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I Eratosthene's explanation of the sun's ray

  1. Apr 25, 2016 #1
    How can we say that when the sun is overhead then its ray will pass through the centre of the earth , does it happens only at the one place on the earth. How they knew that synene is situated in the middle of north and south
  2. jcsd
  3. Apr 25, 2016 #2
    What are you referring to?
    What is 'synene ?
    Light from the Sun reaches all parts of the Earth which are in daytime.
    Daytime comes and goes because the Earth is rotating.
  4. Apr 25, 2016 #3
    I am refering to the explanation given by eratosthenes ( BOOK :CONCEPTUAL PHYSICS BY PAUL G. HEWITS) in finding the radius of the earth. He said that there was a placed named syene near south of alexandria . Where we can see that a veritical stick or a line will not cast any shadow and he declared that at that place sun's ray would pass through the center of the earth. I want to know why or how he declared this statement
  5. Apr 25, 2016 #4
    OK. I understand the question now.
    Yes, the ancient greeks did make measurements of the Sun's shadow, and concluded that the Earth was probably a sphere.
  6. Apr 25, 2016 #5
    But I want to know that how he concluded that when sun is overhead then its ray will pass through the centre, I think its only for that particular place (syene ) which must be situated in the middle of North and South ( if it so, then how they found that it is situated in the middle of North and South).
  7. Apr 25, 2016 #6
    The sun is directly overhead in many places on Earth when that place on the Earth is directly facing the Sun.
    It's common in equatorial regions and can happen as far as 23 degrees north or south of the equator, we call it 'Summer'.
    At the poles it is never directly overhead, but it does get above the horizon.
  8. Apr 25, 2016 #7
    Okkkkkk, but how can we say that its ray will pass through the center of the earth when sun is overhead.
  9. Apr 25, 2016 #8


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    Staff: Mentor

    Doesn't it include a diagram? Can you draw yourself one? It should be obvious from a diagram.
  10. Apr 25, 2016 #9


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    Science Advisor

    "Overhead" is taken to mean perpendicular to the surface of the Earth. Any line perpendicular to the surface of a sphere, and continuing thru the sphere, is a diameter. Try drawing it.
  11. Apr 26, 2016 #10
    Ohh thank u man it waa just a silly question , I forget that
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