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Johannes Kepler, Founder of Celestial Mechanics

Johannes Kepler, founder of celestial mechanics, was born December 27, 1571, at Weil der Stadt, Stuttgart, Germany. His grandfather was lord mayor of the town, but his family had many hardships; his father, Heinrich, was a mercenary who abandoned his family and his mother, Katharina, was an innkeeper’sdaughter tried for witchcraft. Kepler amazed travelers with his mathematical knowledge.

Kepler embraced his studies and proved a bright student. After studying in the Protestant seminary at Adelberg in 1584, he entered the University of Tübingen. He joined the mathematics faculty of the Protestant seminary at Graz, Austria, in 1594.

kepler's book

Kepler studied Nicolaus Copernicus (1473–1543) in depth and wrote the Mysterium cosmographicum (The Sacred Mystery of the Cosmos, 1596), a work defending the Copernican system, which postulated that the Sun—not the Earth—was the center of the universe, and that planets moved in circles in their orbitsaround the Sun.

Kepler is known for his three revolutionary laws of planetary movements, which explained the organization of the solar system. He observed that the orbit of Mars was an ellipse and found similar deductions for orbits of other planets. He realized there was a mathematical explanation, and his first law states that the planets moved in elliptical paths around the Sun.

The second law stipulates that the path the planet travels around the Sun comprises equal areas in equal times as the planet moves its orbit. The first two laws were published in his book Astronomia nova (New Astronomy) in 1609.

His third law of planetary movement states that the square of the time it takes for a planet to revolve once around the Sun is proportional to the cube of planet’s distance from the Sun. The third law was published in 1619 in a book titled Harmonices mundi. The three laws made a seminal contribution to the study of planetary motion.

Kepler made great progress in the development of modern astronomy by abandoning theories held for two prior millennia. However, the reasons behind the laws were discovered by Isaac Newton, who demonstrated that they were the result of the law of universal gravitation.


Religious tensions in Europe forced Kepler to move on more than one occasion. In 1599, he left Graz because of religious persecution and went to Prague at the invitation of Danish astronomer Tycho Brahe (1546–1601). Kepler became the imperial mathematician after Brahe’s death in 1601. Kepler held the post until 1612, when Lutherans were being driven out of Prague. He went to Linz to continue his work in mathematics and stayed there until 1626.

After years of hardship, Kepler died at Regensburg, Bavaria, on November 15, 1630. Kepler the mathematician, astronomer, and astrologer was one of the dominating figures of the scientific revolution that swept Europe.

This animated gif displays Kepler’s second law of planetary motion:

“A line joining a planet and the Sun sweeps out equal areas during equal intervals of time”


The same blue area is swept out in a given time. The green arrow is velocity. The purple arrow directed towards the Sun is the acceleration. The other two purple arrows are acceleration components parallel and perpendicular to the velocity.

Johannes Kepler published his first two laws about planetary motion in 1609, having found them by analyzing the astronomical observations of Tycho Brahe. Kepler’s third law was published in 1619.
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