Astronomer and Astrologer

by Joelle Steele

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Johannes Kepler was born two months premature on January 6, 1572 at 14:37 UT/2:30pm LAT (rectified birth times) at Weil der Stadt (now Weil in Wurttemberg), Germany (48N45 8E52). His father was a petty officer in the army and he was often left in the care of his grandparents. He was a very delicate child and at 4, he contracted smallpox which permanently impaired his eyesight. That didn't stop him from graduatiJohannes Keplerng with his master's degree in August 1591.

In 1594, Kepler became a mathematics professor at Graz where he was expected to prepare annual almanacs giving astronomical and astrological predictions. This led to a diligent study of astrology. But, Kepler was mainly interested in uncovering a regularity between the orbits of the planets, and it was to this that he dedicated much of his energy. In 1596 he published Mysterium Cosmographicum which led him to correspond with Tycho Brahe and Galileo, two of the most eminent astronomers at the time.

Kepler came to Prague in 1600 at Brahe's invitation. When Brahe died a year later, Kepler was appointed to succeed him as court mathematician. He had all of Brahe's research materials at his disposal. Within two years, Kepler published De fundamentis astrologiae certioribus in which he explained the veracity of astrology. He erected charts for Emperor Rudolf II and other dignitaries, but didn't like to practice astrology because he saw it as nothing more than a financial supplement to practicing astronomy, which paid poorly.

In 1609, Kepler's work Astronomia nova was published in Prague. In it, he broke with astronomical traditions of the past 2,000 years when he proposed his first two laws of planetary motion: 1) the planets revolved around the sun in elliptical paths with the sun at one focus; and, 2) the radius vector of each planet described equal areas of the ellipse in equal times which accounted precisely for the irregular velocity of a planet in its orbit.

Kepler's Model of the UniverseIn 1610, the astrological treatise Tertius Interveniens was published. The following year, Kepler published Dioptrice in which he suggested the design of the inverting telescope, a design that was later modified and came into widespread use. In 1613, while court mathematician to the states of upper Austria, Kepler made waves and was met with antipapal prejudice when he advocated the introduction of the Gregorian calendar.

LEFT: Kepler's Model of the Universe.

In 1619, Kepler finally got around to outlining his third planetary law in De Harmonice Mundi. This law proposed the mathematical concept of harmony in the solar system by means of the unification of the intervals in the musical scale with the angular velocities of the planets.

Kepler was the first to apply his three laws of planetary motion to the computation of ephemerides and to use logarithms to do so. In 1627 his Tabulae Rudolphinae ephemeris was first published and was used universally for more than a century. It also included a catalog of 1,005 stars based on Brahe's observations of 777 star positions.

Johannes Kepler died on November 15, 1630 in Ratisbon (Regensburg), Germany. His laws of planetary motion are acknowledged as one of the foundations of modern astronomy.

This article last updated: 08/07/2009.