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Johannes Kepler
1571–1630 · Astronomer & Mathematician
A German astronomer who discovered that planets move in elliptical orbits, laying the groundwork for Newton's theory of gravity.
“I much prefer the sharpest criticism of a single intelligent man to the thoughtless approval of the masses.”
Why is this scientist famous?
Formulating the three laws of planetary motion, including the discovery that planets follow elliptical rather than perfectly circular orbits.
Kepler discovered that planets move in elliptical orbits, not perfect circles, and wrote three laws that describe exactly how they move. Those laws still guide spacecraft navigation — when a probe flies past Jupiter or lands on Mars, engineers use Kepler's maths to plot the course. He showed that the universe follows rules we can discover and write down.
Johannes's story
Early Life
Born in 1571 in Weil der Stadt, Germany, Kepler grew up in a turbulent household — his father was a mercenary soldier who abandoned the family. He won a scholarship to the University of Tübingen, where he studied astronomy and was introduced to the revolutionary idea that the Earth moves around the Sun.
The Mars Problem
Working as an assistant to the great observational astronomer Tycho Brahe, Kepler was given the task of calculating the orbit of Mars. Brahe's observations were the most accurate ever made before the telescope. After years of painstaking calculations, Kepler realized Mars's orbit could only be explained if it was an ellipse, not a circle. This broke with 2,000 years of astronomical tradition.
The Three Laws
Kepler formulated three laws: (1) planets orbit in ellipses with the Sun at one focus, (2) they move faster when closer to the Sun and slower when farther away, and (3) the square of a planet's orbital period is proportional to the cube of its distance from the Sun. These laws were later explained by Newton's law of gravitation.
Renaissance Scientist
Kepler extensively observed and documented the bright new star of 1604, now known as Kepler's Supernova, after it had first been noticed by other observers, and wrote Somnium, a fictional account involving a journey to the Moon that is often discussed as an early work of science fiction, and made improvements to the telescope. He lived through the Thirty Years' War, lost his first wife to illness, and had to defend his mother against accusations of witchcraft — yet he never stopped working.
What did they discover?
First Law: Elliptical Orbits
Showed that planets travel around the Sun in elliptical orbits with the Sun at one focus, replacing the traditional assumption of perfectly circular planetary paths.
Second Law: Equal Areas
Showed that a line from a planet to the Sun sweeps out equal areas in equal times, meaning planets move faster when closer to the Sun.
Third Law: The Harmonic Law
Established that the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.
Kepler's Supernova
Carefully observed and published a detailed study of the new star of 1604, now called Kepler's Supernova, providing further evidence that the heavens were not unchanging.
Amazing facts
Kepler spent years helping defend his mother Katharina after she was accused of witchcraft. She was eventually released in 1621.
He began writing Somnium around 1608. The story, involving an imagined journey to the Moon, was published after his death.
He was so poor that he had to sell his own calculations as astrological predictions to pay for food.
Tycho Brahe's exceptionally precise observations of Mars supplied the data that enabled Kepler to uncover the planet's true orbit.
Newton later showed how Kepler's laws follow from the laws of motion and universal gravitation.
A planet, a crater on Mars, a crater on the Moon, and a NASA telescope are all named after him.
Timeline
Born in Weil der Stadt, Germany on December 27
Began studying at the University of Tübingen on a scholarship
Became assistant to Tycho Brahe in Prague
Succeeded Brahe as Imperial Mathematician after Brahe's death
Observed and documented a supernova, now called Kepler's Supernova
Published Astronomia Nova, containing his first two laws of planetary motion
Published his third law of planetary motion in Harmonices Mundi
Died in Regensburg, Germany on November 15, age 58
Personally helped defend his mother Katharina during her witchcraft case; she was released in 1621
Awards & honours
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