One of my heroes is Eratosthenes of Cyrene, the Greek philosopher, poet and sometime librarian of the Geat Library of Alexandria. He lived between 276 BC and 194 BC (approximately) and his main claim to fame is a remarkable piece of mathematical work that he performed in 240 BC.

Eratosthenes knew that at noon on the day of the summer solstice (about June 19th), the sun at Syene (modern Aswan) was in the zenith, that is, was directly above the earth so that it left no shadows. Accordingly, he measured the angle of the sun at noon on the day of the solstice at Alexandria and found that the angle it made to the vertical was, as he expressed it, one-fiftieth of a circle (7.2° in modern terminology). This, together with a value for the distance between Alexandria and Syene, enabled the philosopher to calculate the circumference of the earth, being the first recorded person to do so.

Erastosthenes obtained a value of 252,000 stadia for the earth’s circumference. The exact length of the stadion is not known with certainty and there have been arguments among scholars as to how accurate this result was. Moreover, Eratosthenes took the sun’s distance as infinite, meaning that its rays were parallel at all points on the earth’s surface. Also, he rounded up his figures to make calculation easier (they had no electronic calculators in those days!). Depending on your choice of values for the units, his result is in the region of 44,100 km (27,400 mi), which differs from the modern value of 40,008 km or 24,860 mi (meridional circumference) by about 10%, a very creditable result.

What I consider important about this famous piece of scientific investigation is that Eratosthenes worked from direct observation and measurement of the phenomena with which he was dealing and did not rely, as was so often the case in past ages, on scholastic or mystical beliefs about how thing “ought to” be. In other words, he proceeded as a scientist. This is a principle which is still often forgotten or glossed over in our own day by people with some sort of axe to grind.

For example, we are told by some that the earth is flat and that the moon landings were a gigantic hoax or that to immunize your child against infectious diseases is to inject them with poison or that evolution is a spurious theory and only the Biblical account is true. To support these views, they have to assert that scientists are talking nonsense or are engaged in a vast worldwide conspiracy to delude the public.

It always amuses me when such a person assures me that the knowledge set forth by scientists is invented rubbish and then pulls out his mobile phone to take a call, send a text or take a photo. Or recounts some programme that he watched last night on TV. How marvellous it is that invented rubbish can achieve such useful results!

Stories like that of Eratosthenes shine like beacons along the road that humanity has travelled from its obscure beginnings to our own day. The darkness seeks always to encroach and we rely on men and women of genius and courage, like the Greek philosopher, to keep it at bay.