Johannes Kepler developed three laws of planetary motion that revolutionized astronomy and led to Isaac Newton’s discovery of the laws of gravity.
Johannes Kepler was born to a poor family in Weil, near Würtemburg, Germany, on December 17, 1571. His father served as a soldier in the duke's army and later owned a tavern where Johannes was employed from ages nine to twelve. His mother, Catherine, who was noted for her fiery temperament, also worked at the tavern and cared for her home and family.
When Kepler was only four years old, he suffered a severe attack of smallpox. The illness did permanent damage to his eyesight and left him very weak. For the rest of his life he was unable to go out at night because of his frail constitution. With outdoor activities severely limited, Kepler turned to reading and writing. He was fascinated by numbers and continually asked questions--especially about the way the universe worked.
At the age of seventeen Kepler enrolled at the University of Tübingen, where he studied astronomy (the study of objects in outer space) and religion and graduated second in his class. In college, he was exposed to the ideas of Nicolaus Copernicus (1473-1543), the first astronomer to assert that the Sun was the center of our solar system. The Catholic Church at that time held that Earth was the center of the universe, so Copernicus's position was strongly opposed. But Kepler believed in Copernicus's model and defended it in debates at his university. He earned a master's degree in 1591, and for the next two years worked within the Lutheran Church. Like his mother, however, Kepler had a fiery disposition, and he clashed with Lutheran authorities over the strictness of the church. In 1594, when the opportunity arose for him to teach astronomy at the University of Gratz, he left the church and became a professor.
Life of a sixteenth-century astronomer
During the sixteenth century, astronomy was closely tied to astrology. At that time, like today, astrology represented a not-quite scientific body of knowledge that asserted that the positions of the stars and planets dictate human destiny and control everything from weather patterns to political events. Therefore, the job of an astronomer in Kepler's time was not only to make accurate calendars but to forecast weather, prepare horoscopes, and make political predictions. Kepler's first years at Gratz were spent pursuing these activities, but he also spent long nights poring over books, making calculations, and guessing as to the nature of the universe.
First book and theory
Kepler sought to find a connection between the number of planets (then it was believed there were only six: Mercury, Venus, Earth, Mars, Jupiter, and Saturn), their times of revolution, and their distance from the Sun. In his 1596 publication Mysterium Cosmographicum, he put forth a mathematical hypothesis (theory) to explain such a connection. Although the hypothesis is now known to be flawed, at the time it was widely acclaimed and helped establish Kepler as a first-rate astronomer and intellectual.
After years of hard work and calculation, Kepler was thrilled at his first discovery and wrote: "The intense pleasure I have received from this discovery can never be told in words. I regretted no more the time wasted; I tired of no labour; I shunned no toil of reckoning, of days and nights spent in calculation, until I could see whether my hypothesis would agree with the orbits of Copernicus, or whether my joy was to vanish in air." The book captured the attention of two famed astronomers, Tycho Brahe (pronounced TEE-ko BRA-hee; 1546-1601) and Galileo Galilei (1564-1642), who would later become very involved in Kepler's life and lead him to his greatest achievements.
Brahe and brilliance
Though Kepler had always been a brilliant thinker, he lacked the data to calculate, develop, and prove his theories. After his first book came out, he began corresponding with Danish astronomer Tycho Brahe, who had spent more than twenty years compiling celestial observations.
Kepler sent Brahe a copy of his book and the two began corresponding by letter. Although Brahe had an idea about the organization of the solar system that was very different from that of Copernicus and Kepler, he greatly respected Kepler's work. For several years, the two had friendly and frank exchanges in which Brahe continually tried to convert Kepler to his "Tychonic System"--which placed Earth at the center of the solar system, with the Sun and other planets revolving around it. Kepler politely but forcefully argued otherwise.
When Kepler was forced to leave his teaching position at Gratz in 1599, he called on Brahe, who was then Imperial Mathematician of the Holy Roman Empire. Brahe was by this time old and weak, and Kepler realized that no one had yet tapped his vast store of knowledge. He offered to examine Brahe's compilation of observations and use them to perfect theories of planetary motion. Brahe eagerly accepted Kepler's proposal, saying: "Come not as a stranger, but as a very welcome friend; come and share in my observations with such instruments as I have with me, and as a dearly beloved associate."
Kepler's first two laws of planetary motion
The timing could not have been better for the meeting of two such brilliant minds. Brahe was near death, and Kepler was able to access his lifetime's accumulation of knowledge before he passed away. Brahe put Kepler to work calculating the movements of Mars. Kepler also worked with Brahe on the Rudolphine Tables (which described planetary movements) and pledged, as the older astronomer's health worsened, that he would complete them. After Brahe's passing in 1601, Kepler was named Imperial Mathematician and continued to work at Brahe's observatory at Benatky Castle.
For six years Kepler struggled to explain the way the planets move. After trying hundreds of hypotheses, Kepler finally came up with a brilliant guess that was verified by Brahe's data. The vision of planets moving in ellipses (ovals) apparently came to Kepler in a dream; when he awoke, he spent the rest of the night and the next day testing his theory. At last he had discovered the first two laws of planetary motion: a planet orbits the Sun in an ellipse, and it moves faster when nearer the Sun and slower when farther away.
Setbacks and hardship
While Kepler searched for the answers to more questions about the planets, he suffered many hardships in his personal life. By 1611, both his wife and his young son had died. He was left alone to care for his two remaining small children. Kepler's life was further complicated by Emperor Rudolf II's death in 1612. The emperor had been his patron, and after he died, Kepler's salary was cut off. In order to provide for his children, Kepler was forced to leave Benatky and set aside work on Brahe's research.
Kepler once again became a professor of astronomy, this time at the university in Linz. To make extra money, he published an almanac that predicted future political events and weather patterns. In 1618, after sixteen years of investigation, Kepler finally hit upon his third law. Kepler's third law of planetary motion states that the square of the time of each planet's revolution is proportional to the cube of its mean (average) distance from the Sun. In other words, using this mathematical formula, you could calculate a planet's distance from the Sun if you knew how long it takes the planet to complete one orbit.
At age forty-seven Kepler had fulfilled his lifelong dream and lived to see it published. However, because his third law was based on the Copernican idea of a Sun-centered solar system, his book was banned by the Church. The church decree adversely affected his career; he was never able to secure a publisher for his work again.
For an entire decade Kepler was unable to publish his own work, so he once again took up the Rudolphine Tables, which Brahe had begun. They were published in 1627, but Kepler had to pay for the printing himself. He had promised Brahe at his death he would see them published and though he was very poor, he fulfilled his great friend's last request.
Unfortunately, Kepler's last years were ones of hardship and struggle. He was owed a considerable amount of money from the Holy Roman Empire and in 1630 he traveled to Prague to petition the government for his past-due wages. However, on the trip he became very ill, and in November 1630, at age fifty-nine, he died. He was buried at St. Peter's Church in Ratisbon, Germany.
- Lodge, Oliver, Pioneers of Science, St. Martin's, 1913.
- Ronan, Colin, The Astronomers, Hill & Wang, 1964.
- Rosen, Edward, ed., Kepler's Conversation with Galileo's Sidereal Messenger, Johnson Reprint Corporation, 1965.
- Tiner, John Hudson, Johannes Kepler, Giant of Faith and Science, Mott Media, 1977.