

We have to think hard to cast our minds back to a time when the earth was not completely understood. But how many of us really understand it without looking up the details in an atlas or on the Internet?

Ask yourself this question: assuming you could walk completely around the earth, how far would you travel? In other words, what is the circumference of the earth?

If you are like most people, you will have absolutely no idea. But even if you don't know the answer, can you think of how you might find out?

Here is one possible solution that's a good lesson on the resourcefulness and determination of one person, and shows us how dependent we've become on sources of information to tell us the answers rather than using our brains to figure them out for ourselves.

Eratosthenes of Cyrene was born in what is today Libya, in North Africa, in 276 BC.

He led a remarkable life by any standard, as he was the chief librarian of the Library of Alexandria - the greatest collection of knowledge in the ancient world - and was also a leading figure in mathematics, geography, and astronomy. He died at age 80 after having made important contributions in all these fields.

The path that led Eratosthenes to calculate the circumference of the earth started when he read a curious account in the Great Library about a water well in Syene (now Aswan, Egypt) that reflected the sun at noon on the day of the summer solstice (around June 21).

The sun shone directly down into the well without casting any shadows from the walls onto the surface of the water. Eratosthenes reasoned that this happened because the sun was directly overhead at noon on that day in Syene, so vertical columns would also not cast any shadows.

However, he also knew that columns in Alexandria, which was almost due north of Syene, did cast shadows at exactly the same time. How was this possible?

The answer he deduced is that the surface of the earth must be curved, so that vertical columns in Syene and Alexandria were actually not parallel.

In a simple and brilliant experiment, he used geometry and shadow lengths to figure out the angle difference (about 1/50th of a circle, or a bit more than seven degrees).

Aristotle had already proposed that the earth might be a sphere, so Eratosthenes reasoned that the distance between the two cities must be 1/50th of the way around the earth.

His own account of his method has been lost, but legend has it that he hired a man to pace out the approximately 800 km distance on foot. This number times 50 then gives an estimated circumference of the earth of about 40,000 km.

In the age of space flight, we have refined this number to 40,008 km around the poles and 40,075 km around the equator.

Imagine - 2,200 years ago, Eratosthenes got almost exactly the same result with little more than brains, feet, imagination and determination.

Now imagine what you could do if you put your mind to it.

See what new things you can learn at www.scienceeast.nb.ca.

Dr. Todd Arsenault is a volunteer member of the executive committee of Science East. He holds a Ph.D. in chemistry. His column appears very fourth Wednesday.