Kepler's laws of planetary motion

The motion of planets is describedby the three Kepler’s laws, which belong to the usual secondary school curriculum. Johannes Kepler (1571–1630), German mathematician and astronomer, whose life was closely connected to Graz and Linz in Austria, and Praha in Bohemia, published the three laws over a period of time in two books. He based his findings on observations of the Danish astronomer Tycho Brahe (1546–1601). First, in 1609 in Astronomia nova, Kepler published two statements that are known as his 1st and 2nd laws of planetary motion.
  • Orbits of all the planets are ellipses with the Sun at one focus (Kepler’s 1st law, the law of ellipses).
  • A line segment from the Sun to a planet sweeps out equal areas in equal time (Kepler’s 2nd law, the law of equal areas).
Then, in 1619 in Harmonices mundi, he added the third law. 
  • The orbital period of a planet is proportional to the three-halves power of the size of the semi–major axis of its orbit (Kepler’s 3rd law, the law of harmonies).