With a concept as simple as it is ingenious: measuring the shadow cast by the sun on poles. All at a time when common sense reigned over science, and nothing was more obvious than thinking that the Earth was flat. But the Greek astronomer Eratosthenes, who lived between 276 BC and 194 BC, had a vision beyond reach: he noticed that lampposts placed in distant cities projected different shadows at midday. This could only happen at the same time if one place was more inclined to the Sun than another.
So the Earth’s surface, thought Eratosthenes, could only be curved, like that of a ball. Bingo! And the Greek went further, using the difference between the angles of these shadows to measure the circumference of the planet. The way was to take a pole to measure the midday shadow in one place and then take the same stick to a far away city – so that the curvature of the planet would be evident. Then, all you had to do was wait for the sun to be overhead, plant the pole there and note the angle of the shadow that appeared.
The difference between the measurements would give the value of the circumference of the Earth. But Eratosthenes had to work around a small problem: sunlight strikes at different angles as the time of year changes, even though the measurement is taken at the same time. So the thing would only work if the angles were measured on the same day and time. Methodical, the Greek did exactly that: he wrote down the angle that the shadow made in a city and a year later, at the same date and time, he did it in another, 800 kilometers to the north.
In the end, the result for the circumference of the Earth was surprising: more or less 40 thousand kilometers, almost on the fly! In Ecuador, the real value is 40,075 kilometers – from north to south, as the poles are flattened, it is a little less, 40,008 kilometers. In any case, there is still controversy as to the margin of error in the Greek calculations. There are different versions about how much the Greek unit of measurement he used, the so-called “stadiums”, is worth in meters. But for all intents and purposes, Eratosthenes got it right.
– What was the Trojan War?
– What are the main Greek gods?
science in race
1 – In the 3rd century BC, the Greek astronomer Eratosthenes rightly assumed that the Sun was something so distant and large that its rays reached the Earth parallel to each other. If the shadows they cast at a specific time but in different places weren’t the same, the Earth couldn’t be flat. Its surface should be curved
2 – With that in mind, he noticed that the midday sun did not cast a shadow on the Egyptian city of Siena, but did on another, Alexandria, 800 kilometers to the north. As the cities were on the same longitude, the problem was not the time zone, but the curvature of the Earth itself. So it was enough to measure the angle of this shadow on the same date and time to calculate the circumference of the planet
3 – The Greek went to measure the shadow in Alexandria. Gave 7 degrees of inclination in relation to the pole. The angle would correspond to an 800 kilometer slice of the planet, which was the distance between the two cities. To complete the 360 degrees of a circle, it would be necessary to multiply these 7 degrees by 50. In the same way, then, the 800 kilometers times 50 would give the circumference of the Earth. It really did: from one extreme to the other, the length of the planet is about 40,000 kilometers