Johannes Kepler (1571 - 1630)

 

With respect to social status and personality, Johannes Kepeler was as far from Tycho Brahe as is possible to imagine.  Whereas Tycho was a wealthy     aristocrat with vast resources and had a voracious appetite for life’s pleasures, Kepler was born into abject poverty and practiced a strict and pious form of Protestantism.  Yet Kepler and Tycho ultimately collaborated to sweep away the ancient concept of perfectly circular motion in the heavens and to replace it with planets moving in elliptical orbits.

 

Kepler developed a fascination with the sky and its movements as a student of mathematics in Tübingen, Germany and became a convert to Copernicus’ new heliocentric system.  He was determined to show how the Copernican system could lead to more accurate predictions than Ptolemy’s.

 

Kepler began working with Tycho in 1600 to take advantage of the fact that Tycho had the most accurate planetary position data available anywhere.  Using this data, he began trying to fit the orbit of Mars into a curve that could be used to predict positions of that planet in the future as well as to specify its position in the past.  Tycho died in 1601, but Kepler stayed with Tycho’s organization and was ultimately successful in demonstrating that planets must move in elliptical orbits.  With that innovation, Copernicus’ heliocentric model was much better at prediction than Ptolemy’s and the number of scholars who believed in a sun-centered universe began to rise.

 

Kepler was able to formulate three laws of motion that describes how planets move about the sun.

 

 

Kepler’s Three Laws of Motion

 

 

 

 

 

 

 

 

 

 

 


  1. Planets move around the sun in elliptical orbits.  The sun is at one of the foci of the ellipse.

 

 

 

 

 

 

 

 

 

 

 


  1. Planets move faster when they are near the sun than when they are at a large distance from the sun.  However, the area defined by two positions of the planet and the sun is always the same if the two points are separated by the same amount of time.

 

 

 

 

 

 

 

 

 

 

 

 

 

 


3. The square of the period P is equal to the cube of a, the average distance between the planet and the sun (a.k.a., the semi-major axis).

                          

                                                         P2 = a3