# Johannes Kepler

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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
Monument of Tycho Brahe and Johannes Kepler in Prague

Johannes Kepler (lived 27 December 1571 – 15 November 1630) was a German astronomer. He was Tycho Brahe's apprentice, or person who learns a trade from another person. Tycho Brahe looked at the way the planets moved in the sky. This motion is very complex. Johannes Kepler found a simple way to say how the planets move. Kepler also studied other things like Kepler's supernova.

## Laws which say 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/ovale or flattened circle that really has two "centers". The Sun is in one of the centers of the ellipse. Before Kepler, astronomers thought that planets moved in different circles which were on top of each other (kind of like a spiral). The Earth was at the middle of the biggest circle.
2. 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.
3. 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 (T2), that number is proportional to the distance (d) of a planet to the Sun multiplied by itself twice (d3).

## Notes and references

• The most authoritative biography of Kepler is Max Caspar's Kepler. Though there are a number of more recent biographies, most are based on Caspar's work with minimal original research; much of the information cited from Caspar can also be found in the books by Arthur Koestler, Kitty Ferguson, and James A. Connor. Kepler's mathematics and cosmological views have been extensively analyzed in books and journal articles, though his astrological work—and its relationship to his astronomy—remains understudied.