Johannes Kepler (1571 – 1630)
A Biography of the German Mathematician, Astronomer and Astrologer
Dec 16, 2008
Johannes Kepler was a German mathematician, astronomer and astrologer who played a vital role in the scientific revolution that occurred in Europe in the 17th century. Kepler is best known for his three laws of planetary motion, laws that were vital to the formulation of Isaac Newton’s theory of universal gravitation.
Formative Years
Johannes Kepler was born in 1571 in Weil der Stadt, Germany. Kepler’s family were fairly poor. His father earned a precarious living as a mercenary but abandoned his family when Kepler was five years old. His mother helped to run the family inn and was also a healer and herbalist who was later tried for witchcraft.
Kepler was a sickly but brilliant child who impressed visitors to the family inn was his mathematical ability. Kepler was also interested in astronomy from an early age, an interest that would last his entire life. Unfortunately, a childhood attack of smallpox left Kepler with weak eyesight and crippled hands and so limited his ability in the observational aspects of astronomy.
Kepler excelled at school and in 1589 enrolled at the University of Tubingen as a theology student. While at university, Kepler proved himself to be an excellent mathematician and earned a reputation as being a skilful astrologer, regularly casting horoscopes for fellow students. During this time Kepler continued his studies of astronomy, becoming a committed Copernican and defender of heliocentrism.
Despite his desire to become a minister, at the age of 23 Kepler accepted a job as a teacher of mathematics and astronomy at a Protestant school in Graz, Austria.
Mysterium Cosmographicum
Kepler’s first major astronomical work, Mysterium Cosmopgraphicum, was also the first published defence of the Copernican system. Kepler claimed that while teaching in Graz about the periodic conjunction of Saturn and Jupiter in the Zodiac, he had the epiphany that regular polygons bound one inscribed and one circumscribed circle at definite ratios which, Kepler reasoned, might be the geometrical basis of the universe.
As indicated in the title of his work, Kepler thought that he had revealed God’s geometrical plan for the universe. Much of his enthusiasm for the Copernican system stemmed from this theological conviction about the connection between the physical and the spiritual.
Laws of Planetary Motion
From 1600 Kepler worked for the astronomer Tycho Brahe. Brahe had compiled intricate tables of the planets motions on the sky and it was through studying Brahe’s table on the motion of Mars that Kepler constructed his fundamental three laws of planetary motion.
Kepler’s first law notes that the planets move in elliptical orbits with the Sun at one of the two foci of the ellipse.
His second law describes how quickly a planet moves around its orbit. As the planet moves along its orbit, it sweeps out an equal segment in an equal time. Since the orbits are elliptical, when the planet is close to the Sun it needs to cover a larger distance to sweep out the same area than when it is far away. So the planet moves faster nearer the Sun than when it is far away. Although Kepler didn’t realise it, this behaviour of the planets is ultimately due to gravity accelerating the planet faster when it is near the Sun’s mass.
Kepler’s third law details how the orbital periods scale up for different sized ellipses at a range of distances from the Sun. It states that the squares of the orbital periods are inversely proportional to the cube power of the longest axis of the elliptical orbit. The larger the elliptical orbit, the slower the time taken to complete an orbit.
Sources:
Sagan, Carl (1983) Cosmos (Abacus)
Westman, Robert ‘Kepler’s Early Physico-Astrological Problematic’ Journal for the History of Astronomy 32 (2001) 227 - 236
Dawkins, Richard & Kerrod, Robin (2001) Oxford Illustrated Science Encyclopedia (OUP Oxford)
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