Sunday, April 29, 2018

More stuff I never knew.......................

     Aristotle, a leader of Greek learning with a passion for encyclopedic knowledge, described the earth as unmoving.  Even so, the Greeks soon discovered that the earth was not what it seemed.  They found that the earth was a sphere, and that was a big surprise.  The Greeks also knew quite accurately just how big our sphere is.
     The Greeks noticed that the slant of the sun's rays changed as one moved from north to south.  The difference of the angle at two points on earth, one directly south of the other, could be easily observed at noon during the summer solstice (when the sun appears to come as far north as it ever does).  If one knows this angle and the distance between the points of measurement, on can determine the circumference of the earth.  Eratosthenes is credited with originating this principle and first applying it.  To measure the angle, he used a perpendicular stick in Alexandria and a well with vertical walls in Syrene (modern Aswan).  The stick in Alexandria cast a shadow that indicated the sun was 7° 12' from the vertical.  At the same time, the sun was shining directly down the wall at Syrene.  The distance between the well and the stick was 493 miles. Since 7° 12' is 1/50 of 360°, the circumference of the earth was calculated to by 50 x 493, or 24,661 miles.  This is only about 150 miles less than today's best measurement.

-Edward Teller,  The Pursuit of Simplicity

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