Johannes Kepler played a key role in the profound changes in human thinking during the scientific revolution. In Kepler’s lifetime:
- religion clashed with religion
- religion clashed with science
- the old ideas of Ptolemy and Aristotle clashed with the new discoveries of Copernicus and Galileo
- the superstition of astrology clashed with the science of astronomy
Kepler reflected the times he lived in. Seen through modern eyes, he had somewhat contradictory ideas. He was:
- an unorthodox Protestant
- a follower of Copernicus and Galileo
- a brilliant mathematician and scientist who discovered that the solar system’s planets follow elliptical paths, not circular paths
- an astrologer, whose horoscopes were sought out as among the best available
Such contradictions were not unusual during the scientific revolution. Isaac Newton, who lived in a later time than Kepler, (1643 to 1727) did not work in the way a modern scientist would. Also a Protestant with unorthodox views, Newton spent more time investigating the true meaning of the Bible’s words and on the pseudoscience of alchemy than he did on mathematics or physics!
Johannes Kepler’s Early Life and Education
Johannes Kepler was born on December 27, 1571, in the town of Weil der Stadt, which then lay in the Holy Roman Empire, and is now in Germany.
His mother, Katharina Guldenmann, was a herbalist who helped run an inn owned by her father. When Johannes was about five his father Heinrich was killed fighting as a mercenary in Holland.
Physically Damaged, Intellectually Strong
The young Johannes Kepler was prone to ill-health. His hands were crippled and his eyesight permanently impaired by smallpox. Despite these difficulties, guests at his grandfather’s inn were astonished by his ability to solve any problem they brought him involving numbers.
His herbalist mother, Katharina, tried to convey her love of the natural world to her son. She made a point of taking him out at night to show him interesting things in the heavens, including a comet and a lunar eclipse.
In doing so, she set her son on a course that would transform our understanding of the solar system and the universe.
Kepler was formally schooled in Latin, the language of academics, lawyers and churchmen throughout Europe. Hoping to become a Protestant minister he attended the Protestant Seminary of Maulbronn.
After successfully completing his studies at Maulbronn he moved to the University of Tübingen. There, although he took courses in Theology, Greek, Hebrew, and Philosophy, it was in Mathematics that he stood out.
He was one of the few students deemed intellectually and mathematically capable of being taught the work of Nicolaus Copernicus. Kepler decided that Copernicus’s heliocentric hypothesis, that the sun lay at the center of the solar system, was correct.
In 1594, aged 23, Kepler became a lecturer in astronomy and mathematics at the Protestant School in the city of Graz, Austria.
Kepler’s Lifetime in Context
Kepler Discovers the Truth about Planets’ Orbits
Kepler thought Copernicus’s heliocentric view of the solar system was right.
His own belief, one which would transform science, was that the sun exerted a force on the planets orbiting it.
In 1596, at age 25, he published a book – Mystery of the Cosmos. His book explained why, logically, the sun lay at the center of the solar system.
Kepler noted that Mercury and Venus always seem to be close to the sun, unlike Mars, Jupiter and Saturn. This is because Mercury and Venus’s orbits are closer to the sun than Earth’s. Kepler said that if the sun and all the planets orbited Earth, there is no reason why Mercury and Venus should always be near the sun.
Kepler Sees the Hand of God
Including Earth, there were six known planets. (Uranus, Neptune and Pluto had not been discovered in Kepler’s era.)
Looking for evidence of ‘God the mathematician,’ Kepler was able to justify six planets and their distances from the sun in terms of the five Platonic solids. These are the five highly symmetrical, regular, 3D solids whose perfect symmetry allows them to be used as dice.
Remarkably, looked at in the way Kepler did, his Platonic solids theory produced a better fit to the planet-to-sun distances that Copernicus had found.
Kepler Seeks Better Data
Thrilled that he had found evidence of God’s hand in the solar system’s design, Kepler now sought better data.
He hoped that more accurate astronomical observations would prove his theory.
By great good fortune, at exactly the same time as Kepler sought better data, one of Europe’s foremost astronomers, Tycho Brahe, was hoping to recruit an assistant to carry out astronomy calculations.
The two started working together, but it was not a match made in heaven! Tycho was notoriously quarrelsome – he had worn a metal nose ever since he lost his real nose in a duel fought over the merits of a particular mathematical formula!
Tycho Brahe’s Data
Early in the year 1600, Kepler moved from Graz to the town of Benatek, which today is in the Czech Republic. There Tycho, who was the imperial mathematician to Holy Roman Emperor Rudolph II, had an observatory.
Tycho gave Kepler access to some of his Mars observations, but not all. The pair fell out and Kepler spent some time in Prague, then Graz, before they resolved their differences and got together again.
During 1601, Kepler carried out calculations of planet movements for Tycho. In October 1601, Tycho died. Kepler replaced him as the imperial mathematician.
Kepler now had unrestricted access to all of Tycho’s astronomical data. With an incredible amount of hard work he began to deduce the laws that govern the movements of the planets. The first law he found is now called his second law!
Kepler’s Second Law – The Law of Areas
This was scandalous! Everyone ‘knew’ that heavenly bodies were perfect, traveling along a path whose shape was perfect – the circle.
It took a long time before people came to terms with Kepler’s shocking discovery and started to believe it. All the more so because the planets’ orbits are actually very close to circular.
It is a tribute to both the precision of Tycho’s observations (made, remember without a telescope) and Kepler’s calculating prowess that Kepler was ever able to discover the truth.
Kepler found Mars was moving in an oval-like orbit and that sometimes it was closer to the sun than at others. When close to the sun, it moved faster than when it was farther away.
In 1602, Kepler deduced what came to be called his second law: a line drawn from planet to sun sweeps out equal areas in equal times.
Kepler’s First Law – The Law of Orbits
Kepler tried to figure out the mathematical shape of Mars’s orbit. After about 40 misses, in 1605, he got it right. Mars follows an elliptical path around the sun.
And now he formulated what would become Kepler’s first law: planets orbit the sun in ellipses, with the sun at one focus.
Kepler’s Third Law – The Law of Periods
Kepler never gave up his idea that regular polygons determine the orbits of the planets. As a fortunate result of this wrong thinking, he continued calculating and theorizing.
In 1618, his continuing research led to his third law of planetary motion:
The square of the period of any planet is proportional to the cube of the semimajor axis of its orbit.
Restated crudely, this law means that if we square the time it takes a planet to complete one orbit around the sun, we’ll find it’s proportional to the planet’s distance from the sun cubed.
Even more crudely: the farther a planet is from the sun, the slower it moves along its orbital path.
Summing up Kepler’s Laws and their Significance
In a 17 year period, when he was aged between 30 and 47, Kepler took Copernicus’s mathematically flawed heliocentric solar system by the scruff of the neck and put it on a rock-solid mathematical footing.
During his own lifetime, the huge significance of his work won little or no recognition.
Later, however, his work provided the platform for Isaac Newton to discover the law of universal gravitation. When Newton (perhaps with double meaning) said that if he had seen further, it was by standing on the shoulders of giants, there can be no doubt that Johannes Kepler was one of the giants.
More of Kepler’s Achievements
Although he greatly admired Galileo’s work, Kepler disagreed with him about the cause of tides on Earth. Galileo believed they were caused by the earth spinning. Kepler, correctly, identified that they were caused by the moon.
Kepler wrote about a ‘magnetic force’ that today we call the force of gravity.
Kepler made important discoveries in optics.
In 1604, Kepler discovered the inverse square law of light intensity.
The consequence of this law is, for example, that because Jupiter is five times farther than Earth is from the sun, each square meter of Jupiter receives only one-twenty-fifth of the sun’s energy compared with a square meter on Earth.
When Newton, guided by Kepler’s Third Law, discovered the law of universal gravitation, he found that gravity also follows an inverse square law. So, if you triple your distance from the sun, the force of the sun’s gravity you feel is divided by nine.
Everything is Upside Down
It was Kepler who discovered that the lenses in our eyes invert images. This means images on our retinas are upside down. Kepler rightly concluded that the inverted image is corrected by our brains.
Designing a Better Telescope
In 1610, Kepler was excited by news from Italy, where Galileo had discovered four moons orbiting Jupiter.
In 1611, Kepler turned his attention to Galileo’s telescope design, improving it by using two convex lenses. This allowed higher magnifications. Kepler’s design is now the standard design for refracting telescopes.
Kepler’s ‘Last Theorem’
Fermat’s Last Theorem, sometimes called Fermat’s Conjecture, took more than 300 years to prove.
Kepler left the world a puzzle that resisted all attempts at formal proof for 400 years: the Kepler Conjecture.
Kepler’s Conjecture looks at how you can pack a bunch of equally sized spheres into the smallest possible space; he says there are two most efficient ways: cubic close packing and hexagonal close packing.
Kepler was right. A formal proof of his conjecture was published in 2015 by Thomas Hales and coworkers.
It turns out that Kepler’s ideas about packing are useful in modern chemistry, where atoms in metals are arranged in close packed arrangements: for example copper’s atoms are cubic close packed, while magnesium’s atoms are hexagonal close packed.
In 1616, Kepler became aware of John Napier’s invention of logarithms.
Using logarithms, any multiplication can be turned into an addition; any division can be turned into a subtraction.
For example, adding 3.5627685 to 3.7736402 by hand is much easier than multiplying 3654 by 5938. Try it and see! And in Kepler’s day, everything had to be calculated by hand! Consulting a table of logarithms allows you to convert the result of the addition or subtraction into the result of the equivalent multiplication or division.
Napier’s logarithms allowed mathematicians to greatly simplify their calculations. Kepler was overjoyed when he learned of them. And no wonder: he carried out a huge number of calculations in his work, and logarithms speeded everything up enormously.
But Kepler went one step further.
Mathematicians had not been able to understand how logarithms worked. They just saw that they seemed to give the right answers. Kepler realized that if logarithms had no firm mathematical basis, his calculations might be discredited someday.
He overcame this hurdle in the way any genius would – he himself proved how logarithms transform multiplications and divisions into additions and subtractions.
Kepler’s proof allowed all mathematicians to use logarithms with no misgivings.
In Kepler’s time, Central Europe was convulsed with tensions between Catholicism and Protestantism – resulting in the Thirty Years’ War.
Symbolic of the paranoia of these times, Kepler’s nature-loving herbalist mother was arrested in Württemberg in 1620, charged with witchcraft. She was held in prison for over a year, and the tortures she faced if she did not confess to witchcraft were described to her in detail. Kepler himself traveled to Württemberg and successfully defended his mother, finally winning her release.
Kepler moved around a lot during the last few years of his life. His job as a mathematician disappeared and, in an effort to make ends meet, he ended up casting horoscopes for a military commander – General Wallenstein.
Johannes Kepler fell ill and died at age 58, on November 15, 1630 in the German city of Regensburg. He was survived by a son and a daughter from his first marriage, to Barbara Müller, who died rather young. He was also survived by his second wife, Susanna Reuttinger, and two sons and a daughter from that marriage.
Today Kepler’s grave is lost. The graveyard he was buried in was destroyed during religious battles a few years after he was buried.
Although Kepler cast horoscopes, his feelings about what was important in his work are summed up in these words:
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