We explain who Johannes Kepler was, what his major contributions to astronomy were, and why his work is so important to modern science.

Johannes Kepler discovered the laws that govern the motion of the planets in their orbits.

## Who was Johannes Kepler?

Johannes Kepler was a** German astronomer, mathematician and physicist**who discovered and formulated the laws governing the motion of the planets around the Sun. Kepler was a collaborator of the Danish astronomer Tycho Brahe (1546-1601), whom he replaced at his astronomical observatory and as mathematician at the court of the Holy Roman Empire.

Kepler’s astronomical studies played an important role in the understanding of the cosmos, since they complemented the heliocentric view of the solar system proposed by Nicholas Copernicus (1473-1543) and** laid the foundations for future discoveries made by scientists such as Isaac Newton (1643-1727)**. Furthermore, Kepler’s ideas conformed to an empirical standard that was rare for his time.

Although Kepler did his best to disseminate his discoveries in such a way that they did not contradict the religious ideas of the society of his time, **his work Epitome astronomiae Copernicanae She was considered suspected of heresy by the Holy Inquisition **and sentenced to her index of banned books.

## Childhood and youth of Johannes Kepler

Johannes Kepler **He was born on December 27, 1571 in the town of Weil der Stadt, in the southwest of the current territory of Germany.**He was one of four children of Heinrich Kepler, a mercenary soldier in the army of the Duchy of Württemberg, and Katharina Kepler, who was a herbalist, healer and manager of an inn. The family, of Lutheran religion, lived in fairly humble conditions.

Johannes was born prematurely, at seven months of pregnancy, and was consequently a child of fragile health. **At the age of three he contracted smallpox, which left him with vision problems.**.

Since his father’s profession required him to travel constantly, Johannes and his three siblings (Margarette, Christopher and Heinrich) were largely raised by their grandparents. Even so, **It was thanks to his father that he discovered astronomy**at the age of six, when they together observed the passage of a comet in Leonberg.

Between the ages of nine and eleven, Johannes worked as a day labourer until he was able to enter the Protestant seminary of Adelberg in 1584 and two years later the Higher Seminary of Maulbronn. His devout character and humble origins earned him funding from the local nobility.

Johannes **He was intelligent, curious and had a great talent for mathematics, so he finished his basic training in 1589 and entered the University of Tübingen.**, where he studied logic, rhetoric, dialectics, astronomy, theology and other human sciences. His plan was to become a theologian.

That same year, his father died on the front lines. Johannes was 18 years old.

## Kepler’s formative years

Kepler married Barbara Müller in 1597.

At university, Kepler **He was one of the best disciples of the German astronomer and mathematician Michael Maestlin (1550-1631)**with whom he discovered the heliocentric model of the solar system, that is, the model proposed by Copernicus thirty years earlier.

**Maestlin reserved this knowledge for his best students.**while teaching the rest the geocentric model of Ptolemy (c. 100-c.170 AD), as mandated by the Church. Kepler and his teacher not only shared this knowledge, but also maintained a friendship that was expressed in decades of correspondence.

**Kepler** **He studied Copernicus’s book in depth From the revolutionary orbium coelestium** (“

*About the revolutions of the celestial orbs*” in Latin) and set out to demonstrate its veracity and express it in both philosophical and religious language. At that time, science and belief were not entirely separate.

He *heliocentrism *proposed by Copernicus is the astronomical theory that understands the solar system as an arrangement of planets that orbit a relatively stationary sun. It was proposed in the 16th century, contradicting the model established and defended by the Catholic Church: the geocentric model, according to which the Earth is the immobile center of the universe and the stars, including the Sun, revolve around it.

Kepler completed his studies at the university and received a master’s degree in 1591. His interest in theology gave way to mathematics and he was sent to a teaching position at a Lutheran school in Graz in 1594. **There he devoted himself to astronomy and astrology, which at that time were one and the same, and to the composition of almanacs and horoscopes.**.

During his years in Graz, Kepler devoted himself to increasingly profound questions about the nature of the universe. **At that time, astral movements were little known and it was thought that they were dominated by divine will.**. And although Kepler was a profound believer, he gave himself permission to ask himself questions about it and look for possible answers in geometry, whose figures were considered based on divinity.

Kepler’s astronomical studies led him to write his first book, *Mysterium Cosmographicum* (“*The cosmographic mystery*”), published in 1596, because he believed he had discovered the “architecture of the cosmos”. He assumed that the proportions between the orbits of the different planets (understood as perfect circles) could be illustrated with certain geometric solids: an octahedron for the orbit of Venus, a dodecahedron for that of the Earth, and so on.

**This first work by Kepler lacks scientific rigor, but it illustrates very well the combination of scientific observation and religious interpretation that characterized the society of the time.**. In addition, Kepler sent copies of his book to two great astronomers with whom he corresponded and even became friends: Galileo Galilei (1564-1642) and Tycho Brahe (1546-1601). The latter, in particular, was interested, since he deduced that Kepler’s calculations were based on the Copernican model, on which Brahe himself was working.

At that time, on the other hand, Kepler needed support to be able to research freely, and his teaching salary was not enough for this. So in 1597 he married Barbara Müller, daughter of a wealthy local merchant, with whom he had five children over the years.

## Kepler’s life in Prague

Kepler and Tycho Brahe were friends and colleagues at the Prague observatory.

In 1600, the Archduke of Austria, Ferdinand II of Habsburg (1578-1637), a staunch Catholic, decreed the expulsion of Protestants from his domains. Kepler and his wife then had to find somewhere to go. Fortunately, the correspondence between Kepler and Tycho Brahe opened the doors of Prague to them.

**Kepler was invited by Brahe to his castle Benátky, outside the city. There they worked together on a new system of planetary observations.**. Brahe maintained that astronomical observation could not occur occasionally, but required frequent measurements using instruments that were as accurate as possible.

However, relations between the astronomers were tense from the start: Kepler was greedy for financial support, ambitious, and only 29 years old, while Brahe, already in his 50s, was jealous of his data and considered Kepler’s claims excessive.

Finally, in 1601, Brahe agreed to intercede on Kepler’s behalf with Emperor Rudolf II of Habsburg (1552-1612). And he did so just in time, for **Brahe died suddenly on 24 October of that same year, and Kepler was chosen as his successor at the astronomical observatory and in the post of imperial mathematician.**.

Kepler inherited from Brahe, among other things, a table of astronomical observations that the Dane had made over the years: the “Rudolphine Tables” (*Tabulae rudolphinae*in honor of Emperor Rudolf II). It was a star catalogue with precise calculations and specific measurements for each planet and each of the main constellations.

All this new knowledge marked the end of Kepler’s mystical stage and oriented him towards a more scientific perspective. Thus, the following year he published *De fundamentalis astrologiae certioribus* (“*On the most accurate foundations of astrology*“), an attempt to give verifiable bases to astrology, supported, however, in the Copernican model and in Brahe’s annotations.

At the imperial court, Kepler enjoyed greater conceptual autonomy and was able to devote himself completely to his studies. **Their efforts focused, first, on understanding the retrograde orbit of Mars.**and realized, in light of the new data available, that his theories about the geometric harmony of celestial orbits did not work.

**Determined to demonstrate the perfection of the divine model, he tried changing ovals for circles to describe the orbits and, finally, he tried ellipses.**. It was thus that he was able, finally, to unravel his first two laws (the third dates from 1619) that appeared in his work. *Nova Astronomy* (“*New Astronomy*”) in 1609.

## Kepler’s laws

Kepler’s laws are the set of mathematical principles through which Kepler described the movement of the celestial bodies, that is, the planets of the solar system, in their respective orbits around the Sun. **This was the first comprehensive theory of planetary motion formulated in history.**.

### Kepler’s first law (1609)

Kepler’s first law states that **All the planets move around the Sun following an elliptical path, that is, in the shape of an ellipse and not a circle as until then thought.**. The Sun, in this sense, is always at one of the focuses of said ellipse.

From this consideration, two points of the planet’s trajectory in its elliptical orbit can also be identified: the aphelion (closest point to the Sun) and the perihelion (the furthest point from the Sun).

### Kepler’s second law (1609)

Kepler’s second law, known as the “law of areas,” **states that the imaginary line ( radio vector) that unites a planet with the Sun at a given point in its orbit, sweeps out equal areas in the same time interval**.

This means that, although the distance of the planets from the Sun varies in length along the orbit, it is possible to draw an imaginary line between the planet and the Sun at two consecutive points in its orbit and study the area arising from both lines. This area will always be proportional to the time elapsed between one point and the other.

This second law allowed Kepler to understand that the planets move faster as they approach the Sun (that is, the aphelion) and more slowly as they move away from it (that is, as they reach the perihelion).

### Kepler’s third law (1619)

Kepler’s third law, known as the “law of periods” or “law of harmonies”, **states that the square of the orbital period…**