Johannes Kepler
1571 - 1630

Johannes Kepler was born to a poor family, whose father finally settled to become a tavern keeper. He was a sickly child, and was withdrawn from school to help in the tavern and as a laborer in the fields. Because his family was Lutheran, Kepler was destined for the ministry. He was sent as a charity student to a protestant seminary, and later to a college where he received a Bachelorís Degree. His evident intelligence earned him a scholarship to the University of Tubingen, where he studied theology and mathematics, and earned a Masterís Degree in Philosophy.

Kepler learned privately of the heliocentric view, which is the belief that the sun is the center of the planetary system, and other Copernican theories at the university. Kepler was forced to study this view privately because the church, as well as society, was adamant in its belief of the Ptolemaic Theory of a geocentric universe. This theory states that the earth is the center around which the heavenly bodies move. Eventually, he became an outspoken supporter in defense of the Copernican system.

After his studies at the university, Kepler became a professor of mathematics. In 1600 he left the teaching profession to work as an assistant to Tycho Brahe, in Prague. It was in Prague, during his tedious study of the orbit of Mars, where Kepler developed his first two laws of planetary motion.


The first law was announced in 1605. The law states that, ďplanets move in ellipses with the sun at one focus.Ē Earlier astronomers and mathematicians defined the ellipse as a regular curve with easily defined properties much like those of a circle. Through his study of the orbit of Mars, Kepler discovered that the simple ellipse would succinctly define itís orbit. The geometric definition of an ellipse helped Kepler disprove the theories of the circular or spherical motion of heavenly bodies, which was believed for over 2,000 years. He, like Copernicus, believed that God had a simple, mathematical plan in his creation of the universe. Keplerís first law exemplified this belief.


The second law states that, ď the radius vector describes equal areas in equal times.Ē Through his observation of the ellipse, Kepler discovered that the planets did not move at a constant speed. A planet closer to the sun in its orbit, moves faster and farther along its elliptical arc. The same planet farther from the sun in its orbit moves slower and covers a smaller distance along the arc of its ellipse.The time, however, is the same for both distances. In Keplerís search for mathematical simplicity, a constant velocity would have been easier to understand, but God clearly chose a less obvious mathematical law to determine the velocity of the planets. This law was published, along with the first, in Keplerís book Astronomia Nova.


His second law proved that the distances of the planets varied greatly in their orbits. The third law resulted as a searched for a principle with that in mind. The third law states that, ďthe squares of the periodic times are to each other as the cubes of the mean distances.Ē Kepler announced this law in 1619, fourteen years after the first two laws. It took him years to find the law to describe the distances of the planets to the sun. After much deliberation, Kepler arrived at the law that if T is the period of revolution of any planet and D is its mean distance from the sun, then T squared is equal to k multiplied by D cubed, where k is a constant, which is the same for all the planets. This third law was published in his book, The Harmony of the World.


  • Kepler formulated eyeglass designs for nearsightedness and farsightedness.
  • He was the first to explain the use of both eyes for depth perception.
  • He explained the principles of how a telescope works.
  • Kepler was the first to explain that the tides are caused by the Moon(Galileo scoffed at him for this belief).
  • He was the first to derive the year of Christís birth, that is now universally accepted.

Contributed by Steve Bixler

  1. Hall, A. Rupert. From Galileo to Newton. Dover, New York, 1981.
  2. Kline, Morris. Mathematics: A Cultural Approach. Addison-Wesley, Mass. 1962.

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