Johannes Kepler
Johannes Kepler  

A 1610 portrait of Kepler by an unknown artist


Born  December 27 1571 Weil der Stadt near Stuttgart, Germany 
Died  November 15 1630 Regensburg, Bavaria, Germany 
Johannes Kepler (27 December 1571 – 15 November 1630) was a German mathematics teacher and astronomer. 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.
 Kepler's first law says that the form of the path is an ellipse, an oval or flattened circle that has two centres. 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 Ptolemaeus with Earth at the middle of the biggest circle.
 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.
 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^{2}), that number is proportional to the distance (d) of a planet to the Sun multiplied by itself twice (d^{3}).
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 16181621)
 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)
Images

The Kepler crater as photographed by Apollo 12 in 1969

Kepler's Platonic solid model of the solar system, from Mysterium Cosmographicum (1596)

Remnant of Kepler's Supernova SN 1604

One of the diagrams from Strena Seu de Nive Sexangula, illustrating the Kepler conjecture

A statue of Kepler in Linz

The GDR stamp featuring Kepler