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Johannes Kepler fascinates me not just because of his scientific accomplishments but because of the world in which he lived. He was born in 1571, only 25 years after the death of Martin Luther. Many people of that time had little education. It seems to me that when people get no education or their education is very ineffective, then the people who are products of that tend toward superstition. This was the world Kepler grew up in. So much knowledge we take for granted today is something people did not have at that time. They did not know that bacteria existed and that this caused the spread of disease. Mathematics and arithmetic had to be worked out by hand. If you sent a letter to someone there was a good chance they would never get it. Most people in Europe believed in the God of the Bible, but there was tension and sometimes outright hatred between Catholics and Protestants. The tensions between Catholics and Protestants also led to different cities using different calendar systems. This meant that if you moved to a different city you could lose days of the year. Johannes Kepler dealt directly with superstition and wrestled with how to balance faith and reason. The world needed to move away from superstition toward reason and knowledge. I think Kepler’s life and his discoveries helped the world make this transition. Also, this seems to have been God’s purpose.

In Kepler’s world superstitious ideas became common regarding many things people did not understand. One area in which this was true was astrology. Another was in fear of witches and magic. People were afraid of even being touched by a witch; they feared they would become ill. In the 1500's and 1600's there were many women executed for witchcraft in German-speaking lands. The trials of witches were often unfair and did not provide much opportunity for defense of the accused. If a person was accused of witchcraft, the problems would affect not only that person but possibly destroy the reputation of the accused person’s family as well. Even in medicine, there were still some superstitious ideas about treating ailments. People experimented with herbs and plants for remedies to various sicknesses without knowing the dangers. Also, some believed that gold could somehow be made from other metals. This was one of the goals of alchemy, but some experimented in alchemy for other purposes. Alchemy involved experimenting with various substances without knowledge of chemistry or the properties of the elements of the periodic table. In all of these matters, mankind’s ignorance was endangering lives and thus there was a great need for scientific knowledge.

Johannes Kepler grew up in this world and was influenced by these superstitions. But he set an example that I think helped people move toward reason and scientific answers. One way Kepler was influenced by superstition was in astrology. In Kepler’s time there were varying views of astrology. Some would accept astrological ideas more completely than others. Some Protestant Christians such as Kepler took a more limited view of astrology. Kepler was employed officially as a mathematician for various rulers and those rulers often required that he publish calendar books that would have been similar to an almanac. They would have the calendar included for a coming year but they would also make recommendations and predictions of what may happen in coming months in world events. Sometimes the weather was predicted using astrology. Outcomes of wars and changes in who would hold certain government positions might be predicted. But Kepler used a method that considered astrology plus other facts about the world. He didn’t predict the weather just from astrology but he also actually made weather observations throughout the year as well. One part of astrology was in the belief that the positions of the planets at the time of your birth could affect tendencies that you would experience in your life. Another aspect of astrology was in astronomical events such as conjunctions of the planets, the Moon and planets, or eclipses, and interpreting some significance from the timing of other astronomical events. When medical doctors were trained in those days, their training included training in astrology because people believed astronomical events could affect people’s health. Kings often considered astrological prognostications for the coming year to be very important and this made the work of their mathematician important.

Astrology is actually of no value and the Bible speaks against it in the Old Testament. In Deut. 4:19 it says not to “worship and serve” the Sun, Moon, and stars because they are created things God has allotted to all people. Isaiah 47:13 essentially says that astrological predictions are useless; they cannot save from anything. In Daniel 2:1-28 it lists various advisors including “magicians” and “sorcerers” that could not offer any insight into King Nebuchadnezzar’s dream. Astrology is very likely to have been part of the “arts” practiced in ancient Babylon. But the prophet Daniel gave good insight to the King without use of astrology. In Johannes Kepler’s day there was a view of astrology that held that God allowed for some astrological predictions to be correct as a kind of accommodation for communicating with men. I think this is a wrong concept also but this was part of Kepler’s understanding.

Johannes Kepler took a kind of “limited” view of astrology. Keep in mind he could not have done his astronomical or mathematical work without producing the calendar books. They were required by his employers throughout most of his life. He rejected using the Zodiac in making horoscopes. He discouraged too much reliance on astrological predictions corresponding to world events or to events in a person’s life. He would say that an individual’s choices can prevent or improve the outcome of events in life, compared to an astrological prediction. So he did not see human beings as “slaves to the planets.” There are various statements Kepler made against astrology even though he did believe in some of it. But he was aware that many had an unhealthy reliance on it that was not good. He made the following statement in a letter to his former astronomy professor: “Truly in all my knowledge of astrology, I know not enough with certainty that I should dare to predict with confidence any specific thing. Astrology is the foolish little daughter of mother astronomy.” On the other hand, he believed that the time of a baby’s birth unconsciously influenced the person their entire life, due to the positions of the Moon and planets on that date. Today this is a strange idea to us but it was widely accepted in Kepler’s day. A Christian view would emphasize that we are the product of the choices we make and what we learn. Many factors influence us from family, society, our education, etc. but the planets just are not a factor that affects our personality or the events of our lives. In spite of these problems with Kepler’s thinking, some of this prompted the efforts to do detailed observations of the planets and this led eventually to people emphasizing the science more and the astrology less. Kepler was able to be of some influence to various political leaders to not overreact to astrological predictions, which were unreliable.

After Kepler had already become well known as an astronomer and mathematician his Mother Katharina was falsely accused of being a witch (in 1615 at age 68) and was imprisoned for some time. This is a long story, but Kepler likely saved his mother from a terrible death. A woman accused Katharina of giving her a witch’s potion that made her ill. Others joined into the accusations, often based only on hearsay. Kepler's mother was a difficult and cantankerous woman who sometimes made people angry with her but I believe she was actually a Protestant Christian, as was most of the Kepler family. It became an escalated legal battle that took six years. Kepler assembled a legal case to defend his mother against the witchcraft charges. Johannes Kepler had to combat superstition, rumors, and dishonest accusations in order to put together a defense of his Mother. Katharina was imprisoned in bad conditions and at one time was threatened with torture, but this was likely a ploy to frighten her into confessing to be a witch. She was not actually tortured. Yet, she steadfastly maintained her innocence and toward the end gave testimony of her Christian faith. The charges against her were eventually dismissed in 1621 but she only lived six months after her release. Kepler went to great effort and expense to defend his mother. This is one of the most impressive things about Johannes Kepler's Christian character to me. I think he deserves recognition for this effort to save his Mother.

Johannes Kepler is most known for his three laws of planetary motion, but there is much more to his scientific work. I’m interested in his concept of intelligent design regarding the orbits of the planets. In some ways he was limited by the limited understanding people of his time had. But in other ways, he really led the way to new insights that are really like modern science. In Kepler’s day, people believed in gravity but the accepted concept, including among astronomers, was that gravity only applied to the Earth and gravity pulled objects to the center of the universe, which was the center of the Earth. However, Kepler believed gravity did operate in outer space, though he did not understand gravity correctly as a force. He looked for a cause for planetary motion as well as a mathematical description of it. He was criticized by other astronomers for trying to explain a physical cause for the motion of the planets. In the following quote, Kepler describes gravity correctly. "If one would place a stone behind the earth and would assume that both are free from any other motion, then not only would the stone hurry to the earth, but also the earth would hurry to the stone; they would divide the space lying between in inverse proportion to their weights."

Kepler’s discoveries about motion became important to Isaac Newton later. When Kepler wrote about the causes of the planets' motions, he thought of it in two components, one a circular revolution around the Sun (which has gravity) and the other a radial effect causing the distance from the Sun to vary. Kepler assumed that the Sun did rotate, though in his day there was as yet no observations from anyone that could prove this. He thought the Sun's rotation somehow pulled the planets around in a circle. But Kepler did not have any awareness Newton's laws of motion; they hadn’t been discovered yet! His understanding of motion was still very influenced by Aristotle.

Kepler had an interesting idea of intelligent design in the motion of the planets. The quote below is from his book ‘Astronomia Nova’ (as translated by William H. Donahue). Kepler addressed the question, ‘What makes the planets follow their paths in the heavens?’ Kepler:

“Now the planet must execute a perfectly circular path in the pure aether [the vacuum of space] by its inherent force . . . . It is therefore clear that the mover is going to have two jobs: first, it must have a faculty strong enough to move its body about, and second, it must have sufficient knowledge to find a circular boundary in the pure aether, which in itself is not divided into such regions. This is the function of mind. Please don't tell me that the motive faculty itself, as a member of the family of simple and brute souls, has a native aptitude for circular motion, exactly like a stone's nature to descend in a straight line. For I deny that God has created any perpetual non-rectilinear motion that is not ruled by a mind.”

So Kepler argued that design was required to get the planets to move in their orbits. Something had to keep the planet on the right path (which he was describing as a circle here). The latter part of this quote says planets do not have a natural tendency to move in a circle, like a stone falling straight downward toward the Earth. But in this last part Kepler was partly wrong. Isaac Newton showed that gravity explains how a planet can move in a non-rectilinear motion (like a circle or an ellipse). Yet Kepler argued that this required a mind (God). I think Kepler was correct about it requiring a mind, but the design was on a level of reality he did not expect. God’s design was in the mathematical form of the force of gravity. Kepler had looked for it in a geometric arrangement. So Kepler figured out part of the puzzle, but Isaac Newton completed many puzzles about motion later. Isaac Newton was born just 12 years after Johannes Kepler died.

Figure 1 Diagram of the Ptolemaic Model showing the Earth (blue), the Sun (yellow) and a planet like Mars (red). The planet would follow the path on the epicycle small circle. But the center of this circle simultaneously moves around a point separated away from Earth, called the equant.

Kepler did a lot to make a case for the Copernican Model of the solar system. Part of this was also explaining it for the scholarly world. Kepler had a very unique position in that he was very educated in the ideas from the Greeks and he could switch his perspective from that of Ptolemy, to that of Tycho Brahe, to that of Copernicus. Figure 1 shows how the Earth, Sun, and a planet are arranged in the Ptolemaic Model. Johannes Kepler had a great advantage over other scholars in being able to relate the mathematics to Brahe’s observations. Brahe’s observations were the best ever done in the world, to that time. In Kepler’s day the mathematics that was available was mainly the geometry from the Greeks, trigonometry, and calculations had to be worked out by hand. Much of Kepler’s important writings on the planets involved geometric and trigonometric proofs. For accuracy in astronomical calculations, this meant hand calculations to many digits, which would be very time consuming. In 1614 and 1617 logarithms were invented by mathematicians. This made calculations quicker by reducing long multiplication and division to addition and subtraction operations. Kepler used logarithms in the compilation of the ephemeris tables called The Rudolphine Tables.

Figure 2 Depiction of the actual path of Mars in the Ptolemaic Model, originally from Johannes Kepler’s book, Astronomia Nova. The diagram graphs the motion of Mars if Earth were stationary in space and Mars moved on its epicycle. Around the outside of the circular diagram are symbols of the zodiac. The point in the center marked “a” is the Earth. To the right of point a is marked the starting position of the observation of Mars in the year 1580. To the upper left of point a is marked the ending position of the observation of Mars in year 1596. Kepler used this diagram to argue against both the Ptolemaic model and the Tychonian model. He argued this was contrary to observations because in this motion Mars would never come back to the same position in the sky, as observed from Earth.

First, Kepler pointed out how the Ptolemaic Model never really fit observations correctly regarding observations of planets. This was true no matter how you tried to adjust it. Mars was the most dramatic case because of its retrograde apparent motion as Earth passes the near point of the orbits. Second, Kepler raised the issue of a plausible physical cause for the motions of the planets. He asks this for the Ptolemaic Model regarding the moving solid spheres (what could make them move?). Then he asks for the Copernican Model, what could make the planets move? The Copernican Model was much simpler.

Kepler showed that by the Ptolemaic and Tychonian models, the Sun and Moon (also Mars) would never come back to the same place in the sky twice as we observe them. He called this motion nonuniform because it made the planet follow a spiraling path he compared to a pretzel. But we can observe the position in the sky to repeat over the correct time scale. For the Sun it's a daily cycle and an annual cycle. For the Moon it is monthly. For Mars it would be over the time for Mars' orbital period. But with the Copernican model Kepler showed how it could be understood so the observed positions repeated as they should. This was brilliant work from Kepler because he worked it out in three dimensional spaces in a way no one had ever done before. He also plotted out what the actual path followed by Mars would look like from the Ptolemaic Model. That is shown in Figure 2 (this also appears in my chapter of the book, The Story of the Cosmos, 2017, p 124).

Another argument for the Copernican Model was to show that Earth’s distance from the Sun varied and that its speed in its orbit varied in the same way as other planets. Kepler also explained Ptolemy’s epicycles better that Ptolemy. He showed by geometric proofs how distances corresponded between the Copernican approach and the epicycle approach. Thus he showed that the epicycles were an imagined addition that was an unreal artifact due to how the Earth’s distance to the Sun varied.

Some of Kepler’s arguments were not really proofs from geometry or physics but more philosophical arguments. This was the case regarding the Sun because in Kepler’s time, it was not possible to do an accurate measurement for the distance from Earth to the Sun. He made geometric arguments that the Sun caused the planets to follow the paths they followed, though he couldn't explain how properly. He also said the Sun did not move as the planets move around it. Kepler said, "And it is more likely that the source of all motion should remain in place rather than move." (This is from Kepler's Astronomia Nova translated by William H. Donahue, p 9.) Also, Kepler indicated he thought it was absurd to think that the Earth could move the Sun. If Earth was in the center, then this implies the Earth moves the Sun but Kepler said no to this.

One other crucial problem regarding observations of Mars would be solved by Johannes Kepler. It has been called the great Martian catastrophe (see the article by Owen Gingerich, “The great Marian catastrophe and how Kepler fixed it,” Physics Today 64, 9, 50, 2011). Tycho Brahe devised a very good observational test to judge whether the Ptolemaic model or the Copernican model was correct. In the Ptolemaic model Mars was always beyond the Sun and thus the nearest distance between Earth and Mars was always 1 A.U. distance when Mars and the Sun were on opposite sides of the Earth. But in the Copernican model, the nearest distance to Mars was closer, about ½ A.U. So, Tycho Brahe wanted very much to measure this. Tycho Brahe had difficulties making this measurement but he tried on multiple occasions. When he finally got his best measurements, correcting for the refraction in Earth’s atmosphere, he found to his shock that the position of Mars in the sky was drastically different. He was off from either the Ptolemaic tables or the Copernican tables, by 5 degrees Longitude for Ptolemaic one direction and 4 degrees Longitude the opposite direction for the Copernican. It turns out what Tycho observed is something that happens only once every 32 years in the orbits of Earth and Mars. Johannes Kepler solves this problem by showing that the planet orbits are not circles as Copernicus thought, but ellipses. Kepler did not completely solve this puzzle until after 1625, when there was another occurrence of Mars’ near pass by Earth. By that time Kepler was more accurate in his determination of Earth’s position. Kepler achieved a very accurate measurement by his Copernican approach with elliptical orbits. He had solved the Martian catastrophe.

Kepler wrote some very good works on optics, and Kepler’s optical work actually improved telescopes and eventually contributed to making better lenses in glasses for people with eye problems. It was Kepler who first figured out that the image in the eye is inverted on the retina in the back of the eye. He determined this from studies of pinhole cameras. He also knew that the intensity of light decreased according to the square of the distance. He knew how concave and convex lenses refract light. He also provided insight into how two eyes could aid in depth perception. Kepler has been called the founder of modern optics as a result these efforts. Kepler also was apparently the first to describe the hexagonal shape and symmetry of snowflakes (in 1611). Kepler also did some work on determining the year of the Birth of Christ. This work may not be available in English but he dated Jesus’ birth at 5 or 4 B.C., which is in agreement with modern scholarship to my knowledge.

To me the culmination of Kepler’s work is in his making a prediction of when and where astronomers would be able to look to see Mercury pass in front of the Sun in 1631. He did this not by astrological prognostication but by mathematical calculation using his laws of planetary motion. However he died before this event on November 15, 1630. This was almost a year before the event. He wrote to astronomers that Venus would pass in front of the Sun in November of 1631 and Mercury about a Month later. He gave a time but he told astronomers to look for Mercury a day early. So Kepler was not certain his Rudolphine Tables would be within a day of accuracy. A man named Pierre Gassendi in Paris confirmed Kepler’s prediction of Mercury passing in front of the Sun, something that had never been observed before. Actually, Kepler’s predicted time was within about 5 hours. This was an unheard of accomplishment for that time.

By late in Kepler’s life his days of producing calendars with astrological predictions were over and such things were not as valued. Some have said Kepler was the last astronomer to believe in astrology. Whether this is true I am not certain but after Kepler the science of astronomy generated more interest than astrology. In Astronomia Nova, Kepler wrote that, “I treat all of astronomy by means of physical causes rather than fictitious hypotheses ....” This was a change of perspective to people of Kepler’s time and it is like modern science. Kepler did not see this as having any conflict with his faith. Kepler lived at a time in history that was right at the cusp of changes from an old medieval understanding of the world to the beginnings of modern science. Kepler’s life shows how God has a purpose for the times and places where we live and that our lives can make a positive impact on others. Kepler also demonstrates a strong commitment to his faith and to Scripture in the midst of his scientific work. He looked for reasonable causes for things and made discoveries that have impacted many lives outside of planetary science, such as in optics. Kepler never stopped praising his Creator and Savior. We can learn from that example even today. I will end with a paragraph from Astronomia Nova, which Kepler entitled, “Advice to astronomers”:

“I too, implore my reader, when he departs from the temple and enters astronomical

studies, not to forget the divine goodness conferred upon men, to the consideration of

which the psalmodist chiefly invites. I hope that, with me, he will praise celebrate the

Creator’s wisdom and greatness, which I unfold for him in the more perspicacious

explanation of the world’s form, the investigation of causes, and the detection of errors

of vision. Let him not only extol the Creator’s divine beneficence in His concern for the

well-being of all living things, expressed in the firmness and stability of the earth, but

also acknowledge His wisdom expressed in its motion, at once so well hidden and so

1) On the trial and defense of Kepler's Mother. This is from someone who examined the actual court records.

2) The great Martian catastrophe and how Kepler fixed it by astronomer Owen Gingerich. Free PDF download.

3) On Kepler's Astrology. This addresses how Kepler took a sort of radical/unconventional view of astrology for his time.

4) Article by Wayne Spencer on Johannes Kepler from 2014, "Great Scientist and Great Christian."