来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  But I have to remind you what elliptical orbit means, right?
  但我想提醒你们椭圆轨道的意义
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  So everyone has to know what an ellipse is.
  每个人应该都知道什么是椭圆
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  So, you draw a circle,
  你要画一个圆
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  you take a thumbtack, you stick it into your table,
  拿一个图钉 把它按进桌子
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  you take a string, form a loop like that,
  找一根细绳 系成一个像这样的圈
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  put a pencil here, and keep the pencil tight and walk around,
  这里放一支铅笔 拿住这个铅笔转一圈
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  keeping the distance, you get a circle.
  保持这个距离 你就得到一个圆了
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  For an ellipse,
  要画一个椭圆
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  you take two thumbtacks and you take a string like this,
  你拿两个图钉和一根弦 像这样
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  keeping that length plus that length constant,
  保持这两个长度的和一定
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  and you move, keeping the string tight,
  然后你移动画笔 保持弦紧绷
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  and the shape you produce is the ellipse.
  你得到的图形就是椭圆