Johannes Kepler, Thinking God’s Thoughts After Him

November marks the death of Johannes Kepler, one of the most important figures of the Scientific Revolution and a scientist who was motivated by his Christian beliefs. The significance of Kepler’s work can only be understood in light of what he faced and risked. The settled science of his day was that the Earth stood at the center of the universe. To challenge that meant to challenge the entire, accepted understanding of physics.  

When Copernicus published On the Revolution of the Heavenly Spheres in 1543, he argued that the universe was centered on the sun rather than Earth. His motivation was to preserve the idea that planets traveled at a constant velocity in perfect circles. In other words, his motivation was more philosophical and aesthetic than it was scientific. Few scientists accepted these ideas that contradicted settled science. As a result, there were only a handful of committed Copernicans prior to 1600. Johannes Kepler was one of them.  

Kepler was a devout Lutheran who planned to become a pastor. However, he excelled at mathematics and had an interest in astronomy. In seminary at the University of Tubingen, he became convinced by Copernicus and defended him on both scientific and theological grounds. After graduating in 1594, he took up 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. He found that his theory worked for all planets except Jupiter. Though he adjusted the theory to make it work, he was convinced the problem would be solved with better observations. As it turned out, the best observational astronomer in the world, Tycho Brahe, lived nearby.  

In 1600, Kepler negotiated with Tycho for access to his data. Tycho recognized Kepler’s genius and eventually agreed to work together. However, a year later, Tycho unexpectedly died. Kepler was appointed his successor as imperial mathematician, which enabled him to continue compiling and analyzing data on planetary motion in order to develop a more accurate model of the universe. 

Tycho’s observations were as good as was possible with the naked eye, and Kepler was determined to use them. Initially, he could not find a formula, whether geocentric or heliocentric, that would work.  

Heliocentrism was close, but not up to the known margin of error of Tycho’s observation. This led Kepler to give up on circles and try ellipses, which fit better, but not perfectly. After playing with some very complicated math, Kepler arrived at a solution that, in the end, proved Copernicus right about the Earth going around the sun. In the process, Kepler discovered his Three Laws of Planetary Motion which stand even to today. 

Kepler’s work was motivated by his Christian faith. He believed that since God is rational, the universe must be as well. Because humans are made in God’s image, we can, as he said, “think God’s thoughts after Him.” In other words, understanding the universe is possible. 

This commitment led Kepler to be a rigid empiricist. Because God had given him Tycho’s data, he was responsible to use it as fully as he could. For example, the earth’s orbit is less than .02% away from being a perfect circle. Even that small amount made Kepler willing to jettison the supposed perfection of circular motion favored by the scientists of his day.  Though most others would have chalked that up to observational error, Kepler knew the margin of error of Tycho’s observations and believed God expected him to honor the quality of the data, rather than conform it to his preconceptions about how it “should” be. 

Kepler knew his theories would be rejected by scientists, but he didn’t care. It had taken eons before anyone discovered how God had structured the universe, so Kepler figured he could wait another century or so to be proven right. His faith in the intelligibility of the universe was grounded in his belief that the world was governed by divine reason, the Logos. This led him to examine the world systematically, to not take shortcuts, to use what God gave him and, in the end, to lay the foundations for modern astronomy and physics. 

This Breakpoint was co-authored by Dr. Glenn Sunshine. 

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