Johannes Kepler

• 1. A Child Of The Stars

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  Johannes Kepler was born on the 27th of December 1571 in the Free Imperial City of Weil der Stadt. His birth came prematurely, and he claimed to have been weak and sickly as a child. Smallpox later left him with weak vision and crippled hands, limiting his ability in observational astronomy. Despite these physical challenges, he often impressed travelers at his grandfather's inn with his phenomenal mathematical faculty. At age six, he observed the Great Comet of 1577, writing that he was taken by his mother to a high place to look at it. In 1580, at age nine, he observed another astronomical event, a lunar eclipse, recording that he remembered being called outdoors to see it and that the Moon appeared quite red. He attended the Grammar School in Weil until 1577, when the family moved to Leonberg in Protestant Württemberg. He then attended two monastic schools from 1584 in Adelberg and from 1586 at the seminary at Maulbronn. In September 1589, Kepler entered the Tübinger Stift at the University of Tübingen. There, he studied philosophy under Vitus Müller and theology under Jacob Heerbrand. Under the instruction of Michael Maestlin, Tübingen's professor of mathematics from 1583 to 1631, he learned both the Ptolemaic system and the Copernican system of planetary motion. He became a Copernican at that time.

• 2. The Search For Order

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  Kepler claimed to have had an epiphany on the 19th of July 1595 while teaching in Graz. He demonstrated the periodic conjunction of Saturn and Jupiter in the zodiac and realized that regular polygons bound one inscribed and one circumscribed circle at definite ratios. He reasoned that these geometrical relationships might be the basis of the universe. After failing to find a unique arrangement of polygons that fit known astronomical observations, Kepler began experimenting with three-dimensional polyhedra. He found that each of the five Platonic solids could be inscribed and circumscribed by spherical orbs. Nesting these solids within one another would produce six layers corresponding to the six known planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. By ordering the solids selectively, Kepler found that the spheres could be placed at intervals corresponding to the relative sizes of each planet's path. His first major astronomical work was Mysterium Cosmographicum, published late in 1596. It established his reputation as a highly skilled astronomer despite not being widely read initially. The book contained an extensive chapter reconciling heliocentrism with biblical passages that seemed to support geocentrism.

• 3. The Orbit Of Mars

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  In October 1604, a bright new evening star appeared, but Kepler did not believe the rumors until he saw it himself. He began systematically observing the supernova and described the new star two years later in his De Stella Nova. This work addressed the star's astronomical properties while taking a skeptical approach to astrological interpretations. The extended line of research that culminated in Astronomia Nova began with the analysis of the orbit of Mars under Tycho Brahe's direction. Kepler calculated and recalculated various approximations of Mars's orbit using an equant. Eventually creating a model that generally agreed with Tycho's observations to within two arcminutes, he was still not satisfied. At certain points, the model differed from the data by up to eight arcminutes. After approximately forty failed attempts, in late 1604 he hit upon the idea of an ellipse. He had previously assumed this to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fit the Mars data, Kepler immediately concluded that all planets move in ellipses with the Sun at one focus. This became his first law of planetary motion. Because he employed no calculating assistants, he did not extend the mathematical analysis beyond Mars.

• 4. The Music Of Spheres

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  In Harmonice Mundi published in 1619, Kepler attempted to explain the proportions of the natural world in terms of music. The central set of harmonies was the musica universalis or music of the spheres. He tried many combinations until he discovered what came to be known as the third law of planetary motion. He stated that the square of the periodic times are to each other as the cubes of the mean distances. Although he gives the date of this epiphany as the 8th of March 1618, he does not give any details about how he arrived at this conclusion. In the final portion of the work, Book V, Kepler dealt with planetary motions and relationships between orbital velocity and distance from the Sun. Similar relationships had been used by other astronomers, but Kepler treated them much more precisely. When conjoined with Christiaan Huygens' newly discovered law of centrifugal force, it enabled Isaac Newton and others to demonstrate that gravitational attraction decreased with the square of the distance. This refuted the traditional assumption of scholastic physics that the power of gravitational attraction remained constant with distance.

• 5. Light And Vision

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  Through most of 1603, Kepler paused his other work to focus on optical theory. The resulting manuscript presented to the emperor on the 1st of January 1604 was published as Astronomiae Pars Optica. In it, Kepler described the inverse-square law governing the intensity of light and reflection by flat and curved mirrors. He also extended his study of optics to the human eye and is generally considered by neuroscientists to be the first to recognize that images are projected inverted and reversed by the eye's lens onto the retina. Later that year, Galileo Galilei discovered four satellites orbiting Jupiter using a powerful new telescope. Kepler responded enthusiastically with a short published reply titled Dissertatio cum Nuncio Sidereo. He endorsed Galileo's observations and offered speculations about their meaning for astronomy and cosmology. To Kepler's disappointment, however, Galileo never published his reactions to Astronomia Nova. Kepler also started a theoretical investigation of telescopic lenses using a telescope borrowed from Duke Ernest of Cologne. The resulting manuscript was completed in September 1610 and published as Dioptrice in 1611.

• 6. A Mother On Trial

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  In December 1615 Kepler received a letter from his family in Württemberg informing him that his mother Katharina had been accused of witchcraft earlier that year. The initial accusation was made by Ursula Reinbold who claimed that Katharina had given her a drink that made her ill. As the case became known, more rumors and accusations circulated. Kepler vowed to defend his mother both by despatches sent to the authorities in Leonberg and by visiting in person. The case dragged on for several years with Katharina held in prison from 1620 to 1621. The final stage was held in Tübingen under the authority of the Duke where it was determined she should be questioned under the threat of torture. She refused to confess saying she trusted God to bring the truth to light. She was then absolved and discharged being released on the 4th of October 1621. She died about six months later. This process has been seen as part of an attack by Lutheran authorities against Kepler himself following his excommunication.

• 7. The Final Years

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  Kepler's wife Barbara contracted Hungarian spotted fever and began having seizures while recovering all three of their children fell sick with smallpox. Six-year-old Friedrich died during this period. Barbara relapsed into illness and died shortly after Kepler's return to Linz. In July 1612 Kepler met Matthias Bernegger who sought him out and became his best friend. They maintained extensive correspondence for nearly twenty years though they never met again. Following the death of his first wife, Kepler considered eleven different matches over two years before returning to Susanna Reuttinger. Their marriage proved much happier than his first. Three more children survived into adulthood: Cordula born in 1621, Fridmar born in 1623, and Hildebert born in 1625. The move to Sagan had not solved Kepler's financial problems. He got little assistance from Wallenstein apart from his salary. On the 8th of October 1630 he set out for Regensburg hoping to collect at least some of the money owed to him. A few days after reaching Regensburg he became sick and progressively worsened. Kepler died on the 15th of November 1630 just over a month after his arrival.