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<\/a><\/p>\nSimple, isn’t it ?\u00a0 If you are surprised, then read on \u2026.<\/p>\n
Eratosthenes & his method<\/h3>\n
Eratosthenes is credited for the first estimation of the size of the Earth, around 240 BC. He was the librarian of the great library of Alexandria in Egypt. He had access to all that was known to the mankind at that time, but he was interested in pushing the limits of knowledge further.<\/p>\n
It was not such a simple thing in those days to believe that the Earth was round! Most people thought it was flat. But Eratosthenes went ahead to measure the sphere he believed the Earth to be. But, he did not use methods like walking around the Earth or digging a hole through its centre to the other side.<\/p>\n
The Earth, if cut into two halves, would show a circular section. The boundary of the circle would be its circumference, which we want to measure. Suppose this circle was a Cake. We cut out a slice that is one-sixth [1\/6] of it. The curved edge of this will then be 1\/6th<\/sup> of the circumference. To put it the other way, the circumference of the cake will be six times the length of the curved edge of the slice.<\/p>\n If we measure the angle made by the other two sides, then we would find that it is 60o<\/sup>. It has to be so, because 60o<\/sup> is 1\/6th<\/sup> of 360o<\/sup> which is the whole circle. Thus if we know the distance between two points on a circumference and know the angle made by them at the centre, we can find out the length of the circle. This means that if we know the distance between two points on the Earth and the angle they make with the Earth\u2019s centre, we can find out the circumference of the Earth.<\/p>\n The distance between two places can be measured. The problem is to measure the angle that they make. This was solved by Eratosthenes.<\/p>\n He had heard that in a place called Syene, south of Alexandria, the walls of a well cast no shadow in the water at noon on the day of the middle of Summer. In other words, there were no shadows by vertical objects\u00a0on June 21st<\/sup>, as the Sun was exactly overhead there at noon.<\/p>\n But on the same date he observed that a vertical pillar in Alexandria cast a small but measurable length of shadow. If the Earth was flat, then the sun-rays coming perpendicular to the flatland would make all vertical objects cast similar shadows anywhere on the Earth. The difference in the length is possible only if the Sunlight comes at different angles at different places. This means that the Earth is a curved surface.<\/p>\n \u00a0 If two sticks perpendicular to the ground in Syene and Alexandria were to extend to the centre of the Earth, they would thus make an angle. Using simple geometry, he found out this angle from the length of the shadow at Alexandria on 21st<\/sup> June. It came out to be about 7o<\/sup> which is 1\/50th<\/sup> of 360o<\/sup>. It is said that he got a troop of the army to walk to Syene [now called Aswan] to measure the distance till there. It came out to be 800 km. Fifty times that i.e. 40,000 km, was the circumference of the Earth!<\/p>\n<\/a><\/p>\n \u00a0 \u00a0\u00a0 \u00a0 <\/a><\/a><\/p>\n