Johannes Kepler and Astrology
Did Kepler he use astrology merely 'to pay the bills' or was he a sceptical astrologer?
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Johannes KeplerJohannes Kepler (27 Dec 1571 - 1630) was an astrologer, astronomer and mathematician. Historian, John North claims 'had he not been an astrologer he would very probably have failed to produce his planetary astronomy in the form that we have it.'[1] However, Kepler's views on astrology were ambivalent.

Kepler earned a living reading horoscopes for the rich and powerful, produced Yearly Almanacs with astrological and other phenomena, introduced three new aspects[2] to astrology and connected the tidal cycle with the Lunar Nodes[3]. In 1601, he wrote De Fundamentis Astrologiae Certoribus in which he "defends on physical grounds concerning the foundations of astrology ... "

Yet, Kepler was also critical of astrologers at a time when judicial astrology appeared to be no more than fortune-telling in an increasingly rational academic world. He described himself to his mentor, Michael Maestlin as a 'Lutheran Astrologer'[4] and claimed that 'Many of the rules of this Arabic art amount to nothing'. In line with the changing times, he wanted a reformation of astrology but, he implored theologians, physicians and philosophers not to 'throw the baby out with the bathwater'. [Tertium Intervens, 1610]
In 1606, Kepler wrote to Oxford scientist, Thomas Harriot:
"I am informed that misfortune came to you from astrology. I ask you if you believe that it could be powerful enough to have such power. Ten years ago, I rejected the division into 12 equal signs, the Houses, dominations (i.e. rulerships), triplicities etc. and I am retaining only the aspects (angles) and am transferring astrology to the science of harmonics."
On the face of it, these widely quoted comments promote a popular myth that Kepler did not 'believe in astrology' and sought to destroy the 'superstition' but had to practice astrology simply to pay the bills. However, Kepler's public comments may not have been a genuine reflection of his personal view of astrology:
  1. Kepler was seeking information from Harriot on his experiments in refraction. He was also aware of Harriot's criticism of astrology and was clearly attempting to recast astrology within what he thought might be a more acceptable Harmonic-Pythagorean framework.[5]
  2. Though Kepler expressed concern to Michael Maestlin that his work on Alamanacks would harm his scientific reputation, Kepler continued to publish these astrological predictions up until 1624. He was not averse to using signs in his analyses. In his prognostication for the Winter of 1618 - Kepler writes ".... that it may be rain or snow. The causes being Venus and Mercury in Sagittarius ..."[6.1]
  3. In 1603, Kepler saw significance in the Jupiter/Saturn conjunction falling in a different element. This confirms that he had not abandoned the triplicitities (and hence the associated zodiacal signs) in the previous century as he had claimed.[6.2]
  4. In Kepler's re-analysis and rectification of the chart for Albrecht Wallenstein, a Bohemian General, on 21 January, 1625 Kepler refers to the Moon in 12th house and Venus square the ascendant.[6.3] In 1999, an astronomer found a horoscope drawn up by Kepler.[7] It clearly shows the positions of the houses and the signs. It was for an Austrian nobleman who was born in 1587. Kepler's client would have had to have been less than 10 years old and Kepler 25, for his claim to Harriot to be true.
  5. After the death of his son in 1611, Kepler corresponded with Michael Maestlin at the University of Tübingen. In two private letters, Kepler discussed the horoscopes of his family and the astrological reasons for his son's death.[8] This private exchange, where no financial gain was involved, reveals that Kepler's interest in astrology was unquestionably sincere.
So Kepler's interest in astrology was sincere and he did not practice it merely to pay his bills. He was however concerned about his reputation in an age of rationalism and positioned himself as a sceptical astrologer. However, despite claims to have rejected astrological techniques other than aspects, he continued to refer to planets in houses and signs in a way that implies significance beyond mere location.

The parts of astrology which Kepler claimed to reject (Signs, Rulerships etc) were later reinterpreted following the discoveries of the outer planets from 1781 onwards.[9] In many ways, Kepler was no different from many professional astrologers today who don't accept all the theories and disapprove of attention-grabbing and crowd-pleasing [10] astrology, which nowadays takes the form of Sun-sign columns in the media and question whether astrology is more about coinciding events rather than causal effects.

Robert Currey


Kollerstrom,Nick Kepler's Belief in Astrology ~
Greenbaum,D.G.; Linde, J.C. & Boner, P. (2010) Kepler's Astrology Culture and Cosmos Vol 14 1&2.


  1. The Fontana History of Astronomy and Cosmology ~ John North [London 1994, p309-326]
  2. Kepler introduced new aspects: the angular relationship between planets. The Quintile (72°), Biquintile (144°)and Sesquidrate (135°). Kepler's new aspects were based on harmonic theory and grounded in empirical observation of astrological effects and research using his collection of over 800 birth charts.
  3. As an example of the "cyclic journeys in the humours of the Earth", Kepler cites "the 19 year period of the Moon" which in sailor's lore affects tides. Kepler is referring to the 18.6-year nodal cycle, the precession of the lunar nodes. "The laws and periods of the cycles should be investigated by collating observations made over many years." It is now known that the Nodal cycle affects tidal levels, harvest and climate. ~ M. Gauquelin, The Cosmic Clocks (Owen, 1969), p. 103
  4. A Lutheran Astrologer: Johannes Kepler ~ J.V.Field. Archive for History of Exact Sciences Vol 31, #3 Sept 1984 p189-272
  5. Kepler's Explanation of a Rainbow ~ Carl Boyer American Journal of Physics, Volume 18, Issue 6, pp. 360-366 (1950). From p363
    "On October 2/11, 1606 Kepler wrote to Thomas Harriot (1560-1621), an Oxford mathematician and scientist, proposing problems in optics and mechanics. (Harriot is one of the earliest scientists to have visited America, for in 1584 he accompanied Sir Walter Raleigh on his expedition to survey and map the Virginia territory.) Kepler had heard of Harriot's work in chemistry and optics (as well as his criticism of astrology!) and hence he requests the latter's views on the Paralipomena. If Harriot will but tell him the cause of color in refraction and send him the measures of refraction in his experiments, Kepler believes that the explanation of the rainbow will be much expedited."
    Harriot's position on astrology is not entirely clear. The Encylopedia Britannica (retrieved Nov.2013) states: "In the turmoil following the discovery of the Gunpowder Plot in 1605, Harriot was arrested upon suspicion of having cast the king's horoscope, though he was soon released."
  6. Greenbaum,D.G.; Linde, J.C. & Boner, P. (2010) Kepler's Astrology Culture and Cosmos Vol 14 1&2.
    1. Prognostication for the Year 1618. p.181
    2. Kepler on the New Star: De Stella Nova. pp.209-215 At the same time as Kepler wrote to Harriot about rejecting signs and houses, he expanded on this point when he wrote about the supernova of 1604 in De Stella Novea in pede Serpentarii (1606). Kepler defends the use of aspects while accepting much of Pico dell Mirandola's critique of astrology. But while apparently rejecting all other parts of astrology, he writes about the contemporary Fiery Trigon - a period of around 200 years when Jupiter and Saturn conjunct in fire signs (Aries, Leo and Sagittarius). Kepler comments "It is truly remarkable that the most significant epochs occur in nearly these same periods of time." He drew up a table showing 9 periods of Fiery Trigons with events that occurred every 800 years: Charlemagne (800 AD), Birth of Christ (0), Isaiah/Babylonians (800 BC), Moses/Exodus (1600 BC), etc. He points out, like many modern astrologers,that these are coinciding occurrences and not effects. This suggests that Kepler rejects any causal relationship between signs and events but appears not to reject signs as significators of events.
    3. Kepler on Wallenstein. p.137. The translators comment "Here is another instance of Kepler using a standard astrological technique which he claims to disdain on philosophical and scientific grounds: using the houses in interpretation"
  7. Horoscope drawn up by Kepler was found by Dr Anthony Misch, astronomer from the Lick Observatory [managed by the University of California at Santa Cruz]. The subject of the birth chart was an Austrian nobleman, Hans Hannibal Hutter von Hutterhofen.
  8. Kepler's personal astrology: two letters to Michael Maestlin ~ Dorian Greenbaum, 2008 [Lecture for Cambridge Centre for Western Esotericism at Warburg Institute, London WC1] Also in Kepler's Astrology - trans. Greenbaum & Linde (2010) Culture and Cosmos Vol.14. Kepler's delineation of his Family's Astrology pp.55-64 and Kepler and Michael Maestlin on their Son's Nativities<, 1598 pp.65-78/i>
  9. It is of note that Kepler claimed to reject the very parts of astrology that were reformulated and much enhanced by the discovery of the outer planets. Uranus was discovered 150 years after Kepler died and is now accepted by most astrologers as the ruler of Aquarius. Pluto and Neptune have been assigned as planetary rulers for Scorpio and Pisces. In the past, these signs shared their ruling planets with Aries and Sagittarius respectively. Besides transforming the rulerships and hence the understanding of the signs, it has improved astrologer's understanding of the related Houses. However, the part of astrology that Kepler embraced: the aspects (angles) between planets remained unchanged after the discovery of the outer planets.
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Astrology is often judged on the basis of a 'secondary or tertiary' opinion such as a teacher or a text book. What students or readers don't know is that this apparent 'authority' lacks the expertise on the subject and often uncritically recycles a mix of prejudice or hearsay. This is most evident when 'scholars' try to rewrite history to impress students that they are supported by the opinions of great scholars such as Cicero, Newton, Liebniz or Einstein. So biographies are often framed so that they appear to favour the author's world view.

While it is important to establish historical truth and gratifying to find that astrology occupied so many geniuses in the arts, culture and science, their opinions have little or no bearing on the veracity of astrology. Science and astrology has advanced (with new planets, techniques and evidence) in the years after their lifetime.

Here we challenge the assertion that Kepler only practiced astrology to pay the bills.
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Kepler, Harmonics Mundi, chapter 7
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Horoscope drawn up by Johannes Kepler.
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