Ancient Greece’s wisest scientists did not go along with the generally accepted thinking of their ancestors – that the Earth was a circular plate supported by four elephants standing on a huge sea turtle. They had already concluded that it was a sphere. The idea was mooted about 500 BC by followers of Pythagoras, who considered the sphere to be the perfect shape.
The first man to be credited with measuring the Earth’s circumferenace was the Greek astronomer Eratosthenes in 230 BC. He reasoned that if the Earth was a sphere, then the line joining two places was an arc of a great circle. If he could measure the distance as both a length and as a proportion of 360 degrees (a complete circle), he would have an arc from which he could calculate the total circumference.
It had been recorded that at noon at the summer solstice (about June 21), the Sun was directly overhead at Syene (modern Aswan) because it shone vertically down a deep well. So at the same time of year at Alexandria, many miles to the north-west of Syene, Eratosthenes measured the angle of the Sun from the vertical and found it was one-fiftieth of a complete circle – exactly 7.2 degrees in today’s measure.
He then need to know the crow’ flight distance between Syene and Alexandria. One of the ways in which he may have calculated this was by the journey time of camels. A laden camel keeps up a steady pace, and could travel at around 100 stadia a day – today 1 stade is usually regarded as about 605ft (185m). Since a camel caravan took 50 days to travel between Alexandria and Syene, Eratosthenes made the distance 5000 stadia – about 575 miles (925km). This produced the figure of 250,000 stadia, or roughly 28,740 miles (46,250km), as the circumference of the Earth.
Bearing n mind his lack of equipment and his rule of thumb measurements, it is remarkable that he arrived at a figure that is less than 15 per cent too long compared with modern measurements. Had he known the exact distance between Syene and Alexandria – 526 mile (847km) as the crow flies – his answer would have been just under 230,000 stadia, about 26,440 miles (42,550km), and only 6 per cent out.
Today, we know that the Earth is flattened at the North and South Poles, and the circumference at the Equator is about 24,900 miles (40,075km).
Picture Credit : Google
