page_title: Kepler's Laws
hierarchical_section_title: Kepler's Laws
context_page_description: Kepler's laws of planetary motion are three laws that describe the motion of planets around the sun:
  Planets move around the sun in elliptic orbits. The sun is in one of the two foci of the orbit.
  A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
  Johannes Kepler found these laws, between 1609 and 1619.
context_section_description: Kepler's laws of planetary motion are three laws that describe the motion  of planets around the sun:
  Planets move around the sun in elliptic orbits. The sun is in one of the two foci of the orbit.
  A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
  Johannes Kepler found these laws, between 1609 and 1619.
caption: Figure 1: Illustration of Kepler's three laws with two planetary orbits.
   (1) The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1. 
  
   (2) The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2. 
  
   (3) The total orbit times for planet 1 and planet 2 have a ratio a13/2 : a23/2.