Johannes Kepler

Portrait of Johannes Kepler by Jacob van der Heyden; between 1601 and 1633.
 Credit: Germanisches Nationalmuseum Nürnberg. Wikimedia Commons

Johannes Kepler, a German astronomer, astrologer and mathematician, was born in Weil der Stadt, Württemberg, on December 27, 1571. He was one of the most significant representatives of the so-called Scientific Revolution of the 16th and 17th centuries, and was committed to heliocentric astronomy. Kepler is best known for his three laws of planetary motion, and his books Astronomia nova, Harmonice Mundi, and Epitome Astronomiae Copernicanae. These works also provided one of the foundations for Newton's theory of universal gravitation.

   Kepler attended Tübingen University from 1591 to 1594. There, he became a convinced follower of the heliocentric astronomy created by Nicolaus Copernicus (1473–1543). Kepler was pursuing a doctorate in theology, but before finishing he became a mathematics teacher at a seminary school in Graz. 

   In Graz, Kepler developed his first original ideas in astronomy, which he published in the Mysterium Cosmographicum in 1596. In his work, he presented a geometric model of the planetary system that explained both the number of planets and their sequential distances from the Sun by nesting the five Platonic solids (octahedron, icosahedron, dodecahedron, tetrahedron, and cube) within the six spheres encompassing the planetary orbits. 

   In 1600, Kepler became an assistant to the astronomer Tycho Brahe (1546–1601) in Prague, and, after Tycho's death in 1601, the Imperial Mathematician to Emperor Rudolf II and his two successors Matthias and Ferdinand II. 

   In 1609, in Astronomia nova, Kepler published the first two laws of planetary motion. He published the third law, the so-called “harmonic law”, for the first time in his Harmonice mundi (1619). Based on the very precise observations of Mars’ orbit made by Tycho Brahe, and developed in the Copernican system, they describe the shape of the orbit of the planets around the Sun as an ellipse (first law), explain how the planets move around the Sun (second law) and give a relationship between the extent of the orbit and its period (third law).

• Kepler’s first law:
 The path of each planet around the Sun is an ellipse, with the Sun at one of the foci.

• Kepler’s second law:
 Each planet moves in such a way along its orbit that a line drawn from the Sun to the planet sweeps an equal area in an equal amount of time.
 
 • Kepler’s third law:
 The ratio of the cube of the semimajor axis of orbit to the square of the orbital period is the same for all planets.
   With his laws, Kepler broke with the principle of uniform circular motion for celestial bodies, which Copernicus had tried to uphold. Though Kepler hadn't known about gravitation when he came up with his three laws, they were instrumental in Isaac Newton (1643–1727) deriving his theory of universal gravitation, which explains the unknown force behind Kepler's Third Law. Kepler and his theories were crucial in the better understanding of our solar system dynamics and as a springboard to newer theories that more accurately approximate our planetary orbits.

   In 1606, in his book De Stella nova in pede Serpentarii, Kepler described his observations of supernova SN 1604, also known as Kepler's Supernova. In 1627 he published Tabulae Rudolphinae (Rudolphine Tables), planetary tables and star catalog based principally on the observations of Tycho Brahe. The best of the pretelescopic catalogs, it is accurate to a few minutes of arc and contains positions for 1,005 stars and tables and directions for locating the planets.

   Kepler also did fundamental work in the field of optics (Astronomiae Pars Optica, 1604), invented an improved version of the refracting (or Keplerian) telescope (Dioptrice, 1611), and published works about astrology. 
 
   In 1608 Kepler wrote Somnium, published posthumously in 1634. It is an imaginative account of a visit to the Moon. Somnium is now considered as the first serious scientific work about lunar astronomy and one of the earliest works of science fiction. 
 
   Kepler died in Regensburg on November 15, 1630.


"Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse."
 
— Johannes Kepler, Dissertatio cum Nuncio Sidereo (1610)
 

 

Illustration of Kepler's three laws with two planetary orbits. The orbits are ellipses, with focal points F1 and F2 for the first planet and F1 and F3 for the second planet. The Sun is placed at focal point F1. 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. The total orbit times for planet 1 and planet 2: t1/t2 have a ratio (a1/a2)3/2. Credit: Wikimedia Commons


References:

Apt, Adam Jared. The Biographical Encyclopedia of Astronomers. Editor-in-Chief Thomas Hockey. 2007 Springer Science+Business Media, LLC.
Encyclopedia Britannica: Johannes Kepler
The Stanford Encyclopedia of Philosophy: Johannes Kepler

 

© 2025, Andrew Mirecki

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