来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  and if you want to get the circle back from the ellipse,
  如果你想要把椭圆变回圆
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  you just move the two focal points to a single point,
  你只要移动两个焦点到一个点上
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  and you'll get the circle.
  你就可以得到一个圆
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  Whereas a circle has only a single number R,
  圆只有一个参量 R
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  the radius characterizing it,
  也就是半径
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  the ellipse has what's called a major axis and a minor axis;
  而椭圆有长轴和短轴
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  and major axis, 2a,
  长轴是 2a
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  and a is called a semi-major axis, which is that distance.
  a 被称作半长轴 就是这段距离
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  So, do you know how you can get an ellipse?
  你们知道怎么样获得一个椭圆了吗
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  Why the Greeks were studying ellipses?
  为什么古希腊人要研究椭圆呢
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  Where they come from? Yes?
  它们怎样得到的 请讲
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  Right. They'll be ellipse, circles;
  是的 它们是椭圆 圆