Johannes Kepler

**Johannes Kepler** (27 December 1571 – 15 November 1630) was a German mathematics teacher, astronomer, optician, natural philosopher, astrologer and Lutheran theologian.

He was Tycho Brahe's apprentice. Tycho Brahe looked at the way the planets moved in the sky. Johannes Kepler found a simple way to say how the planets move. Kepler also studied other things like Kepler's supernova.

## How the planets move

A planet moves along a path called an orbit. Kepler used three laws to say what form the path has and how fast the planet moves:

1. Kepler's first law says that the form of the path is an ellipse, an oval or flattened circle that has two centers. The Sun is in one of the centers of the ellipse. Before Kepler, astronomers thought that planets moved in circles within circles (epicycles) according to the system of Claudius Ptolemy with Earth at the middle of the biggest circle.
1. Kepler's second law says how fast the planet moves around the ellipse. When the planet is closer to the Sun, it moves faster. When it is farther from the Sun, it moves slower. If there is a line between the planet and the Sun, the line sweeps out an area as it follows the planet. The area it sweeps out in one day is always the same. Before Kepler, astronomers thought that planets always moved at the same speed along the circles.
1. Kepler's third law says how fast different planets move. A planet that is farther from the Sun moves slower than a planet that is closer to the Sun. If a person multiplies the time (_T_) it takes for a planet to go around the Sun by itself (_T_<sup>2</sup>), that number is proportional to the distance (_d_) of a planet to the Sun multiplied by itself twice (_d_<sup>3</sup>).

Kepler published the first two laws in 1609, and the third in 1619.

## Writings by Kepler

- _Mysterium cosmographicum_ (_The Sacred Mystery of the Cosmos_) (1596)
- _Astronomia nova_ (_New Astronomy_) (1609)
- _Epitome astronomiae Copernicanae_ (_Epitome of Copernican Astronomy_) (published in three parts from 1618 to 1621)
- _Harmonice Mundi_ (_Harmony of the Worlds_) (1619)
- _Mysterium cosmographicum_ (_The Sacred Mystery of the Cosmos_) 2nd Edition (1621)
- _Tabulae Rudolphinae_ (_Rudolphine Tables_) (1627)
- _Somnium_ (_The Dream_) (1634)

## Related pages

- Galileo Galilei
- Nicolaus Copernicus
- Tycho Brahe
- Kepler (spacecraft)

## Sources

- Caspar, Max. 1993. _Kepler_. transl. and ed. by C. Doris Hellman; with a new introduction and references by Owen Gingerich; bibliographic citations by Owen Gingerich and Alain Segonds. New York: Dover.
- Gingerich, Owen. 1993. _The Eye of Heaven: Ptolemy, Copernicus, Kepler_. American Institute of Physics.
- Gingerich, Owen 1973. "Kepler, Johannes". In _Dictionary of Scientific Biography_, Volume VII. Charles Coulston Gillispie, editor. New York: Scribner's.

## Other websites

- Kepler and the "Music of the Spheres"
- Gale E. Christianson- Kepler's Somnium: Science Fiction and the Renaissance Scientist
- Kepler's Belief in Astrology by Nick Kollerstrom
- References for Johannes Kepler
- Johannes Kepler Citizendium

## Infobox (scientist)

- **name**: Johannes Kepler
- **image**: JKepler.jpg
- **caption**: Portrait of Kepler, 1620
- **birth date**: December 27, 1571
- **birth place**: Weil der Stadt, Free Imperial City of Weil der Stadt, Holy Roman Empire, (now Germany)
- **death place**: Regensburg, Free Imperial City of Regensburg, Holy Roman Empire
- **field**: Astronomy, astrology, mathematics and natural philosophy
- **nationality**: German
- **alma mater**: Tübinger Stift, University of Tübingen
- **doctoral advisor**: Michael Maestlin
- **known for**: Kepler's laws of planetary motion
Kepler conjecture
_Rudolphine Tables_
- **influences**: Nicolaus Copernicus
Tycho Brahe
- **influenced**: Sir Isaac Newton
- **signature**: Unterschrift Kepler.svg