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Historic Mystery

  1. Dec 22, 2008 #1
    Hi all :D,
    I was wondering if someone could help me. My physics teacher once told me about this unsolved mystery - and I have a vague memory of it. I was hoping someone could help me find out what it is.

    Basically it was something to do with astronomy and about a Greek astronomer. I think he made simultaneous astronomical observations at great distances apart maybe in Egypt. The mystery is how he did it simultaneusly

    Who was this person and what did he do?

    I know its vague - googling hasnt helped - maybe someone with more general knowledge thna me could help me out. If not - then oh well.

    Thanks

    Q
     
  2. jcsd
  3. Dec 22, 2008 #2

    turbo

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    Eratosthenes did not make simultaneous measurements. He knew that a particularly deep well in Syene was fully illuminated by sunlight one day a year and he knew that on that same day, a vertical stick in Alexandria would cast a shadow. By measuring this shadow, and using geometry to calculate the arc created by the curvature of the Earth between Syene and Alaxexandria, he came up with a surprisingly good estimate of the circumference of the Earth. Google Eratosthenes for the whole story and the calculations.
     
  4. Dec 22, 2008 #3
    Thanks for the info. Im not sure if its him but you might be right.
     
  5. Dec 22, 2008 #4
    Hi. Just read up on it. It is him. The mystery is how he measure the distance between the cities. Am i right?
     
  6. Dec 22, 2008 #5

    turbo

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    Distances were measured by very accurate pacing. Such distances were of military importance if you had to consider the logistics of moving troops from one city to to another.
     
  7. Dec 22, 2008 #6

    russ_watters

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    AFAIK, the only mystery is the conversion between the units he used and what we use today.
     
  8. Dec 22, 2008 #7

    turbo

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    IIR, that's the big one, Russ. What is a stadia in modern units?

    It's for sure that the Greeks knew what they were measuring, though. The calculations involved in getting mounted scouts from one place to another and deciding what kind of troop-staffing they could get with chariot-based teams, supply-wagons, infantry, etc in what times.... stuff like that could make or break a dynasty.
     
  9. Dec 22, 2008 #8
    Im not too good with physics even less with history - but how could he have timed it right?
     
  10. Dec 22, 2008 #9

    turbo

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    The Egyptians and the Greeks were very precise about their calendars. No mystery.
     
  11. Dec 22, 2008 #10
    Dont wanna sound arrogant or anything - but I still dont get it. :confused:

    How could he know the its midday in one city while being in another city. Were their clocks that sophisticated?
     
  12. Dec 22, 2008 #11

    Vanadium 50

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    He picked two cities that were (almost) on a north-south line: Alexandria and Syrene.
     
  13. Dec 22, 2008 #12

    mgb_phys

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    It didn't have to be synchronised.
    You can visit Alexandria on midsummers day (which the Egyptians and Greeks knew) and measure the angle, then you either visit Syrene any another year on midsummers and measure the angle - or you know from reading the guidebooks about the well in Syrene and just pick Alexandria to do the other test.
     
  14. Dec 22, 2008 #13
    But how would he know precisely at what moment the well is directly under the sun when he is in another city? If he goes to both cities in misummer then both places would not produce any shadows. I dont quiet understand.
     
  15. Dec 22, 2008 #14

    mgb_phys

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    Syrene is almost on the tropic of cancer and so the sun is perfectly overhead on midsummer's day. Alexandria is about 31deg north and so on midsummers day the minimum angle would be around 7.5 deg (31-23.5deg)

    All you have to be able to do is find midsummers day - which is easy with just a stick in the sand.
     
  16. Dec 23, 2008 #15
    Just say he knew the day the well wouldnt have a shadow in one city. Then he would be at the other city on that day. Now - how would he know at what point in time the well is shadowless?
     
  17. Dec 23, 2008 #16

    turbo

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    He didn't have to know the time of day via a clock or some other machine. The Sun is a pretty good indicator of mid-day. When it is as overhead as it can get at your latitude, it is noon-time at your location. This is NOT a mystery, though we should be impressed by his rigorous and inventive techniques that he used to obtain his estimate.
     
  18. Dec 23, 2008 #17
    Im sorry. Im probably not getting it or something. There are two cities. We need to measure the shadow in one when its noon in the other. Now - how do you know at what point it is noon in the other city while you are in the second city (where theres a stick in the ground)? Maybe im confusing myself or something.

    Edit
    Ofcourse its easy to know when its noon where you are. But how would he know its noon somewhere else?
     
  19. Dec 23, 2008 #18

    turbo

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    If you are at the same longitude, it's noon at the same time every day, regardless of your latitude. Maybe it's a mystery how some Greek scientist figured out how to make use of this information 2000 years ago, but I'd rather lay this to human ingenuity and perception. This is not an especially tough problem. Kids replicate this measurement every year as an academic exercise, as you will discover if you dig a bit.
     
  20. Dec 23, 2008 #19
    Thanks. I get it now.
     
  21. Dec 23, 2008 #20

    russ_watters

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    It's noon when the sun is at it's highest, so it is easy to find local noon.
     
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