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Johannes Kepler

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Johannes Kepler

Johannes Kepler (December 27, 1571November 15, 1630), a key figure in the Scientific Revolution, was a German astrologer, astronomer, and mathematician. He is best known for his laws of planetary motion. He is sometimes referred to as "the first theoretical astrophysicist", although Carl Sagan also refers to him as the last scientific astrologer.

Kepler was a professor of mathematics at the University of Graz, court mathematician to Emperor Rudolf II, and court astrologer to General Wallenstein. Early in his career, Kepler was an assistant to Tycho Brahe. Kepler's career also coincided with that of Galileo Galilei.

Table of contents
1 Kepler's life
2 Scientific work
3 Kepler on God
4 Writings by Kepler
5 External links

Kepler's life

Johannes Kepler was born on December 27, 1571 at Weil der Stadt in the German province of Swabia (now a part of the German state Baden-Württemberg). His grandfgather had been mayor of that town, but by the time Johannes was born, the Kepler family fortunes were in decline. His father earned a precarious living as a mercenary, and abandoned the family when Johannes was 17. His mother, an inn-keeper's daughter, had a reputation for involvement in witchcraft. Born prematurely, Johannes is said to have been a weak and sickly child, but despite his ill health, he was precociously brilliant.

Though he excelled in his schooling, Kepler was frequently bullied, and was plagued by a beliefs that he was physically repulsive, thoughoughly unlikable and, compared to the other pupils, an outsider. This ostracizing probably led him to turn to the world of ideas, as well as an abiding religious conviction, for solace.

He was introduced to astronomy at an early age, and developed a love for that discipline that would span his entire life. At age six, he observed the Comet of 1577, writing that he "...was taken by [his] mother to a high place to look at it." At age nine, he observed another astronomical event, the Lunar eclipse of 1580, recording that he remembered being "called outdoors" to see it and that the the moon "appeared quite red."

In 1787, Kepler began attending the University of Tübingen, where he proved himself to be a superb mathematician. Upon his graduation from that school (1591), he went on to pursue study in theology, becoming a part of the Tübingen faculty. However, before he took his final exams he was recommended for the vacant post of teacher of mathematics and astronomy at the Protestant school in Graz, Austria. He accepted the position in April of 1594, at the age of 23.


In April 1597, Kepler married Barbara Muehleck. She died in 1611 and was survived by two children.

In December 1599, Tycho Brahe wrote to Kepler, inviting Kepler to assist him at Benatek outside Prague. After Tycho's death, Kepler was appointed imperial mathematician to the Hapsburg Emperor in November 1601.

In October 1604, Kepler observed the supernova which was subsequently named Kepler's Star. In January 1612 the Emperor died, and Kepler took the post of provincial mathematian in Linz.

On May 15, 1618 Kepler discoverd the distance-cubed-over-time-squared, or 'third', law of planetary motion (he first made the discovery on March 8 of that year, but at first rejected the idea).

In August of 1620, Katherine, Kepler's mother, was arrested in Leonberg as a witch; she was imprisoned for 14 months. She was released in october 1621, after attempts to convict her failed. Even though she was subjected to torture, she refused to confess to the charges.

On November 15, 1630 Kepler died of a fever in Regensburg.

Scientific work

Like previous astronomers, Kepler initially believed that celestial objects moved in perfect circles. These models were consistent with observations and with the Platonic idea that the sphere was the perfect shape. However, after spending twenty years doing calculations with data collected by Tycho Brahe, Kepler concluded that the circular model of planetary motion was inconsistent with that data. Using Tycho's data, Kepler was able to formulate three laws of planetary motion, now known as Kepler's laws, in which planets move in ellipses, not circles. Using that knowledge, he was the first astronomer to successfully predict a transit of Venus (for the year 1631).

Kepler discovered the laws of planetary motion while trying to achieve the Pythagorean purpose of finding the harmony of the celestial spheres. In his cosmovision, it was not a coincidence that the number of perfect polyhedra was one less than the number of known planets. Having embraced the Copernican system, he set out to prove that the distances from the planets to the sun where given by spheres inside perfect polyhedra, all of which were nested inside each other. The smallest orbit, that of Mercury, was the innermost sphere. He thereby identified the five Platonic solids with the five intervals between the six known planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn; and the five classical elements.

In 1596 Kepler published Mysterium Cosmographicum, or The Cosmic Mystery. Here is a selection explaining the relation between the planets and the Platonic solids:

… Before the universe was created, there were no numbers except the Trinity, which is God himself… For, the line and the plane imply no numbers: here infinitude itself reigns. Let us consider, therefore, the solids. We must first eliminate the irregular solids, because we are only concerned with orderly creation. There remain six bodies, the sphere and the five regular polyhedra. To the sphere corresponds the heaven. On the other hand, the dynamic world is represented by the flat-faces solids. Of these there are five: when viewed as boundaries, however, these five determine six distinct things: hence the six planets that revolve about the sun. This is also the reason why there are but six planets…

… I have further shown that the regular solids fall into two groups: three in one, and two in the other. To the larger group belongs, first of all, the Cube, then the Pyramid, and finally the Dodecahedron. To the second group belongs, first, the Octahedron, and second, the Icosahedron. That is why the most important portion of the universe, the Earth—where God's image is reflected in man—separates the two groups. For, as I have proved next, the solids of the first group must lie beyond the earth's orbit, and those of the second group within… Thus I was led to assign the Cube to Saturn, the Tetrahedron to Jupiter, the Dodecahedron to Mars, the Icosahedron to Venus, and the Octahedron to Mercury…

To emphasize his theory, Kepler envisaged an impressive model of the universe which shows a cube, inside a sphere, with a tetrahedron inscribed in it; another sphere inside it with a dodecahedron inscribed; a sphere with an icosahedron inscribed inside; and finally a sphere with an octahedron inscribed. Each of these celestial spheres had a planet embedded within them, and thus defined the planet's orbit.

On October 17, 1604, Kepler observed that an exceptionally bright star had suddenly appeared in the constellation Ophiuchus. (It had appeared on October 9 previous.) The appearance of the star, which Kepler described in his book De Stella nova in pede Serpentarii ('On the New Star in Ophiuchus's Foot'), provided further evidence that the cosmos was not changeless; this was to influence Galileo in his argument. It has since been determined that the star was a supernova, the second in a generation, later called Kepler's Star or Supernova 1604. No further supernovae have since been observed with certainty in the Milky Way, though others outside our galaxy have been seen.

In his 1619 book, Harmonice Mundi or Harmony of the Worlds, as well as the aforementioned Mysterium Cosmographicum, he also made an association between the Platonic solids with the classical conception of the elements: the tetrahedron was the form of fire, the octahedron was that of air, the cube was earth, the icosahedron was water, and the dodecahedron was the cosmos as a whole or ether. There is some evidence this association was of ancient origin, as Plato tells of one Timaeus of Locri who thought of the Universe as being enveloped by a gigantic dodecahedron while the other four solids represent the "elements" of fire, air, earth, and water.

To his disappointment, Kepler's attempts to fix the orbits of the planets within a set of polyhedrons never worked out, but it is a testimony to his integrity as a scientist that when the evidence mounted against the cherished theory he worked so hard to prove, he abandoned it.

His most significant achievements came from the realization that the planets moven in elliptical, not circular ,orbits. This realization was a direct consequence of his failed attempt to fit the planetary orbits within polyhedra. Kepler's willingness to abandon his most cherished theory in the face of precise observational evidence also indicates that he had a very modern attitude to scientific research. Kepler also made great steps in trying to describe the motion of the planets by appealing to a force which resembled magnetism, which he believed emanated from the sun. Although he did not discover gravity, he seems to have attempted to invoke the first empirical example of a universal law to explain the behaviour of both earthly and heavenly bodies.

Kepler also made fundamental investigations into combinatorics, geometrical optimization, and natural phenomena such as snowflakes, always with an emphasis on form and design. He was also notable for defining antiprisms. In addition, since he was the first to recognize the non-convex regular solids (such as the stellated dodecahedra), they are named Kepler solids in his honor.

In 1632, only two years after his death, his grave was demolished by the Swedish army in the Thirty Years' War.

Kepler on God

"I was merely thinking God's thoughts after him. Since we astronomers are priests of the highest God in regard to the book of nature," wrote Kepler, "it benefits us to be thoughtful, not of the glory of our minds, but rather, above all else, of the glory of God."


Writings by Kepler

External links