The rate of sweeping out of area is a constant-this is one of three Keplerlaws.
单位时间扫过的面积为常量，这就是开普勒三定律之一。
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On August 6th America’s space agency, NASA, announced that its Kepler planet-detector (named after the man who worked out the laws of planetary motion, as this article explains) is also behaving well.
Kepler went on to show that an elliptical orbit is sufficient to explain the movement of the other planets and to devise the laws of planetary motion that Newton built on.