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School’s High Noon Solstice celebrations

At high noon on Midsummer's Day Truro High School students were making the most of the sun - not on the beach or out for a picnic, but using its rays to calculate the circumference of the earth.

With the guidance of Truro High School Head of Science, Jon Dean, and the Director of the Roseland Observatory, Brian Sheen, the girls used a solar telescope, the shadow angle of the sun and some basic trigonometry to estimate that the circumference of the earth is 39,605km.

“We were recreating the Eratosthenes Experiment,” explained Jon. “Eratosthenes lived around 2,000 years ago. He studied in Alexandria in Egypt and became librarian of the Great Library there – the centre of science and learning in the ancient world. His experiment was one of the most important of the period and his estimated circumference was accepted and used until man went into space.”

Before they began the experiment Brian told the girls how Eratosthene arrived at his figure. He knew that on one day of the year the sun shone to the bottom of a well in Aswan and that every day a shadow was cast by the Great Obelisk in Alexandria. He measured the angle of the obelisk’s shadow at the exact time the sun hit the bottom of the well and, using the angle measured with the distance between the obelisk and the well, estimated the circumference of the earth.

After completing their experiment the Truro High School girls linked up with hundreds of Egyptian students at the library in Alexandria – who had also been using the same calculation method – to compare results.

“Our results were pretty accurate,” said Jon. “It’s a great experiment to complement the GCSE Maths syllabus. The girls learned a lot, including that if Columbus had used Eratosthenes estimate he might not have miscalculated his epic journey which left him believing he was in the Far East when in reality he’d only reached America!”

Published by: Darren Stevens

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