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Kepler and part of his heliocentric Solar System,

How Did the Skeptical Astrology of Johannes Kepler Contribute to Our View of the Cosmos?


Johannes Kepler, a key figure in the scientific revolution and keen astrologer, paid the bills by writing horoscopes for the rich and famous. The namesake of NASA’s space observatory was a German scholar who wrote extensively on how the patterns of the stars had profound effects on planets such as Earth as well as on individuals’ lives. The classroom legacy of Kepler focuses on the improvements he made to the telescope and his laws of planetary motion. However, these were merely byproducts of the man’s lifelong ambition to improve the science of astrology so that it could stand up to the light of scrutiny during the age of reason.

Back dropped by the blackness of space and Earth's horizon, the European Space Agency's "Johannes Kepler" Automated Transfer Vehicle-2 (ATV-2) docks to the aft end of the International Space Station's Zvezda Service Module

Back dropped by the blackness of space and Earth's horizon, the European Space Agency's "Johannes Kepler" Automated Transfer Vehicle-2 (ATV-2) docks to the aft end of the International Space Station's Zvezda Service Module. ( Public Domain )

Differences Between Astronomers and Astrologers

First, one must understand the difference between an astronomer and astrologer. Today, one profession is considerably more respected than the other, but in Kepler’s time there was much controversy over whether the two were distinct fields of research or not. An astronomer “studies everything outside of the earth's atmosphere, such as planets, stars, asteroids, galaxies; and the properties and relationships of those celestial bodies. Astronomers base their studies on research and observation” (AAS, 2016).

In contrast, an astrologer “interprets the influence that the Sun and Moon have while they are in a specific sign of the zodiac. According to astrology, there are 12 divisions in the sky, measuring exactly 30 degrees each… These are the 12 signs of the zodiac. Astrologers believe that the position of the Moon and Sun within these twelve divisions influence what happens on Earth, from the personalities of people born under a certain sign to the likelihood of certain kinds of undertakings to be successful” (Weatherbee, 2010).

Kepler was arguably the last astronomer to believe in astrology, at least in the Western world.

Portrait of Johannes Kepler, 1610.

Portrait of Johannes Kepler, 1610. ( Public Domain )

Kepler’s Astrological Almanacs

Kepler drew up some 800 horoscopes (not all of them flattering) for friends and patrons. While serving as the court mathematician of Holy Roman Emperor Rudolph II, Kepler calculated the horoscopes of famous people such as Emperor Augustus Caesar and the Prophet Mohammed in order to explain why their fates developed as they did. This served as both an enjoyable and educational form of entertainment.

Despite this, Kepler’s renown as an astrologer mostly derived from his annual almanacs, which he published for over three decades. Similar to today’s farmer’s almanac, 17th century almanacs were “expected to include such things as war and pestilence, weather and harvest, the religious and political upheavals, and when to sow or bleed or purge and the like” (Kollarstrom, 2006). Kepler derided much of this as “Arabic art amounting to nothing,” however, he did manage to predict a winter of severe cold and possibly a Turkish invasion (Kollarstrom, 2006).

The Cosmos with overlay of the Astrological Zodiac.

The Cosmos with overlay of the Astrological Zodiac. ( Youtube Screenshot )

The Skeptical Astrologer

Kepler lived and wrote in a time when general opinion was shifting away from the closed Ptolemaic view of the world and towards the infinitely open Copernican view. Like any good rationalist in the 17th century, Kepler did not buy wholesale the soothsaying and fortune telling that infested the study of astrology. Thus, he earned the sobriquet ‘the skeptical astrologer.’ Nonetheless, he felt passionately that predictions could be made by studying the heavens. He warned ‘“theologians, physicians and philosophers against throwing out the baby with the bathwater, and thereby maltreating their profession’ (Note: that seems to be the first use of the phrase about babies and bathwater)” (Kollarstrom, 2006).

Like the Ptolemaic worldview, the ancient practice of astrology was geocentric. Kepler sought to reconcile astrology with a heliocentric worldview. It is difficult to say how well he succeeded. However, as the historian John North notes, “Had he not been an astrologer he would very probably have failed to produce his planetary astronomy in the form that we have it” (North quoted in Currey, 2010).

Model of Kepler’s Solar System Theory, Technisches Museum, Vienna

Model of Kepler’s Solar System Theory, Technisches Museum, Vienna ( CC BY-NC SA 2.0)

Kepler wrote vast tomes on astrology and astronomy as well as on music, geometry, and arithmetic. Yet, until recently, his writings on astrology were not translated into English, perhaps because “these beliefs of his are normally dismissed in the history books as mere vestiges of a medievalism from which he was regrettably unable to free himself” (Kollarstrom, 2006).

The view on astrology has softened as of late, largely in part because so many famous thinkers and innovators believed so strongly in it. Whatever one may think of the practice, astrological sentiments helped shape many of history’s greatest minds. Kepler himself recognized that the scorn heaped upon astrologers was a temporary phenomenon and that one day, he would be vindicated:

“Now because 18 months ago the first dawn, 3 months ago broad daylight but a very few days ago the full sun of the most highly remarkable spectacle has risen nothing holds me back. I can give myself up to the sacred frenzy, I can have the insolence to make a full confession to mortal men that I have stolen the golden vessel of the Egyptians to make from them a tabernacle for my God far from the confines of the land of Egypt. If you forgive me I shall rejoice; if you are angry, I shall bear it; I am indeed casting the die and writing the book, either for my contemporaries or for posterity to read, it matters not which: let the book await its reader for a hundred years; God himself has waited six thousand years for his work to be seen.”  (Kepler’s Harmonices Mundi Book V quoted in Kollarstrom, 2006)

Monument to Tycho Brahe and Kepler in Prague, Czech Republic

Monument to Tycho Brahe and Kepler in Prague, Czech Republic ( Public Domain )

Top Image: Kepler and part of his heliocentric Solar System, Source: Mjcv

By Kerry Sullivan


AAS. "What's the Difference between Astronomy and Astrology?"  Education - General Astronomy . American Astronomical Society, 26 Feb. 2016. Web. https://aas.org/faq/whats-difference-between-astronomy-and-astrology

Currey, Robert. "Johannes Kepler and Astrology."  Astrologer. Equinox, 2010. Web. http://www.astrologer.com/tests/kepler.htm

Kollerstrom, Nick. "Kepler's Belief in Astrology."  SkyScript. The Astrological Association, 2006. Web. http://www.skyscript.co.uk/kepler2.html

Weatherbee, Caleb. "Astrology vs. Astronomy: What's the Difference?"  Farmers' Almanac . Farmers' Almanac, 5 Apr. 2010. Web. http://farmersalmanac.com/astronomy/2010/04/05/astrology-vs-astronomy/

Kerry Sullivan's picture

Kerry Sullivan

Kerry Sullivan has a Bachelor of Science and Bachelor of Arts and is currently a freelance writer, completing assignments on historical, religious, and political topics.

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