Johannes Kepler (1571-1630)

Christians who Changed their World

This is part of an on-going series of articles about largely unknown Christians who had an enormous impact on society by faithfully living out their biblical worldview in various areas of life.

The 1500s saw enormous changes in Europe. Columbus had sailed, but it was still unclear in 1500 what exactly he had found. Within decades, however, the first global empires were founded. Gunpowder sparked a military revolution that would change warfare and set up European domination of the world over the next centuries. The Reformation split the Western church leading to religious war and persecution across Europe. And the scientific revolution had its beginnings with the publication of Nicholas Copernicus’s On the Revolution of the Heavenly Spheres (1543).

Contrary to popular belief, Copernicus’s theory was less about modern ideas of science and more about philosophy. The old Ptolemaic system, which had the earth as the center of the universe (“geocentric”), needed a lot of adjustments to make it work, including having planets move in circles at varying speeds. Copernicus found this idea distasteful, since his aesthetic sense said that in the heavens, motion should be perfect—that is, that everything should move in circles at a constant speed. Further, he was influenced by the Renaissance discovery of ancient Greek Pythagoreanism, a philosophical/religious system which worshipped the sun.

Because of these considerations, Copernicus proposed a sun-centered (“heliocentric”) model of the universe, where everything moved in perfect circular motion. Since the observable movements of the planets don’t align with this, he followed the older model’s use of epicycles (circles on circles) and even double epicycles. In the end, he had more circles in his system than Ptolemy’s did.

Copernicus’s system was easier to use mathematically than Ptolemy’s, though it was just as complicated; it was also no more accurate since the Ptolemaic system could be adjusted to make it every bit as accurate as Copernicus’s.

Further, the idea that the earth moved violated Aristotle’s physics, which was based on the idea that earth as the densest element naturally sinks to the center of the universe. Without that idea, the rest of Aristotle’s physics does not work. And since all physics in the period was based on Aristotle, the price of making the sun the center of the universe was losing all of physics, with simpler math as the only benefit. This was a tradeoff few scientists would take, and so prior to 1600, there were very few committed Copernicans.

Johannes Kepler was one of those Copernicans. Kepler was born in Weil-der-Stadt near Stuttgart in 1571. Although his grandfather had been the town’s mayor, the family had fallen on hard times. His father had intermittent work as a mercenary and left the family when Kepler was five, never to return. He is presumed to have been killed in battle. His mother was the daughter of an innkeeper who made a living as a healer and herbalist.

Kepler was born prematurely and was sickly as a child. He even had a bout of smallpox, which left him with poor eyesight and crippled hands. He grew up in poverty and never managed to achieve significant financial success at any point in his life.

Kepler was a devout Lutheran and planned to become a pastor. He excelled at mathematics, however, and had an interest in astronomy from childhood. At the university, he learned both Ptolemaic and Copernican astronomy. He was convinced by Copernicus and defended it on both scientific and theological grounds at the university. When he graduated in 1594, he was recommended for a position teaching mathematics at the Protestant school at Graz (now the University of Graz) in Austria.

While in Graz, Kepler began to develop a theory about the number of planets and the relative size of their orbits. Without going into detail, he found that his theory worked for all the planets except Jupiter. He decided to make an adjustment to the theory to make it work, but he was convinced the problem would be solved if he had better observations to work with. And the best observational astronomer in history to that point was living at Benátky nad Jizerou, about 20 miles from Prague. His name was Tycho Brahe.

In 1600, Kepler began negotiating with Tycho for access to his data. Tycho for his part recognized Kepler’s genius, and after some ups and downs he agreed to allow Kepler to work with him. Religious conflicts in the area made it difficult for Kepler to move to Bohemia, however. Meanwhile, the Counter-Reformation was systematically eliminating Protestantism in Austria; Kepler’s refusal to convert to Catholicism led to his family being banished from Graz. Fortunately, conditions were right for the family to move to Benátky nad Jizerou, and so Kepler was able to join Tycho.

Kepler and Tycho were working together on a set of tables of planetary motion when Tycho died unexpectedly in 1601. Kepler was appointed Tycho’s successor as imperial mathematician by Holy Roman Emperor Rudolf II, which gave him some protection against religious pressures. Kepler continued Tycho’s work on compiling the tables of planetary motion, and began to analyze the data to develop a more accurate model of the universe.

Tycho’s observations were as good as they could be in the day of naked eye observation, and Kepler was determined to use them to the full. Initially, he could not find any formula, whether geocentric or heliocentric, that would work. Heliocentrism was close, but not up to the known margin of error of Tycho’s observation. So Kepler decided to give up on circles and try ellipses; this was a much better fit, though the data suggested that the planets changed speeds in their orbits. So he then gave up on uniform motion and worked out the equal area rule, which describes how the planets’ speeds change in the course of their orbits, and the inverse square law, which defines the relationship of the size of the orbit with the time it takes for the planet to complete its circuit. In other words, Kepelr gave up the key aesthetic considerations that drove Copernicus to reject geocentrism, and in the process discovered his Three Laws of Planetary Motion.

While doing all that, Kepler also worked on optics, among other things improving the refracting telescope.  He also studied the human eye, and was the first to figure out that its lens projects an inverse image onto the retina. And he made important advancements in mathematics and in theories of motion.

When Rudolf was forced to abdicate as emperor (1611), his successor Matthias kept Kepler on as imperial mathematician. Religious tension led the emperor to relocate Kepler to Linz (1612), where he was welcome to worship but kept from the Eucharist because of suspicions that he had Calvinist sympathies.

The Thirty Years’ War began in 1618, however, and when Linz was besieged in 1626 Kepler was forced to withdraw to Ulm. As imperial astronomer (and astrologer), Kepler became an advisor to Wallenstein, the emperor’s most important general. Interestingly enough, although the war was fought in part over religion, both Wallenstein and Kepler were Protestants supporting the Catholic imperial side. Kepler spent the last three years of his life traveling and acting as an advisor to the emperor and to Wallenstein.

Kepler’s work was motivated by a complex set of ideas. First and foremost, he was a Christian natural philosopher: he believed that a rational God created a rational universe, and we as beings made in God’s image are also rational and can therefore work out the laws governing the universe. In many ways, this was the governing principle behind all of his work.

This commitment to understanding the mind of God through the creation led him to be an absolutely rigid empiricist. He believed that God gave him Tycho’s data, and thus it was his responsibility to use it to the full. Particularly remarkable is how far he was willing to go with this. For example, the earth’s orbit is only 1 part in 6,000 away from being a perfect circle. For that small amount, Kepler was willing to jettison circular motion. Most other natural philosophers would have chalked that up to observational error, but Kepler knew the margin of error of Tycho’s observations and he believed God expected him to honor the quality of the data, not to make it conform to his preconceptions about how it “should” be.

Kepler was also the last great Pythagorean. He believed that God was a geometer, and geometry held the key to understanding the universe. And of course, he was a committed Copernican. These motivations, along with Kepler’s mathematical genius, all interlocked to lead Kepler to discover the laws of planetary motion.

He was also a man of his era. He practiced astrology as an element of applied astronomy, for example—in the period, the words were used interchangeably—and for this reason he was appointed as an advisor to the emperor. That said, his advice generally had more to do with common sense than astrological theory.

But he also led the way to the future, turning Copernicanism into mathematical science, joining physics (which had been a branch of philosophy) with mathematics and geometry, and laying a foundation for the work of later scientists, including particularly Isaac Newton.

Kepler knew his theories would be on shaky ground with most scientists. But he didn’t care: he figured that it had taken God 5,000 years before anyone discovered how He structured the universe; Kepler could wait a century to be proven right. His faith in the intelligibility of the universe governed by divine reason, the Logos, led him to examine the world systematically, not take short cuts, and to use everything God gave him to the full.


Dr. Glenn Sunshine is a professor of history at Central Connecticut State University and a Senior Fellow at the Colson Center for Christian Worldview.

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