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How far away is a plane in the sky?

  1. May 24, 2010 #1
    Ok, my brother & I were trying to work this out.

    Without directly knowing the angles is it possible to work out how far away a plane is in the sky?

    For example, a boeing 747 is about 70m long. It appeared to be about 0.001m holding out a ruler about 80cm from my eye.

    I look forward to reading your replies.

    Thank you.
  2. jcsd
  3. May 24, 2010 #2


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    Staff: Mentor

    Welcome to PF.

    Airliners at cruise are flying at an altitude between 30,000 feet and 40,000 feet 90% of the time.
  4. May 24, 2010 #3
    Apply the rule of similar triangles.
  5. May 24, 2010 #4
    As a purely practical thing, the mesurement method used makes any answer he's going to get from similar triangles meaningless. The error is likely to be an order of magnitude out. (at least)

    But yeah, in a textbook case, similar triangles is the method.
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