Johannes Kepler: Unlocking the Secrets of Planetary Motion

When Johannes Kepler was born in the late 16th century, people thought that planets in the solar system traveled in circular orbits around Earth. An occasional problem — such as Mars appearing to suddenly reverse course — was solved by the addition of miniature circles, or epicycles, to planetary paths. But Kepler not only adamantly defended the idea that planets orbit the sun, he also revealed that their paths were not perfect circles. His descriptions of planetary motions became known as Kepler's laws. Today, these laws not only describe planetary motion but also determine the orbits of satellites and space stations.

[See also our overview of Famous Astronomers and other scientists who have contributed to the rich history of discoveries in astronomy.]

Coded correspondence

Born in December 1571, in Weil der Stadt in Swabia, in southwest Germany, young Johannes Kepler was a sickly child of poor parents. He was awarded a scholarship to the University of Tübingen, where he studied to become a Lutheran minister. While there, he was introduced to the work of Nicolaus Copernicus, who had written that the planets orbited the sun rather than Earth.

In 1594, Kepler became a professor of mathematics at a seminary in Graz, Austria, as well as district mathematician and calendar maker. In his spare time, he continued to study astronomy and astrology (which were virtually the same thing back then). In 1596, Kepler wrote the first public defense of the Copernican system. This was a dangerous stance, given that in 1539, Martin Luther, founder of the Lutheran church, derided the theory when he first heard it, while the Catholic church deemed such a position heretical in 1615 (they later subjected astronomer Galileo Galilei to house arrest for his publication on the subject, though the arrest may have had more to do with the fact that he had insulted the pope).

German astronomer Johannes Kepler used mathematics to calculate the path of the planets, finding that they traveled not in circles, as long expected, but in ellipses. (Image credit: Johnnes Kepler Gesammelte Werke , C. H. Beck, 1937)

"The era in which Kepler lived was one of tremendous upheaval and change," Dan Lewis, curator of the history of science and technology at the Huntington Library in San Marino, Calif., said on NASA's JPL website. "Religious leaders were reluctant to relinquish their ideas about the heavens. Talk by astronomers of a sky filled with objects moving in non-circular orbits and other phenomena that went against an Earth-centric model threatened their beliefs."

Actually, Kepler's motivation was not to upset the church. Rather, his defense of the Copernican model was not on physical or mathematical grounds — Kepler's argument was religious, astrophysicist Paul Sutter wrote in a “Expert Voices” column. According to Sutter, Kepler said that since the son of God was at the center of the Christian faith, the sun ought to be at the center of the universe. 

Nevertheless, according to Lewis, Kepler and his first wife, Barbara, created a code with which to write letters to each other so that their correspondence would not put them at risk of persecution.

In search of the most detailed notes about the paths of the planets, Kepler contacted astronomer Tycho Brahe. A wealthy Danish nobleman, Brahe built an observatory in Prague where he tracked the motions of the planets and maintained the most accurate observations of the solar system at the time. In 1600, Brahe invited Kepler to come work with him.

Brahe, however, became suspicious and unwilling to share his notes with his assistant. Instead, he assigned Kepler to solve the mystery of Mars, one of the most puzzling problems in astronomy at the time. Ironically, the detailed records of the challenging planet were the tools Kepler needed to understand how the solar system functioned.

When Brahe died in 1601, Kepler managed to acquire Brahe's observations before his family could use them to their financial benefit.

Murder suspect

In 1901, scientists opened up Brahe's grave and claimed to find mercury in his remains. Kepler was posthumously accused of poisoning his benefactor in order to get hold of his closely guarded notes. But when Brahe's body was exhumed again in 2010, tests revealed that the mercury content in his body was not high enough to kill him.

"In fact, chemical analyses of the bones indicate that Tycho Brahe was not exposed to an abnormally high mercury load in the last five to ten years of his life," researcher Kaare Lund Rasmussen, an associate professor of chemistry at the University of Southern Denmark who analyzed mercury levels in Brahe's beard and bones, said in a statement.

Accused of murder long after his death, Kepler's name was finally cleared.

Kepler's laws

The Martian problem, which Kepler said he would solve in eight days, took nearly eight years. Astronomers had long struggled to figure out why Mars appeared periodically to walk backward across the night sky. No model of the solar system — not even Copernicus' — could account for the retrograde motion.

Using Brahe's detailed observations, Kepler realized that the planets traveled in "stretched out" circles known as ellipses. The sun didn't sit exactly at the center of their orbit, but instead lay off to the side, at one of the two points known as the foci. Some planets, such as Earth, had an orbit that was very close to a circle, but the orbit of Mars was one of the most eccentric, or widely stretched. The fact that planets travel on elliptical paths is known as Kepler's First Law.

Mars appeared to move backward when Earth, on an inner orbit, came from behind the red planet, then caught up and passed it. Copernicus had suggested that observations made from a moving Earth (rather than a centrally located one) could be a cause of the retrograde motion, but the perfect circular orbits he posited still required epicycles to account for the paths of the planets. Kepler realized that two planets, traveling on ellipses, would create the appearance of the red planet's backward motion in the night sky.

Kepler also struggled with changes in the velocities of the planets. He realized that a planet moved slower when it was farther away from the sun than it did when nearby. Once he understood that planets traveled in ellipses, he determined that an invisible line connecting the sun to a planet covered an equal amount of area over the same amount of time. He posited this, his Second Law, along with his first, which he published in 1609.

Kepler's Third Law was published a decade later, and recognized that the relationship between the period of two planets — the time they take to orbit the sun — is connected to their distance from the sun. Specifically, the square of the ratio of the period of two plants is equal to the cube of the ratio of their radius. While his first two laws focus on the specifics of a single planet's movement, his third is a comparison between the orbit of two planets.

"It was this law, not an apple, that led Newton to his law of gravitation. Kepler can truly be called the founder of celestial mechanics," NASA says in its Kepler biography.

h2>Other notable discoveries

Though Kepler is best known for defining laws regarding planetary motion, he made several other notable contributions to science. He was the first to determine that refraction drives vision in the eye, and that using two eyes enables depth perception. He created eyeglasses for both near and farsightedness, and explained how a telescope worked. He described images and magnification, and understood the properties of reflection.

Kepler claimed that gravity was caused by two bodies, rather than one, and as such, the moon was the cause of the motion of tides on the Earth. 

"If the Earth ceased to attract the waters of the sea, the seas would rise and flow into the moon," Kepler wrote, according to Michael Fowler, a physics professor at the University of Virginia. "If the attractive force of the moon reaches down to the Earth, it follows that the attractive force of the Earth, all the more, extends to the moon and even farther."

He suggested that the sun rotates, and created the word "satellite." He tried to use his knowledge of the distance Earth travels to measure the distance to the stars. Kepler also calculated the birth year of Christ.

In recognition of his contribution to his our understanding of the motion of the planets, NASA named their planet-finding telescope after the German astronomer.

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Nola Taylor Tillman
Contributing Writer

Nola Taylor Tillman is a contributing writer for She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd