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Johannes Kepler

Johannes Kepler: His Life, His Laws and Times

"... the ways by which men arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves"

- Johannes Kepler

IYA Kepler

A Short Biography

A List of Kepler's Firsts

Kepler's Laws of Planetary Motion

People and Events Contemporary to Kepler (1571-1630)

Articles about Kepler

Biographies and books

Web Sites (music, drama, animations, lectures, museums, sites with biographies)

Johannes Kepler
Picture courtesy of Sternwarte Kremsmünster, Upper-Austria

A Short Biography

Johannes Kepler was born about 1 PM on December 27, 1571, in Weil der Stadt, Württemberg, in the Holy Roman Empire of German Nationality. He was a sickly child and his parents were poor. But his evident intelligence earned him a scholarship to the University of Tübingen to study for the Lutheran ministry. There he was introduced to the ideas of Copernicus and delighted in them. In 1596, while a mathematics teacher in Graz, he wrote the first outspoken defense of the Copernican system, the Mysterium Cosmographicum.

Kepler's family was Lutheran and he adhered to the Augsburg Confession a defining document for Lutheranism. However, he did not adhere to the Lutheran position on the real presence and refused to sign the Formula of Concord. Because of his refusal he was excluded from the sacrament in the Lutheran church. This and his refusal to convert to Catholicism left him alienated by both the Lutherans and the Catholics. Thus he had no refuge during the Thirty-Years War.

The Holy Roman Empire of German Nationality at the Time of Kepler
The Holy Roman Empire of German Nationality at the Time of Kepler

Kepler was forced to leave his teaching post at Graz due to the counter Reformation because he was Lutheran and moved to Prague to work with the renowned Danish astronomer, Tycho Brahe. He inherited Tycho's post as Imperial Mathematician when Tycho died in 1601. Using the precise data that Tycho had collected, Kepler discovered that the orbit of Mars was an ellipse. In 1609 he published Astronomia Nova, delineating his discoveries, which are now called Kepler's first two laws of planetary motion. And what is just as important about this work, "it is the first published account wherein a scientist documents how he has coped with the multitude of imperfect data to forge a theory of surpassing accuracy" (O. Gingerich in foreword to Johannes Kepler New Astronomy translated by W. Donahue, Cambridge Univ Press, 1992), a fundamental law of nature. Today we call this the scientific method.

In 1612 Lutherans were forced out of Prague, so Kepler moved on to Linz. His wife and two sons had recently died. He remarried happily, but had many personal and financial troubles. Two infant daughters died and Kepler had to return to Württemburg where he successfully defended his mother against charges of witchcraft. In 1619 he published Harmonices Mundi, in which he describes his "third law."

In spite of more forced relocations, Kepler published the Epitome Astronomiae in 1621. This was his most influential work and discussed all of heliocentric astronomy in a systematic way. He then went on to produce the Rudolphine Tables that Tycho had envisioned long ago. These included calculations using logarithms, which he developed, and provided perpetual tables for calculating planetary positions for any past or future date. Kepler used the tables to predict a pair of transits by Mercury and Venus of the Sun, although he did not live to witness the events.

Johannes Kepler died in Regensburg in 1630, while on a journey from his home in Sagan to collect a debt. His grave was demolished within two years because of the Thirty Years War. Frail of body, but robust in mind and spirit, Kepler was scrupulously honest to the data.

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

A List of Kepler's Firsts
  • First to correctly explain planetary motion, thereby, becoming founder of celestial mechanics and the first "natural laws" in the modern sense; being universal, verifiable, precise.

In his book Astronomia Pars Optica, for which he earned the title of founder of modern optics he was the:

  • First to investigate the formation of pictures with a pin hole camera;
  • First to explain the process of vision by refraction within the eye;
  • First to formulate eyeglass designing for nearsightedness and farsightedness;
  • First to explain the use of both eyes for depth perception.

In his book Dioptrice (a term coined by Kepler and still used today) he was the:

  • First to describe: real, virtual, upright and inverted images and magnification;
  • First to explain the principles of how a telescope works;
  • First to discover and describe the properties of total internal reflection.

In addition:

  • His book Stereometrica Doliorum formed the basis of integral calculus.
  • First to explain that the tides are caused by the Moon (Galileo reproved him for this).
  • Tried to use stellar parallax caused by the Earth's orbit to measure the distance to the stars; the same principle as depth perception. Today this branch of research is called astrometry.
  • First to suggest that the Sun rotates about its axis in Astronomia Nova
  • First to derive the birth year of Christ, that is now universally accepted.
  • First to derive logarithms purely based on mathematics, independent of Napier's tables published in 1614.
  • He coined the word "satellite" in his pamphlet Narratio de Observatis a se quatuor Iovis sattelitibus erronibus

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

Kepler's Laws of Planetary Motion

Kepler was assigned the task by Tycho Brahe to analyze the observations that Tycho had made of Mars. Of all the planets, the predicted position of Mars had the largest errors and therefore posed the greatest problem. Tycho's data were the best available before the invention of the telescope and the accuracy was good enough for Kepler to show that Mars' orbit would precisely fit an ellipse. In 1605 he announced The First Law:

Planets move in ellipses with the Sun at one focus.

The figure below illustrates two orbits with the same semi-major axis, focus and orbital period: one a circle with an eccentricity of 0.0; the other an ellipse with an eccentricity of 0.8.

Circular and Elliptical Orbits Having the Same Period and Focus
Circular and Elliptical Orbits Having the Same Period and Focus

Prior to this in 1602, Kepler found from trying to calculate the position of the Earth in its orbit that as it sweeps out an area defined by the Sun and the orbital path of the Earth that:

The radius vector describes equal areas in equal times. (The Second Law)

Kepler published these two laws in 1609 in his book Astronomia Nova.

For a circle the motion is uniform as shown above, but in order for an object along an elliptical orbit to sweep out the area at a uniform rate, the object moves quickly when the radius vector is short and the object moves slowly when the radius vector is long.

On May 15, 1618 he discovered The Third Law:

The squares of the periodic times are to each other as the cubes of the mean distances.

This law he published in 1619 in his Harmonices Mundi . It was this law, not an apple, that led Newton to his law of gravitation. Kepler can truly be called the founder of celestial mechanics.

Also, see the article on "Kepler and Mars - Understanding How Planets Move" by Edna DeVore

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

People and Events Contemporary to Kepler (1571-1630)

Johannes Kepler timeline
Credit: NASA Kepler Mission/David Koch

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

Articles about Kepler

2008 Oct 5. Searching Heaven and Earth for the Real Johannes Kepler. by Dava Sobel, Discover Magazine Nov 2008 issue. Galileo may be science's most famous martyr, but it was Kepler who solved the mystery of the planets. Excerpt: Zielona Gora, Poland—The great German astronomer Johannes Kepler (1571–1630) arrived in this forested region to serve his last employer exactly 380 years ago—reason enough for some two dozen science historians to gather here and celebrate with a conference. For five days in late June, they regaled each other with the fruits of their own recent research into their hero’s achievements ....

His Rudolphine Tables of 1627 (his “crowning publication,” according to Gingerich) enabled him to predict the first observable transits of Mercury and Venus—the passages of those planets across the face of the sun—both in 1631. Kepler, however, never witnessed either event. He died in 1630, on a frustrated journey to collect payments owed him by several patrons....

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

Biographies and Books on Johannes Kepler

Kepler by Max Caspar, Dover Publications, 1993, 441pp. ISBN 0-486-67605-6 (paperback).
This is the most complete and authoritative biography on Johannes Kepler. It is a recent translation by C. Doris Hellman with an introduction, bibliography and list of textual citations by Owen Gingerich.

Kepler's Witch: An Astronomer's Discovery of Cosmic Order Amid Religious War, Political Intrigue, and the Heresy Trial of His Mother by James Connor, Harper SanFrancisco, To be published Apr. 2004. 416pp.$24.95 ISBN: 0-06-052255-0

Tycho & Kepler: The Unlikely Partnership that Forever Changed Our Understanding of the Heavens by Kitty Ferguson, Walker New York, 2002, 402pp., $28.00 ISBN: 0-8027-1390-4 (hard cover)

Johannes Kepler: Short Biography, Encyclopedia Britannica
by Robert S. Westman, Professor of History and Science Studies, UC San Diego. Westman presents a concise biography of Johannes Kepler: his life, discoveries, and publications. A bibliography offers further references. Free viewing online; by subscription for download.

The Sleepwalkers: A History of Man's Changing Vision of the Universe by Arthur Koestler, Penguin Books, 1959, 623pp. ISBN 0-14-019246-8 (paperback).
It also includes material on Copernicus, Tycho and Galileo.

Johannes Kepler, John Tiner, Mott Media, 1977, 202pp. ISBN 0-915134-11-X (paperback) ISBN 0-915134-96-9 (hard cover)
For high school level reading, a biography which reads more like a story.

Johannes Kepler: And the New Astronomy, by James R. Voelkel, Oxford University Press, 1999
144pp., ISBN: 0195116801 (hard cover) ; ISBN: 019515021X (paperback)

The Composition of Kepler's Astronomia Nova, by James R. Voelkel, : Princeton University Press, 2001, 308pp. ISBN: 0691007381 (hard cover)

In German:

Johannes Kepler, Max Caspar, Verlag für Geschichte der Naturwissenschaften und der Technik, Stuttgart, 1995, Vierte Auflage (4th ed.) 591pp. ISBN 3-928186-28-0 (For English translation, see above.)

Johannes Kepler Er veränderte das Weltbild, Günter Doebel, Verlag Styria, Graz, 1983, 256pp. ISBN 3-222-11457-9

Johannes Kepler Dokumente zu Lebenszeit und Lebenswerk, by Walther Gerlach and Martha List, Ehrenwirth Verlag, München, 1971, 243pp. ISBN 3 431 01421 6

Johann Kepler Sein Leben in Bildern und eigenen Berichten, by Justus Schmidt, Rudolf Trauner Verlag, Linz, 1970, 308pp. ISBN 3 85320 258 6

Other Information:

There is a play on Kepler and Tycho "Reading The Mind of God" by Patrick Gabridge

There is a web site for the Kepler Museum in Weil der Stadt by the Kepler-Gesellschaft e. V.

See also Andrew Fraknoi's resource guides:

Short Biography -|- Kepler's Firsts -|- Kepler's Laws -|- People and Events in Kepler's Time -|- Articles
Biographies and books -|- Web Sites -|- IYA Kepler

Links to Other Sites Relating to Johannes Kepler

Music (IYA - 2009) - Listen to AstroCappella songs by The Chromatics with themes about Kepler and Galileo:

  • Dance of the Planets - about extrasolar planets
  • Shoulders of Giants, commissioned by the Johannes Kepler Project, written and arranged by Padi Boyd and performed by The Chromatics, specifically for the International Year of Astronomy. We hope the song will strike a harmonious cord with people and be used in a variety of other IYA projects around the globe. To this end, we would like to make this song freely available for such use, asking only that the appropriate credits be given to the composer (Padi Boyd), the performers (The Chromatics/AstroCappella) and the Johannes Kepler Project. (use of the song as part of a commercial project will require a copyright release from the Project.)

Museum: (June 2000) - Kepler Museum in Weil der Stadt by the Kepler-Gesellschaft e. V.

Drama: (June 2000) - There is a play about Kepler and Tycho "Reading The Mind of God" by Patrick Gabridge

Sites describing Kepler's Laws

Sites with biographies, biographical material

  • - a comprehensive and up-to-date collection of Kepler related resources, articles and community discussions on the web. Includes Somnium (Dream) by Johannes Kepler, perhaps the first ever work of science fiction, in which an employee of Tycho Brahe takes a journey to the moon with the help of his bewitched mother in order to study the lunar environment and its inhabitants.
  • Short Biography MacTutor History of Mathematics Archive Univ St Andrews. Has several portraits. (Caution: There are only four known portraits done in Kepler's lifetime. I have not been able to definitely identify which are which. The biograghy by Max Caspar discusses the issue at the very end, but does not reproduce them. The cover of the Dover publication by Caspar is a modern drawing.)
  • Outline Biography Rice University
  • Not so Short Biography by Galileo Project
  • Golden Age of Astronomy in Prague
  • Kepler's House in Linz 1625
  • Johannes Kepler University-Linz Austria
  • Kepler Astronomy Picture of the Day
  • Weil der Stadt, Kepler's birth place. Click on Sehenswürdigkeiten. Then click on the red dot for marktplatz to see the Kepler monument in Weil der Stadt.

I much prefer the sharpest criticism of a single intelligent man to the thoughtless approval of the masses.

--Johannes Kepler