Biography of Johannes Kepler the first astronomer who tried to explain the phenomena of nature through observation and measurements, to develop appropriate models.

Johannes Kepler is a key figure in the scientific revolution.

Astronomer and mathematician, known for his laws on the motion of the planets in their orbits around the Sun.  He was a collaborator of Tycho Brahe.

This admirable great man was able to brilliantly describe the movements of the planets, but he could not explain why they move around the Sun and what is the cause of their keeping in their orbits.

This is a task that he left to Isaac Newton.

Kepler’s childhood and studies

Kepler was born in Würtemberg, Germany, in the year 1571.

His parents aroused his interest in astronomy.

At the age of five, his mother took him to a high place to observe the comet of 1577.

At the age of nine, his father made him contemplate the lunar eclipse of January 31, 1580. He always remembered that the Moon looked quite red. Between the ages of nine and eleven he had to work as a laborer in the fields.

Comet 1577
Comet lithograph in the year 1577. Credit: Wikipedia

Between the ages of nine and eleven he had to work as a laborer in the fields.

Kepler in his youth

In 1584 he entered the Adelberg Protestant Seminary.

In 1589 he entered the University of Tübingen.

There he studied ethics, dialectics, rhetoric, Greek, Hebrew, and astronomy.

In that year, his father left again for the war and disappeared from his life forever.

His math teacher, the astronomer Michael Maestlinle, taught the Copernicus heliocentric system whose explanation was reserved for the best students.

The other students took as true the geocentric system of Ptolemy, which stated that the Earth was immobile occupying the center of the Universe; and that the Sun, the Moon, the planets and the stars revolved around it.

Kepler became a convinced Copernican and maintained a very close relationship with his teacher; later, he did not hesitate to ask her for help or advice for his work.

University of Tuebingen, Germany. Credit: web “”

Kepler working life

While he was planning to become a Lutheran minister, he learned that the Protestant school in Graz was looking for a math teacher.

He then abandoned his studies in theology to take up the post, in 1594.

In Graz, Johann Kepler studied the motion of the planets and published almanacs with astrological predictions.

Engraving of the city of Graz, made in 1670 by Georg Matthäus Vischer. Credit: Wikipedia.

Kepler spent most of his life trying to understand the laws of planetary motion.

At first, he considered that the movement of the planets had to comply with the Pythagorean laws of harmony or the music of the celestial spheres.

He tried to show that the distances from the planets to the Sun were given by 6 spheres successively nested inside each other.

In these spheres would be the 6 planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn.

In 1596, he wrote a book in which he expounded his ideas (“The Cosmic Mystery”).

Kepler collaborated with Tycho Brahe

Four years later, in 1600, he accepted the collaboration proposal made by Tycho Brahe, an astronomer for Emperor Rudolph II.

Brahe had set up the best astronomical observation center of that time, at Benatky Castle, near Prague.

The castle in Benátky. Credit: Wikipedia. Author: H. Raab

In the castle of Benátky (Czech Republic), Tycho Brahe lived from August 1599 to June 1600.  In February 1600, Tycho Brahe and Johannes Kepler met here for the first time.

Tycho Brahe then had the best planetary observational data, much more accurate than those managed by Copernicus, but he refused to share them with Kepler.

Only when he was already on his deathbed did Tycho agree to bequeath Kepler the data on the orbits of the planets that he had collected for years.

Thanks to these data, the most accurate and abundant of the time, Kepler was able to deduce the real orbits of the known planets.

The Kepler’s Star

On October 17, 1604, Kepler observed a supernova in our own Galaxy, which would later be called Kepler’s star.

The star could be observed with the naked eye for 18 months after its appearance.

The supernova is only 13,000 light years from us. No subsequent supernovae have been observed in historical times within our own galaxy.

Kepler's Star
This is the remnant of Kepler’s supernova, the famous explosion that was discovered by Johannes Kepler in 1604. Credit: NASA/CXC/NCSU/M.Burkey et al; Optical: DSS

The red, green and blue colors show low, intermediate and high energy X-rays observed with NASA’s Chandra X-ray Observatory, and the star field is from the Digitized Sky Survey.

Kepler’s mother was accused of witchcraft

A little later, in 1615, his mother was already 68 years old and she ran a boarding house.

She but she also worked as a healer and was accused of witchcraft.

Persuaded of her innocence, Kepler spent six years defending her in court and writing numerous pleadings.

She spent a year locked up in the tower of Güglingen although, thanks to her son, she escaped torture.

Finally, she was released on September 28, 1621. She Weakened by the harsh years of trial and incarceration and she died six months later.

Kepler’s planetary model

Kepler initially tried to make his planetary model with the circumference, being the most perfect of the trajectories, but the observed data did not fit that model correctly.

Fortunately, Tycho had focused on Mars, which has a very pronounced elliptical; otherwise it would have been impossible for Kepler to realize that the orbits of the planets were elliptical and not circular.

Unable to accept that God had not ordained that the planets describe the simplest geometric figure, he devoted himself with limitless determination to trying all sorts of combinations of circles.

When he was convinced that it was impossible to do it with circles, he tried ovals.

Disgruntled, by failing to fit a stubborn eight-minute difference of arc between the actual data and his theoretical model, he eventually found that the ellipse-based model fit perfectly with his and Tycho’s measurements.

Kepler’s three laws

This is how he came to write Kepler’s first law: “The planets describe elliptical movements around the Sun, the latter being located in one of the foci of the ellipse.”

Kepler law 1
First Kepler Law. Credit: web “”.

After that important mental leap, where for the first time the facts came before the wishes and the existing prejudices about the nature of the world, Kepler simply devoted himself to observing the data and drawing conclusions without any preconceived ideas.

He went on to check the speed of the planet through the orbits, arriving at the second law: “The planets, in their journey through the ellipse, sweep equal areas in the same time.”

Kepler law 2
Second Kepler Law. Credit: web “”.

For a long time, Kepler was able to confirm these two laws only on the other planets.

Still it was a spectacular achievement; but it was necessary to relate the trajectories of the planets to each other.

After several years of observations and work, he discovered the third and most important law of planetary motion: “The square of the periods of the planets is proportional to the cube of their mean distance from the Sun“.

This law, also called the harmonic law, together with the other laws already made it possible to unify, predict and understand all the movements of the stars.

Kepler law 3
Thirth law of Kepler. Credit: web “”

Kepler’s final years and death

These three laws astonished the world and made him the most famous astronomer of his time, although he did not stop feeling uneasy about the failure of his original intuition of simplicity (why ellipses, having circumferences?).

If he had been able to witness, three centuries later, when Einstein showed in his Theory of General Relativity that in the four-dimensional geometry of space-time the celestial bodies follow straight lines, he would have been satisfied with his search for simplicity; and there was still a simpler figure than the circumference: the line.

In 1627, Kepler published the Tabulae Rudolphine, to which he devoted enormous effort, and which for more than a century were used throughout the world to calculate the positions of planets and stars.

Using the laws of planetary motion he was able to predict satisfactorily the transit of Venus in 1631, thus allowing his theory it was confirmed.

Venus transit
Venus Transit 2012 Sun Crossing. Credit: youtube “”

Kepler died in 1630 in Bavaria, at the age of 59.

In 1632, during the Thirty Years’ War, the Swedish army destroyed his grave and his works were lost.

However, in 1773 they were recovered by Catherine II of Russia. They are currently at the Pulkovo Observatory in St. Petersburg.