Using the sun’s rays in a 2000-year-old science experiment, Truro High School pupils have been linking up over 100 Egyptian students at the library in Alexandria – who will also be using the same calculation method on Midsummer’s day – to share their findings and calculate the circumference of the earth.

With the guidance of Physics teacher, Mrs Clare Hallam, Maths teacher, Miss Cath Harding, and the Director of the Roseland Observatory, Brian Sheen, the girls used the shadow angle of the sun and some basic trigonometry to estimate that the circumference of the earth is 39,000 km.

Mrs Hallam explained that the girls were recreating the Eratosthenes Experiment which has been undertaken by scientists for nearly 2000 years.  

Based in the Great Library at Alexandria, Eratosthenes lived at the centre of science and learning in the ancient world. His experiment is regarded as one of the most important of the period; the accepted figure today is only a few percent different from Eratosthenes’ calculations, impressively close for an ancient astronomer without modern tools.

Before they began the experiment Brian told the girls how Eratosthenes 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.

“It’s great to see an incredibly simple experiment produce such an impressive measurement,” said Mrs Hallam. “The girls were thrilled with their results and can’t wait to share them with the students in Alexandria – it’s a fantastic opportunity to take part in science with an international perspective and share in the spirit of enquiry together.”

The school is a firm believer in educating girls about the many opportunities Astronomy can offer and regularly undertake special astronomical projects involving school children from across the county and beyond. The School’s pupils also have the opportunity to undertake GCSE Astronomy as an extra-curricular activity in Year 8 and 9.

In partnership with the Roseland Observatory, Truro High is home to mainland UK’s only solar observatory.

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