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From the Astronomical Dictionary

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Johannes Kepler

Johannes Kepler (1571 - 1630) defined the three Laws of Kepler that properly explained the motion of the planets around the Sun for the first time. The three Laws of Kepler are:

  1. Each planet moves around the Sun in an orbit that is an ellipse, with the Sun in one of the foci of the ellipse.
  2. The straight line segment from the Sun to the planet covers the same amounts of area of the ellipse in the same amounts of time.
  3. The third power of the semimajor axis of the orbit of a planet is proportional to the second power of the orbital period of the planet around the Sun.