Never Mind the Bullocks, Here's the Science by Karl Kruszelnicki Page A

circumference as being 40,253 km—within 0.5% of the current figure of 40,075 km.
Measure Size of Earth with Sticks
Eratosthenes was a Greek who lived in Alexandria in the 3rd century BC. Using just a stick and some maths, he measured the circumference of the Earth.
He had been told by travellers of something wonderful in the Egyptian town of Syene (situated near the giant dam on the Nile, today it is known as Aswan). On just one day of the year, the Summer Solstice, the light of the Sun would reflect off the water in the bottom of a well, for a few moments around midday. That meant that on the Summer Solstice, the Sun was vertically overhead (and that Syene was on the Tropic of Cancer). So on the same day, 21 June, Eratosthenes set up an experiment in his home town of Alexandria. He set up a stick to be perfectly vertical and, around midday, measured the smallest shadow that it threw. The shadow was about 7.2° away from the stick. Now, 7.2° is about one-fiftieth of the 360° that make up a circle.
So that meant that the distance between Syene and Alexandria was one-fiftieth of the circle that makes up the Earth (roughly 800 km).
All Eratosthenes had to do was find the north-south straight line distance between Syene and Alexandria. This came to about 5,000 stadia (one stadia was the length of a foot race in a stadium).
If 5,000 stadia represented one-fiftieth of the circumference of the Earth, then the full circumference was 250,000 stadia.
But a few assumptions were made.
First, Syene was not exactly on the Tropic of Cancer, but slightly north of it. So the Sun was not exactly vertical on the day of the Summer Solstice.
Second, Syene was not exactly south of Alexandria, but a little to the side—so the measured north-south distance between them was a little inaccurate.
Third, the Sun is not a point infinitely far away, so its rays are not exactly parallel. Indeed, over the distance between the Earth and the Moon, they diverge by one-sixth of a degree.
Fourth, it is really difficult to maintain accuracy when you have to pace out a distance of 800 km.
Fifth, how big was a stadia in those days? How many ruined stadiums do you have to average? The Greek historian Herodotus (c. 484 BC-c. 425 BC) reckoned that one stadia was 600 feet. But how big is a foot? Depending on the purpose of the measurement, and which culture measured it, a stadia could range between 157 m and 209 m. So the circumference of the Earth would range from about 40,000 km to about 46,000 km.
Anyhow, depending on the exact measurements used and other factors, Eratosthenes got to within 0.5-17% of the true value—which is pretty good using just a stick as the measure!
Eratosthenes—Always Second Best
Eratosthenes spread himself over many fields. Besides being an astronomer, a geographer and a mathematician, he was also a poet and an athlete. He worked out the circumference of the Earth, and the tilt of its spin axis. He also devised a system of latitude and longitude, and a calendar that included leap years.
His colleagues called him ‘beta’ (the second letter of the Greek alphabet), because they reckoned he was the second best in almost any field.
    Myth of Flat Earth
    So, for the past 2,500 years, in Europe and in the Middle East, the Flat Earthers were in a very small minority.
    At least, this is what the historian Jeffrey Burton Russell, of the University of California at Santa Barbara, believes. His book Inventing the Flat Earth: Columbus and Modern Historians claims that since the 3rd century BC, practically all educated people in the Western world believed in a spherical Earth.
    Looking into the historical record as an historian, he found tens of thousands of Christian theologians, poets, artists and scientists who believed that the Earth was a sphere. On the other hand, he could find only five Christian authorities who believed in a Flat Earth. Dr Russell wrote: ‘In the first fifteen centuries of the Christian era, five writers seem to have