When Johannes Kepler was born in the late sixteenth century, scientists believed that planets in the solar system traveled in circular orbits around the Earth. The occasional problem was solved by the addition of miniature circles test — 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. [See also our overview of Famous Astronomers and great scientists from many fields who have contributed to the rich history of discoveries in astronomy.]
Here is a brief biography of Kepler:
Johannes Kepler Facts
Born in December of 1571, young 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 studied the work of Nicolaus Copernicus, who taught that the planets orbited the sun rather than the Earth, though he had no observational evidence to offer as proof.
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).
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, proved to be suspicious and unwilling to share his detailed 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.
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 to walk backwards 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 focus. 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.
Other notable discoveries
Though Kepler is best known for his 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. 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.
Johannes Kepler, by Michael Fowler of the University of Virginia
— Nola Taylor Redd