# Johannes Kepler

### Garrett Dunn

## Johannes Kepler

Born December 27, 1571 to poor parents in Weil der Stadt, Württemberg in the Holy Roman Empire in present-day Germany, Kepler was a sickly but intelligent child. His superior intellect delivered him a scholarship to study for the Lutheran ministry at the University of Tübingen, where he studied the ideas of Copernicus delightfully.

Kepler later wrote his first defense of the Copernican system while a mathematics teacher in Graz, the *Mysterium Cosmographicum*. As a Lutheran, he agreed with the Augsburg Confession, a Lutheran defining document, but did not accept the Lutheran position on real presence. This and his refusal to sign the *Formula of Concord* made him lose the Lutheran sacraments, and his refusal to convert to Catholicism alienated him during the Thirty-Years War, with no refuge among Lutherans or Catholics.

Forced to leave his teaching position at Graz because of the Counter Reformation, he moved to Prague to work with Tycho Brahe, renowned Danish astronomer. In 1601, Kepler inherited the position of Imperial Mathematician after Brahe's death. Using Brahe's precise data, Kepler concluded that the orbit of Mars was elliptical. He published *Astronomia Nova *in 1609, describing his discoveries, today called Kepler's first two laws of planetary motion. The book was the first time a scientist documented how he used the scientific method to come to a conclusion.

In 1612, Kepler moved to Linz after Lutherans were forced out of Prague. His two sons and his wife had recently died, but Kepler happily remarried. He had many financial and personal problems, however. His two infant daughters died, and Luther had to return to Württemberg to defend his mother against charges of witchcraft, which he did successfully. *Harmonices Mundi* was published by him in 1619, in which his third law is described.

Despite many forced moves for being Lutheran, Kepler published *Epitome Astronomiae*, his most influential book that systematically describes all of heliocentric astronomy, in 1621. He then fulfilled his old colleague Brahe's vision by creating the *Rudolphine Tables*, which included logarithmic calculations he developed, and also tables for calculating past or future planetary positions. In 1630, Kepler set out from his home in Sagan to collect a debt. However, he died along the way in Regensburg, leaving behind a legacy and information still used today.

Kepler meets Castiglione's requirements in that he was well-educated, well-mannered, master of many areas, and knew literature and history. He was an eloquent speaker, able to defend his mother from charges of witchcraft. However, he was not overactive or athletic. In fact, he was a sickly child and a rather physically frail adult. Despite this, he was part of one of the most astronomical movements in history.

## Kepler's Three Laws of Planetary Motion

__The Law of Ellipses (The First Law)__

Kepler's first law, described in his book *Astronomia Nova* along with his second law, states that the path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. An ellipse is a shape which resembles a stretched circle. It is essentially an oval with a mathematical equation. It has two focus points, or foci, and the center of the sun is one of the foci of the path of the planets. The closer the foci are together, the closer the ellipse is to becoming a circle. As opposed to common thought of the day, Kepler was proposing that planets had an elliptical orbit rather than a circular one.

__The Law of Equal Areas (The Second Law)__

Also described in *Astronomia Nova*, Kepler's second law states that an imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal amounts of time. As planets move in their elliptical orbit, the closer they are to the sun at any given time makes them move along their orbit faster at that moment. The further away from the sun a planet is, the slower it will move in its orbit. This works because when the planet is further away from the sun it moves slower, and when it is closer it moves faster, so the distances and speeds balance out to create equal area for any given planetary position and amount of time.

__The Law of Harmonies (The Third Law)__

Kepler's third law, described in his *Harmonices Mundi* states that the ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. This means that when you square the amount of time it takes for two planets to orbit the sun completely, the ratio of one planet to the other is equal to the cubes of the respective planets' average distances from the sun. This can also mean that the ratio of time to orbit the sun squared to the cube of the average distance from the sun is equal among all planets.

## Resources

"Kepler's Three Laws." Kepler's Three Laws. The Physics Classroom, n.d. Web. 22 Apr. 2015. <http://www.physicsclassroom.com/class/circles/Lesson-4/Kepler-s-Three-Laws>.

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