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The Man Who Unlocked the Secrets of the Solar System

The Man Who Unlocked the Secrets of the Solar System

 The Man Who Unlocked the Secrets of the Solar System


SIXTEENTH-CENTURY Europeans regarded comets with awe. So when a comet made famous by Danish astronomer Tycho Brahe was visible in the night sky, Katharina Kepler got her six-year-old son, Johannes, out of bed to see it. Over 20 years later, when Brahe died, whom did Emperor Rudolf II appoint to replace him as imperial mathematician? At 29 years of age, Johannes Kepler became the imperial mathematician to the Holy Roman Emperor, a position he held for the rest of his life.

Mathematics is not the only science in which Kepler is held in high esteem. He distinguished himself in the fields of optics and astronomy. Kepler, small in stature, had an astonishing intellect and also a resolute character. He suffered discrimination when he would not convert to Roman Catholicism, even under great pressure.

Mathematical Genius

Johannes Kepler was born in 1571 in Weil der Stadt, a small town on the edge of the German Black Forest. The family was poor, but scholarships from the local nobility secured Johannes a good education. He studied theology at the University of Tübingen, as he planned to become a Lutheran minister. But his genius for mathematics was recognized. When in 1594 a mathematics teacher at the Lutheran high school in Graz, Austria, died, Kepler replaced him. While there, he published his first major work, Cosmographic Mystery.

 The astronomer Brahe had spent years keeping a painstaking record of planetary observations. When he read Cosmographic Mystery, Brahe was impressed with Kepler’s grasp of mathematics and astronomy, and he invited Kepler to join him in Benátky, near Prague, now in the Czech Republic. Kepler accepted the invitation when religious intolerance forced him to leave Graz. And, as described previously, when Brahe died, Kepler succeeded him. In place of a meticulous observer, the imperial court now had a mathematical genius.

Milestone in Optics

To benefit fully from Brahe’s collection of planetary observations, Kepler needed to understand more about the refraction of light. How is reflected light coming from a planet refracted when entering the earth’s atmosphere? Kepler’s explanations were contained in Supplement to Witelo, Expounding the Optical Part of Astronomy, which expanded on the work of the medieval scientist Witelo. Kepler’s book was a milestone in optics. He was the first man to explain the workings of the eye.

Kepler’s main pursuit was not optics, however, but astronomy. Early astronomers believed the sky to be a hollow globe with stars stuck to the inner surface like sparkling diamonds. Ptolemy viewed the earth as the center of the universe, whereas Copernicus believed the planets all revolved around a stationary sun. Brahe suggested that the other planets revolved around the sun, which in turn orbited the earth. Since in relation to the earth, all other planets were heavenly bodies, they were viewed as perfect. The only form of motion considered appropriate for them was a perfect circle, each planet traveling at a constant speed. This was the environment in which Kepler took up his work as imperial mathematician.

Beginnings of Modern Astronomy

Equipped with Brahe’s tables of planetary observations, Kepler studied cosmic movements and drew conclusions based on what he saw. His genius with figures was matched by a strong will and a restless curiosity. His voracious capacity for work is evidenced by the 7,200 complex calculations he completed in studying the observation tables of Mars.

And it was Mars that first caught Kepler’s eye. Punctilious study of the tables revealed that Mars orbited the sun but not in a circle. The only orbital shape that fitted the observations was an ellipse with the sun as one of its foci. Kepler sensed, however, that the key to unlocking the secrets of the heavens was, not Mars, but planet Earth. According to Professor Max Caspar, “Kepler’s inventiveness moved him to a touch of genius.” He put the tables to an unconventional use. Instead of  using them to investigate Mars, Kepler imagined himself standing on Mars looking back at the earth. He calculated that the speed of the earth varied in inverse proportion to its distance from the sun.

Kepler now understood that the sun is not simply the center of the solar system. The sun also acts like a magnet, rotating on its own axis and exercising a force on the movement of the planets. Caspar writes: “This was the grand new concept that guided him from then on in his research and led him to the discovery of his laws.” To Kepler the planets were all physical bodies harmoniously governed by a uniform set of laws. What he learned from Mars and Earth must be true of all planets. He thus concluded that each planet travels around the sun in an elliptic orbit at a speed that varies in relation to its distance from the sun.

Kepler’s Laws of Planetary Motion

In 1609, Kepler published New Astronomy, which is recognized as the first book on modern astronomy and one of the most important books ever written on the subject. This masterpiece contained the first two of Kepler’s laws on planetary motion. His third law was published in Harmonies of the World in 1619, when he was living in Linz, Austria. These three laws define the basics of planetary motion: the shape of a planet’s orbit around the sun, the speed of a planet’s movement, and the relationship between a planet’s distance from the sun and the time it takes to complete a circuit.

How did Kepler’s fellow astronomers react? They failed to grasp the impact of Kepler’s laws. Some were even aghast in disbelief. Perhaps they were not entirely to blame. Kepler shrouded his works in a Latin prose that was almost as impenetrable as the clouds around Venus. But time was on Kepler’s side. Some 70 years later, Isaac Newton used Kepler’s work as a basis for his laws of motion and gravity. Today Kepler is recognized as  one of the greatest scientists ever​—one who helped to drag astronomy out of the Middle Ages and into modern times.

Europe Engulfed in Religious War

The same month that Kepler formulated his third law, the Thirty Years’ War broke out. During that period (1618-48), Europe was decimated by religious murder and plunder and Germany lost a third of its population. Witch-hunts were widespread. Kepler’s mother was charged with witchcraft and narrowly escaped execution. Whereas Kepler’s salary at the court was reportedly paid irregularly before the war, during the war it was hardly paid at all.

Throughout his life Kepler, a Lutheran, experienced religious persecution and prejudice. He was forced to leave Graz​—which meant loss and hardship—​because he refused to become a Roman Catholic. In Benátky he met with further efforts to persuade him to convert. But Kepler could not accept the worship of images and saints; such practices were, for him, the works of the wicked one. In Linz, disagreement with his fellow Lutherans who believed that God is omnipresent led to his being excluded from their Evening Meal. (See pages 20-1 of this magazine.) Religious intolerance was anathema to Kepler, who believed that the harmony among the planets ought to be evident among humans. He stuck to his beliefs and suffered willingly. “Suffering along with many brothers for the sake of religion and for the glory of Christ by enduring harm and disgrace, by leaving one’s house, fields, friends, and home​—I would never have believed all of this could be so agreeable,” Kepler wrote.​—Johannes Kepler, by Ernst Zinner.

In 1627 he published the Rudolphine Tables, which he regarded as his major astronomical work. Unlike his earlier books, this one was widely applauded, soon becoming indispensable to astronomers and navigators. Finally, in November 1630, Kepler died in Regensburg, Germany. One of Kepler’s colleagues was continually amazed to find in Kepler “such well-founded learning and such a wealth of knowledge of the most profound secrets.” A worthy tribute to the man who unlocked the secrets of the solar system.

[Blurb on page 26]

Kepler is recognized as one of the greatest scientists ever​—one who helped to drag astronomy out of the Middle Ages and into modern times

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Religious intolerance was anathema to Kepler, who believed that the harmony among the planets ought to be evident among humans

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Kepler’s Astrology and Theology

While Johannes Kepler established a brilliant reputation for his discoveries in the field of astronomy, it must be acknowledged that he was influenced by the prevalent religious ideas of his day. Thus he wrote extensively on astrology, although he rejected “much of what was claimed to be known about stellar influence.”

He was also a firm believer in Christendom’s Trinity. “One of the ideas to which he was most strongly attached​—the image of the Christian Trinity as symbolized by a geometric sphere and, hence, the visible, created world—​was literally a reflection of this divine mystery (God the Father::centre; Christ the Son:: circumference; Holy Spirit:: intervening space).”​—Encyclopædia Britannica.

In contrast, what did Sir Isaac Newton have to say about the Trinity doctrine? He denied the Trinity teaching. His principal reason for rejecting it was that when he sought to verify the statements of the creeds and the church councils, he found no support for the doctrine in the Scriptures. In fact, he believed strongly in the supreme sovereignty of Jehovah God and the Scriptural position of Jesus Christ as inferior to his Father. *​—1 Corinthians 15:28.


^ par. 30 See The Watchtower, April 15, 1977, pages 244-7.

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Kepler’s Laws of Planetary Motion

Kepler’s laws of planetary motion are still regarded as the starting point for modern astronomy. They can be summarized as follows:

1 Every planet follows an elliptical orbit around the sun, which is one focus of the ellipse

← Sun ←

↓ ↑

↓ ↑

Planet ● ↑

→ → →

2 Each planet moves faster when it is closer to the sun. Regardless of the planet’s distance from the sun, a line drawn from the center of the sun to the center of the planet sweeps out an equal area in an equal length of time

Planet moves faster

Planet moves slower

A ● B

↓ ↑

↓ Sun


● B


● B

Thus, if the time it takes the planet to travel from point A to point B is the same in each example, then the shaded areas are equal

3 The time taken for each planet to complete one orbit around the sun is known as that planet’s period. The squares of the periods of any two planets are proportional to the cubes of their average distances from the sun


Planet Mercury

Distance from Sun * 0.387

Period in years 0.241

Period2 0.058 *

Distance3 0.058 *

Planet Venus

Distance from Sun 0.723

Period in years 0.615

Period2 0.378

Distance3 0.378

Planet Earth

Distance from Sun 1

Period in years 1

Period2 1

Distance3 1

Planet Mars

Distance from Sun 1.524

Period in years 1.881

Period2 3.538

Distance3 3.540

Planet Jupiter

Distance from Sun 5.203

Period in years 11.862

Period2 140.707

Distance3 140.851

Planet Saturn

Distance from Sun 9.539

Period in years 29.458

Period2 867.774

Distance3 867.977


^ par. 61 Relative distance compared with Earth’s. For example, Mars’ distance from the Sun is 1.524 times Earth’s distance.

^ par. 63 Notice that on this chart these two numbers are equal or nearly equal for each planet. The difference increases the farther the planet is from the sun. Later Isaac Newton, in his law of universal gravitation, adjusted Kepler’s law, providing the necessary corrections by including the mass of the respective planet and the sun.

^ par. 64 Notice that on this chart these two numbers are equal or nearly equal for each planet. The difference increases the farther the planet is from the sun. Later Isaac Newton, in his law of universal gravitation, adjusted Kepler’s law, providing the necessary corrections by including the mass of the respective planet and the sun.

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Kepler’s telescope and books

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[Credit Line]

Courtesy of NASA/JPL/Caltech/USGS

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Copernicus and Brahe: Brown Brothers; Kepler: Erich Lessing/Art Resource, NY; Jupiter: Courtesy of NASA/JPL/Caltech/USGS; Planet: JPL

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Venus: Courtesy of NASA/JPL/Caltech; Planet: JPL

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Telescope: Erich Lessing/Art Resource, NY; Neptune: JPL; Mars: NASA/JPL; Earth: NASA photo