Johannes Kepler; Astronomer & Astrologer

Setting the Historical Record Straight Copyright © 2005 All Rights Reserved

by David Cochrane


Johannes Kepler is one of the greatest scientists of all time. Kepler's 3 laws of planetary motion are sometimes cited as a mark a transition to a scientific age where mathematics combines with careful observations to explain the rational basis of a great many things that previously were regarded as arbitrary or the will of the gods.

Kepler also practiced astrology. Different arguments have been presented as to why Kepler practiced astrology. Some claim that he did it in order to survive; he needed the money. Some claim that Kepler was an expert in the classical methods of astrology and was fully an astrologer. Others may argue that Kepler practiced astrology but a radically different kind of astrology than most astrologers and that Kepler did not endorse the majority of astrological concepts. Who is correct? Is the historical evidence clear?


Whether Kepler's practice of astrology was an expression of his genuine interest in astrology and a belief in astrological ideas is much more than a minor footnote in history. Skeptics of astrology and those who represent the scientific community sometimes trivialize Kepler's involvement in astrology. The effect of this is to distinguish the greatness of real astrologers from the rabble of the deluded astrologers. The scientist draws a clear fence around the rational sciences and the superstitions of astrology. Sometimes these presentations are overt and sometimes more subtle. This is important because if Kepler practice of astrology was sincere, then those who trivialize Kepler's involvement in astrology have distorted the historical record. This distortion of history creates a fictitious rigid historical boundary between rational science and astrology, and suggests that great scientists and great astrology have remained segregated for over 400 years, thus suggesting that only weak minds gravitate towards a serious study of astrology.

The irony of this situation is that in cases where Kepler's involvement in astrology is trivialized, it is the result of poor scholarship on the part of the person who makes this case, typically an academic who regards astrologers as lacking in scholarship and critical thinking. The historical record, as we see below, is clear on this point. Kepler fully believed in, and endorsed, astrology as a meaningful and important study. He did not pursue astrology solely for the purpose of making money.

Consider, for example, the otherwise excellent article on Brahe and Kepler at

One could hardly find a more authoritative source than information from and yet it implies that Kepler's practice of astrology was motivated primarily by economic need. A search for the words "astrology", "astrologer" and "horoscope" bring up these three statements in this article:

"However, Tycho died one year later, and even though Kepler was appointed astronomer to the court, he found so little official support for his position that he had to survive by making astrological predictions for noblemen who wanted their fortunes told."

"The contributions to science by these two astronomers from radically different backgrounds was set against a time of great turmoil in European history — the early 1600's. It was a time of upheaval, superstition, and fear — a time when court astrologers were powerful, and the stars were thought to predict and guide one's destiny."

"Kepler finished his days in poverty, writing horoscopes for noblemen in order to survive. Tycho Brahe and Johannes Kepler had totally disparate backgrounds and temperaments. In spite of this, Tycho's painstaking and detailed observational data of the planet Mars, combined with Kepler's mathematical genius, allowed Kepler to derive the three laws of planetary motion. Both Tycho and Kepler made significant contributions to the change in the prevailing world view of a geocentric universe. It was the beginning of a systematic study that transformed Medieval thinking — alchemy became chemistry and astrology led to astronomy."

The authors make it clear that Brahe and Kepler were important contributors to the transition from medieval superstitious beliefs like astrology to scientific thinking. The authors acknowledge that Kepler practiced astrology but "he had to survive by making astrological predictions for noblemen who wanted their fortunes told." The desire for having fortunes told appears to be a clear example of the "upheaval, superstition, and fear - a time when court astrologers were powerful."

The overall impression is that we can be grateful to Kepler and Brahe for helping rescue us from the deep and dark delusions of medieval thinking. Even Kepler had to partake in this superstitious nonsense to survive in this environment." However, this article gives an overly simplified and distorted view of the actual history. The story sounds plausible and makes for great reading. There is just one problem. It is not true. Furthermore, this distorted view of the actual historical record serves to reinforce the a priori belief systems of the authors rather than challenge their actual understanding by facing the actual facts. Ironically, the self-proclaimed ambassadors of scientific thinking are themselves lacking in careful analysis in this case.


Throughout Kepler's adult life he consistently argues for the validity of a kind of astrology based on harmonic theory. The academic historians are correct that Kepler was at times adamantly opposed to superstitious thinking and he regarded a large amount of astrological ideas as being incorrect. However, he consistently believed in the power of one of astrology's vitally important variables: geocentric angular distances between planets, known by astrologers as aspects.

Astrologers regard planets that are 60, 90, 120, or 180 degrees apart from each other or aligned in the same direction as having an impact on human behavior, weather, and other activity on Earth. No scientific reason has yet been found to account for why these angular distances are important but there is not a single known case (that we are aware of) where Kepler denied the power of aspects.

Kepler's opinion of other astrological variables such as zodiac signs and house placements appears to vary at different points in his life. Sometimes he uses signs and house placements and other times he argues that they are purely nonsense. We do not know if his opinion changed or he was being careful to protect his professional image or had some other motivation. We cannot read the minds even of people today, let alone someone who lived 400 years ago. However, doubting that Kepler believed in the power of astrological aspects is absurd. He practiced, wrote, and labored over the influence of aspects at length. He attempted to integrate harmonic theory with astrological aspects by creating the so-called minor aspects, he predicted weather based on aspects, he analyzed charts of family members, historical figures, and clients based on aspects and often other astrological variables as well.

Those who perpetuate the image of Kepler as a rational scientist who practiced astrology only to survive must give some account of how they can defend this preposterous proposal. In contrast to the many hundreds of cases where Kepler employs astrological aspects at least one case of him denying their importance needs to be presented. Perhaps most importantly, there was no monetary remuneration for some of Kepler's astrological work! His analysis of the horoscopes of biblical figures could seem to have little market! Astrologers were not just the darlings of nobility as the article at suggests. Kepler's colleague Thomas Harriot, an astronomer in England was briefly imprisoned and threatened with death and torture for allegedly practicing astrology and other "witchcraft". There were risks involved in practicing astrology as well. Horoscopes of biblical figures was especially frowned upon by the Church!

Kepler was not a scientist living in a superstitious age. He was simultaneously a scientist and a devout person striving to understand the mysteries of life. He found truth in some astrological doctrines and fiction in others. He was striving to get to the truth of astrology just as he had also found some of the truth of astronomy. He was sorting out exactly what worked and did not work in astrology. He made grand leaps to integrate harmonic theory and astrology. He investigated and moved forward with his astrological insights as well astronomical insights.

Kepler was an astrologer and an astronomer. In astronomy he succeeded in discovering the laws of motion. In astrology he was unable to reach as clear a conclusion and as clear an understanding, and his legacy has been distorted, trivialized, and confused by those who have followed after him. Kepler deserved better treatment than this and greater respect. Given below are some of Kepler's contributions and efforts in astrology.


Astrologer Rumen Kolev (see identifies a system of primary directions developed by Kepler. A technical issue in primary directions is how hot equate 1 degree of diurnal movement of the celestial sphere with a year of life for forecasting. Some astrologers equate about 4 minutes of time to a year of life but there are also rather elaborate systems for identifying the relationship. Kepler developed his own system. There is no plausible reason that comes to mind as to why Kepler became so immersed in the technical details of astrological forecasting other than to assume that he found astrology sufficiently compelling to be worthy of this detailed study.

Kepler also introduced new aspects like the quinitle aspect and he speculated on their possible relationship to weather. Again we see Kepler working out technical details of astrology as h tries to refine and improve his understanding of how astrological variables work.

(for which there is no monetary remuneration)

Kepler analyzed the astrological charts of many biblical figures. Analyzing horoscopes of biblical figures could run one into conflict with the Church and have dire consequences. Kepler took this risk and there appears to be no potential financial reward for this activity. One would be hard-pressed to identify an alternative explanation for Kepler's work in this area other than his sincere interest in, and belief in, astrological principles.

(for which is no monetary remuneration)

Astrologer Robert Currey pointed out to me that after the death of a child, Kepler interpreted his chart. Kepler was a deeply religious man and had a profound respect for the sanctity of life. He would not undertake this study lightly. There is no financial remuneration for this interest. Kepler was clearly an astrologer as well as an astronomer. Contrary to the statements made by skeptics, non-believers, and cynics of astrology, Kepler did not analyze only the charts of royalty, He also analyzed astrological influences to predict weather and some of his speculations on the relationship of quintile aspects to rainfall, etc. clearly were beyond the scope needed to produce almanacs.


Kepler wrote a small booklet entitled "Concerning the More Fundamental Aspects of Astrology" in which he summarizes his view of certain astrological variables which he is reasonably certain actually work! Here he plainly and directly states his conclusions regarding the astrological variables that he finds to work. How anyone can deny Kepler's sincere interest, and belief in, astrology given this booklet is difficult to fathom.


In 1606 Kepler wrote to Thomas Harriot, a British astronomer and mathematician, who also studied astrology (and historical evidence suggests believed in astrology and used astrology):

"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."

In this letter to Harriot we see Kepler's conclusion that most astrological theory is not valid but that he states that he uses only aspects. Kepler at times did employ other astrological variables in his analyses but he consistently and unambiguously endorses the use of aspects through his entire adult life. The letter to Harriot was presumably a private letter rather than for public consumption. Harriot and Kepler respected each other and were leading thinkers of their times. This is a frank communication among friends and colleagues.

The following excerpt from Tertius Interneniens is an extraordinarily passionate and intense statement by Kepler, which is consonant with his letter to Harriot:

"It should not be considered unbelievable that one can retrieve useful knowledge and sacred relics from astrological folly and godlessness. From this filthy mud one can glean even an occasional escargot, oysters or an eel for one's nutrition; in this enormous heap of worm-castings there are silk-worms to be found; and, finally, out of this foul-smelling dung-heap a diligent hen can scratch up an occasional grain-seed — indeed, even a pearl or a gold nugget."

Thus, Kepler reiterates this point that a great amount of astrology is inaccurate but that there are gems to be found in astrology. As he points out to Harriot, the gems are the aspects. Because Kepler is inconsistent regarding astrological variables other than aspects, we must conclude that either his opinion regarding other variables, such as house and zodiac sign placements, varied throughout his life or he adapted his comments to the context of the situation. However, he clearly is an astrologer who uses aspect relationships throughout his adult life as well as an astronomer.

David Cochrane AUTHOR: David Cochrane