Physics School

Johannes Kepler Biography and Laws of Planetary Motion

Johannes Kepler Biography

Johannes Kepler was a 17th century German mathematician and scientist. He is known for his laws of planetary motion which are referred to as Kepler’s Laws. He is one of those who introduced modern astronomy to the world of science.

Where was Johannes Kepler Born?

Johannes Kepler was born in Weil der Stadt, Swabia, Germany on December 27, 1571. His father, Heinrich Kepler, was a mercenary soldier and he left his family when Johannes Kepler was still around five years old. His mother, Katharina Guldenmann, was an innkeeper’s daughter. Johannes Kepler lived with his mother in his grandfather’s inn and he had impressed inn lodgers with his extraordinary mathematical skills. Although Johannes Kepler was born as a sickly child, he still grew up as an intelligent child.

Johannes Kepler's Love of Astronomy

Johannes Kepler had the privilege to observe the Great Comet of 1577. In 1580, he witnessed a lunar eclipse. These incidents had triggered his interest in the field of astronomy. He studied in a local grammar school and then in a seminary before he got admitted to study philosophy at the University of Tübingen in 1589. Johannes Kepler excelled in mathematics and it was Michael Mästlin, his astronomy teacher, who introduced and taught to him the Ptolemaic system and Copernicus’ heliocentric cosmological system. These systems of advanced astronomy were only taught by Michael Mästlin to a chosen few under him.

He accepted the Copernican system as true. In April 1594, he got the position as mathematics teacher in Graz, Austria. His first major publication was in defense to the Copernican system.

Johannes Kepler and Religion

Johannes Kepler was a religious man and he believed that God created nature with numerical relationships. It was his association with the authorities in the University of Tübingen that compromised his religious beliefs and this made him excommunicated in 1612. It was even Michael Mästlin who kept him from being ordained by persuading him to be a post mathematics teacher in Graz.

In Prague, Johannes Kepler became an assistant to a famous astrologer, Tycho Brahe. In 1601, Tycho Brahe died and Johannes Kepler replaced his position. Being in charge of Brahe’s studies collections, Johannes Kepler was able to devise his theory about the planets’ orbits and this eventually led to his statements and laws. He had also worked as a mathematician to Rudolph II who was a Holy Roman Emperor.

Who was Johannes Kepler Married To?

Johannes Kepler was married to Barbara Müller on April 27, 1597. They had two children named Heinrich and Susanna who died in their infancy but they had one daughter and two sons who survived to grow up whose names were Susanna, Friedrich, and Ludwig. His son Ludwig died after acquiring smallpox and it was not long before his wife Barbara died of illness. On October 30, 1613, he was married to Susanna Reuttinger who was the fifth of his eleven prospects. Among his children with Susanna, three died in childhood and three whose names were Cordula, Fridmar, and Hildebert survived to adulthood.

Johannes Kepler spent his last years travelling over Europe until he came to Regensburg and became ill. He died on November 15, 1630 leaving the world of science his laws and theories that made him a figure of change in the history of science and astronomy.

Kepler’s Laws of Planetary Motion

Johannes Kepler stated the three laws of planetary motion. These laws are what are known today as Kepler’s laws and they describe the motion of the planets. He discovered these laws through the help of Tycho Brahe’s studies collections because he was in charge of them when Brahe died.

Kepler's First Law

The first law states that “Every planet has an elliptical orbit with the Sun at one focus.” Having an elliptical orbit means that the sum of the distances of each orbital point to a point called the focus is constant. The first law states that the Sun is a focus of the orbit of every planet in the solar system.

The claims of the Ptolemaic and Copernican systems stated that the orbits of the planets follow a circular path or a superposition of circular paths. Johannes Kepler discovered the elliptic path of the orbits of the planets after studying Tycho Brahe’s observations and study collections and he proved this through his calculations.

The elliptical orbit of the planets is an effect of the force of gravity. Most of the planets follow an elliptical orbit with low eccentricity. Other heavenly bodies such as asteroids also follow an elliptical orbit, with high eccentricity, with the Sun at one of its foci. The eccentricity of an ellipse describes its shape, that is, an ellipse with low eccentricity has a geometric shape that is close to a circle and an ellipse with high eccentricity has a geometric shape that is close to an oval or a stretched circle.

Kepler's Second Law

The second law states that “For equal intervals of time, the line that joins the planet to the Sun sweeps out equal areas.” Consider that when the planet earth is following its elliptical orbit for one day or 24 hours, an area is covered by the earth’s path in its orbit and it is described by the line that joins the earth to the sun.

The second law states that for a 24-hour time interval, this area is always the same wherever the earth on the point of its orbit is. Since the Sun is at one focus of the elliptical orbit of every planet, the planets move faster when they are closer to the sun because the areas that they sweep are the same when they are farther from the sun.

Kepler's Third Law

The third law states that “The square of the period of a planet’s orbit is directly proportional to the cube of the semi-major axis of the planet’s orbit.” This law is described by the equation P2=ka3 where P is the planet’s orbital period, k is the proportionality constant and a is the planet orbit’s semi-major axis. The proportionality constant is the same for all planets.

This law implies that the time it takes the planet to orbit around the sun or what is known as its orbital period increases as its distance from the sun, which is directly related to its orbital path’s semi-major axis, also increases. Mercury, which is the nearest planet to the sun, takes the shortest time to orbit around the sun and Pluto, which is the farthest plant from the sun takes the longest time to orbit around the sun.

The third law was what other scientists and astronomers used to calculate for orbital periods of satellites and other heavenly bodies.

Indeed, Kepler’s Laws of Planetary Motion have been very useful for other scientists and astronomers to understand the orbital paths of the planets, satellites, and moons.