Johannes Kepler: an astronomer

Bill Dennison ·
29 April 2014
Science Communication | 

'Scientists who made a difference' series

Johannes Kepler was a German Lutheran astronomer who created laws for planetary motion, developed an important improvements for telescopes, and laid the foundations for Newtonian physics. Kepler was born near Stuttgart, Germany in 1571. Johannes and his two older brothers and sister were raised by their mother, as their father left home to fight as a mercenary. Johannes was born prematurely and was sickly as a child. Yet he lived a full life, was married twice and fathered six children who lived beyond infancy, and lived until 1630.

A 1610 portrait of Johannes Kepler. Source: Wikipedia

Johannes developed a love of astronomy from an early age and studied philosophy and theology at the University of Tubingen. He learned about the Copernican system of heliocentric movements of planets around the sun and became an adherent. Following his university studies, he joined the faculty at the University of Graz, Austria's second oldest university, to teach mathematics and astronomy.

Kepler married a widow, Barbara Muller, and had three children with her. He published his first book, The Cosmographic Mystery in 1596. In this book, Kepler provided the first published defense of the Copernican theory which Copernicus had originally published half a century earlier. Kepler made his case both from both an astronomical as well as a theological perspective. In 1600, Kepler views on the Copernican theory brought him into religious strife at the University of Graz, and when he refused to convert to Catholicism, he contacted Tycho Brahe, a Danish-born astronomer based in Prague, the capital of Bohemia. Brahe was impressed with Kepler's astronomical and mathematical acumen and underwrote his move to Prague. And then when Brahe died unexpectedly in 1601, Kepler took over his job as the imperial mathematician to the Emperor Rudolph II, who was the King of Bohemia, as well as the Holy Roman Emperor.

Kepler's Platonic solid model of the Solar system from Mysterium Cosmographicum. Source: Wikipedia

Kepler lived and worked productively in Prague for 12 years. He wrote an important treatise on optics (The Optical Part of Astronomy) in which he explained the physics of human vision and he developed the principles of parallax for astronomy. Parallax is the difference in the position of an object viewed along two different sight lines, and is an important tool used by astronomers in calculating distances to celestial objects. Kepler observed and described a bright new evening star (supernova) that appeared in 1604. He also wrote 'A New Astronomy', which described laws of planetary motion, including the elliptical orbits of planets around the sun. When Kepler received a copy of Galileo's 1610 book, 'Starry Messenger', he endorsed Galileo's work and set about to improve the telescope that Galileo described. Kepler succeeded in producing a telescope with more magnification and this improvement in telescope technology ushered in a wave of new astronomical discoveries.

Johannes Kepler's original drawing depicting the location of the stella nova. Source: Wikipedia

In 1612, Kepler's benefactor Emperor Rudolph II died, and, tragically, his six year old son and wife also died. Kepler remained the imperial mathematician to Rudolph's successor, but Kepler moved to Linz, Germany, remarried a twenty four year old woman, and had three more children who survived infancy. While in Linz, Kepler published a three volume book called the 'Epitome of Copernican Astronomy' in which he described his three laws of planetary motion:

  1. The orbit of every planet is an ellipse with the Sun at one of the two foci
  2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time
  3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit

Illustration of Kepler's three laws with two planetary orbits. Source: Wikipedia

Kepler's mother, who was a herbalist, was accused of sorcery and tried as a witch, and he helped represent her in court. Kepler's mother was found innocent in the end. Kepler was able to continue with his work after the trial and he published a book 'Harmony of the World' in 1619. This book attempted to explain the proportions in the natural world, and he related the proportions to music. Religious tension which resulted in the bloody Thirty Years War (1618-1648) forced Kepler to move from Linz in 1626 to Ulm and then to Regensburg where he died in 1630.

Johannes Kepler wrote his own epitaph which was as follows:

I measured the skies, now the shadows I measure

Skybound was the mind, earthbound the body rests.

Impression of the Kepler telescope. Source: Wikipedia

Johannes Kepler has been commemorated by a wide variety of means, with schools, a university, buildings, mountains and even moon and Mars craters named after him. One of the most appropriate legacies is the Kepler Mission launched in 2009 by NASA to search for earth-like planets. In fact, just this month, the Kepler space photometer discovered an earth-like planet that was named Kepler-186f orbiting a red dwarf.

About the author

Bill Dennison

Dr. Bill Dennison is a Professor of Marine Science and Vice President for Science Application at the University of Maryland Center for Environmental Science (UMCES). Dr. Dennison’s primary mission within UMCES is to coordinate the Integration and Application Network.

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