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Word Count: 631
What is the circumference of earth How was it measured Imagine that you are located at a point on earth where at some time of the day the length of the shadow cast by a vertical pole is zero You also have a friend located some 500 kilometers due north of you monitoring the length of the shadow of a vertical pole of the same height If you ask your friend over the telephone to measure the length of the shadow he is monitoring then a simple geometric calculation would give you the circumference of the earth and hence its diameter This is more or less what Eratosthenes seems to have done over two centuries before Christ You would notice that all you need is the distance between two points located on the same meridian and the valid assumption that the rays of the sun at the two locations are parallel Of course a telephone connection would be useful Eratosthenes avoided the need of a telephone by making his measurements at the time of the year when the shadows at the two points were minimum In other words he chose the day of the summer solstice The story goes that Eratosthenes knew that at Syene in Egypt the summer solstice sun shone vertically down into a well He then proceeded to measure the angle to the vertical of the rays causing the minimum shadow at Alexandria located 787 kilometers to the north and came out with a remarkable result still nearly valid Incidentally the present value of the average radius of the earth is 6380 kilometers If true this is a story depicting extreme creativity in science The constellation Ursa Major Saptarishi revolves clockwise around the pole star Then what will be the revolving speed of these stars Ursa Major or other constellations do not actually revolve around the Pole Star They appear to be so revolving because our earth rotates
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