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Johannes Kepler

developer of three laws of planetary motion

Johannes Kepler

Johannes Kepler was born on December 27, 1571, in Weil (near Stuttgart), Germany. It was while studying theology and the classics at the University of Tübingen that he learned about the heliocentric theory of planetary motion first developed by Nicolaus Copernicus. From 1594 to 1600, he held the chair of astronomy and mathematics at the University of Graz, Austria.

While at Graz, Kepler worked out a complex geometric hypothesis to account for distances between the planetary orbits. He then proposed that the Sun emits a force that diminishes inversely with distance and pushes the planets around in their orbits. He published his account in a treatise entitled Mysterium Cosmographicum (Cosmographic Mystery) in 1596.

Kepler left Graz in 1597 rather than undergo compulsory conversion to Roman Catholicism. While seeking another post, he became assistant to the Danish astronomer Tycho Brahe in the latter's observatory near Prague. On the death of Brahe in 1601, Rudolph II, the Holy Roman Emperor, appointed Kepler to be Brahe's successor as imperial mathematician.

Kepler made his most significant discoveries when he tried to find an orbit to fit Brahe's observations of Mars. Earlier astronomers believed a planet's orbit was a circle or a combination of circles, but Kepler could not find a circular arrangement to agree with Brahe's observations. He realized that the orbit could not be circular, and resorted to an ellipse in his calculations. The ellipse worked, and Kepler destroyed a 2,000-year-old belief. In 1609, he published Astronomia Nova (New Astronomy), which contains two of his laws of planetary motion: (1) every planet follows an oval-shaped path around the Sun, called an ellipse, with the Sun at one focus; and (2) an imaginary line from the center of the Sun to the center of a planet sweeps out the same area in a given time, meaning that planets move faster when they are closer to the Sun.

the apparent movements of the then-known planets as observed from the Earth
the apparent movements of the individual planets, as seen from the Earth

In 1612, Kepler became mathematician to the states of Oberösterreich (Upper Austria). In 1619, while living in Linz, he published his Harmonica Mundi (Harmony of the World), the final section of which contains his third law of planetary motion: (3) the ratio of the cube of a planet's distance from the Sun and the square of the planet's orbital period is a constant and is the same for all planets.

Epitome Astronomiae Copernicanae (Epitome of Copernican Astronomy), published between 1618 and 1621, brought all of Kepler's discoveries together in a single volume, and became the first textbook of astronomy to be based on Copernican principles. Tabulae Rudolfinae (Rudolfine Tables), published in 1625, consists of new tables of planetary motion based on Brahe's data.

Kepler also founded modern optics by giving the earliest acurate account of the way in which the human eye works. He correctly identified the pupil as a diaphragm through which light rays enter the eye and described the way in which they are refracted. He suggested that people with blurred vision had an optical defect which meant that rays were focused either in front of or just behind their retina. He then went on explain how telescopes work by refracting light and brining an enlarged image to focus in the eyepiece. His work provided the theoretical basis for subsequent improvements in telescope design and for the development of opthalmic optics as a recognized branch of medicine.

Johannes Kepler died in Regensburg, on November 15, 1630.

The Galileo Project

Nicolaus Copernicus
Tycho Brahe

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This page was last updated on 11/14/2017.