Kepler and the Universe: How One Man Revolutionized Astronomy
Johannes Kepler (1571-1630) and Galileo Galileo (1564-1642) are the two names that pop up in most people's mind when it comes to the acceptance of the heliocentric planetary system that was previously announced by Nicolaus Copernicus (1473-1543). The ancient Greek tried to explain the trajectories of the Sun and the planets as circles with our Earth in the center. When that didn't fully explain things, Apollonius proposed epicircles and Ptolemy refined this a bit by placing the Earth just outside the center of the circles. That system was the general belief, until Copernicus picked up an idea of Aristarchus (3rd century BCE) and proposed a model where the Earth and the planets circled around the Sun. Some accepted this system not necessary as a reality but rather as a simplification for the computations to make predictions. There was also a compromise system proposed by the Danish astronomer Tycho Brahe where the planets were all circling around the Sun, but the Sun and the Moon were circling around a stationary Earth.
Kepler was born in a divided Germany where the religion of a region depended on the choice of ruler of the region. This was an arrangement to bring some peace between Lutherans and Catholics. Kepler was Lutheran which is important to get his sacrilegious theories published. The heliocentric visions were considered unbiblical and were severely prosecuted by the Catholics as Galileo experienced in an Italy dominated by the Catholics. The Lutherans were a bit more tolerant, but still such theories were frowned upon also by them. Kepler had an unhappy youth in a wrecked family and he had many health problems. He writes in a later autobiography that many people seemed to hate him. He was a brilliant student and thanks to his mentor Michael Maestlin he got a position in Tübingen in pursuit of a clergy position. However, because of his Copernican ideas and Calvinist tendencies, he was expelled to a lower teaching position in Graz where he wrote his Mysterium Cosmographicum. He tries to answer three questions: why are there only 6 planets, why are they at a certain distance from the Sun, and why do more distant planets move more slowly? He (wrongly) thought to have found a solution for the first two questions by placing the five Platonic solids in between the planets. This shows that the ideas of the ancient Greeks were still strongly present in the Kepler's mind.
The publication of this book brought him some fame and he got in contact with Galileo and Brahe. Especially the latter was important because Brahe had the most accurate observations available at that time. However Brahe was very protective about his data because he previously had a bad experience with another visitor, Nicolaus Reimer, who claimed Brahe's compromise solar system model as his own, resulting in a lifelong fight about plagiarism between these two. After Brahe died from benign prostatic hyperplasia, Kepler was appointed by emperor Rudolph II as a replacement for Brahe in Prague to work on the Rudolphine Tables that should allow to predict planetary positions. Now Kepler had access to Brahe's data and this led to his major work the Astronomia Nova. This is the work where Kepler formulates his famous second law, describing the speed of the planets on their elliptic trajectories around the Sun sweeping equal areas in equal time. Love gives a step by step analysis of this voluminous work of 600 pages. In this Prague period Kepler was very productive and wrote about optics, the human eye, observed a supernova that was named after him, he computed the precise date of Jesus' birth, and analysed the six-pointed snowflake. He described even space travel is a science fiction-like fashion.
The first telescopes were produced which improved the accuracy of the observations considerably. Galileo published his Sidereus Nuncius (The Starry Messenger) based on telescope data which strongly supported the Copernican system. Kepler was very supportive of Galileo's work. His letter with commentaries was published as a book (Kepler was never concise) Conversation with Galileo's Messenger from the Stars. The discovery of four moons of Jupiter by Galileo led Kepler to the prediction that Mars should have two moons and Saturn six or eight. This was based on a wrong arithmetic or geometric progression that he thought to be present in our solar system. On the other hand he formulated several other speculations that turned out to be rather close to reality. For example his argument used to explain Olbers' paradox: the fact that it is dark at night can be explained if we accept a finite universe. Galileo announced more of his findings in the form of anagrams (to claim later priority since scientific journals did not exist in those days). Some of these were wrongly deciphered by Kepler. Such misinterpretations of facts seemed to have happened frequently to Kepler. He sometimes stuck to a wrong idea and predicted something based on wrong arguments that later turned out to be surprisingly accurate.
Kepler's third law was written up in his book Harmonices Mundi that he wrote while he was in Linz. This law relates the distance of a planet from the Sun with the period of its revolution. This finally settled the answer to the third question he had when writing his Mysterium Cosmographicum. After that he eventually finished and published the Rudolphine Tables during the last period of his life that he spent in Ulm.
All this work he did notwithstanding all the misfortune he had in his personal life. He married and was widowed twice. He had twelve children of which only three reached adulthood. He had problems with the heirs of Brahe, and he had to watch his steps on the thin ice of religion and politics. His mother died shortly after she was acquitted of witchcraft and his life ended in the middle of the Thirty Years War that eliminated one third of the German population. He died probably in a bout of quartan fever of which he suffered intermittently most of his life.
Love brings this biography in a very readable form, also for somebody who is not a historian or a specialist in celestial mechanics. He is obviously an admirer of Kepler, but does not shy away to expose where Kepler has erred. The main thread of the book is not Kepler's personal life, but the subdivision in chapters and the chronology is directed by Kepler's scientific results that eventually have led to his three laws of celestial mechanics. There are many illustrations and at the end an epilogue is added prolonging the post-Keplerian history of astronomy and cosmology via Newton and the eighteenth till the twenty-first century. This allows Love to mention the Kepler crater on the moon and NASA's Kepler mission launched in 2009 to discover earth-like planets in our universe.