Astronomy and Astrology: The Siamese Twins of the Evolution of Science. By Thony Christie

Thony Christie (Twitter handle @rmathematicus) is a British-born historian of early modern science and mathematics currently living in Franconia, Germany. He runs the blog, The Renaissance Mathematicus, and he was the editor of Whewell’s Gazette.

Supporters of science, especially those who believe that empirical science is the only purveyor of truth in the world, like to poke fun at astrology as the prime example of a load of old rubbish with pretensions to non-scientific truth. Whilst as a non-believer I have some sympathy, I often ask myself if those harsh critics are aware of the central role that astrology played in the historical evolution of science in general and astronomy in particular, from the early days in ancient Babylon down to the end of the seventeenth century. Over a period of something between three and four thousand years, astronomy and astrology were not rivals or competitors but two sides of the same coin, Siamese twins joined at the hip only separated in the final phases of the so-called scientific revolution.

Babylon and Greece

Like the chicken and the egg it’s impossible to say which came first, astrology or astronomy (though I recently read that evolutionary biologists have determined that the egg came first). I think that what first happened is that people looked up at the night sky – don’t forget that there was no light pollution –, and went wow! Continuing to observe over a longer period, they noticed that the stars revolved in regular circles and a handful of special ones, the planets, weaved a sort of regular path through the others. Awed by this majestic display of dancing lights, they began to wonder if they in some way influenced or controlled life on the earth down below. However, to determine that influence the observers had to track the patterns of those celestial objects and so astronomy was born.

The Babylonians developed their study of the stars and their influences reaching a point where they could accurately follow the paths of the planets and predict with accuracy eclipses of the moon, all done with algebraic algorithms. Eclipses of the sun are somewhat more difficult, and the Babylonians could predict when they might take place but couldn’t identify the ones that wouldn’t because of failing alignments. These extraordinary achievements in what we call astronomy were all driven by the Babylonian obsession with what we call astrology. In the second half of the first millennium BCE the ancient Greeks adopted the whole package, astrology and astronomy, from their Babylonian neighbours.

Shifting from algebraic to geometrical models the Greeks continued to develop astronomy, like the Babylonians, in the service of astrology. This development reached its zenith in the second century CE in the work of Claudius Ptolemaeos, antiquity’s greatest astronomer. Ptolemaeos, known better by the English version of his name, Ptolemy, wrote the definitive volumes on both Greek astrology, his Apotelesmatiká (Ἀποτελεσματικά) or Tetrabiblos, and Greek astronomy, his Mathēmatikē Syntaxis (Μαθηματικὴ Σύνταξις) or Almagest.  In the former, Ptolemy wrote that the science of the stars has two aspects, one that determines the position of the stars and the other that determines their influence. We would call the first astronomy and the second astrology, but from the time when the Greeks took over the disciplines from the Babylonians down to the seventeenth century, the two terms were used indiscriminately for either of its two branches or both together.

The Middle Ages

Following Ptolemy, learning in general went into decline in Europe and with it astronomy. When during the Early Middle Ages astronomy was kept afloat, even if only on a very low level, it was because of a continuing interest in astrology. However, both disciplines were kept more than just alive outside of Europe in the Islamic Empire.

The story of how the young Islamic Empire began to search out and translate Greek knowledge into Arabic in the eighth century CE is legendary. What however is less well known is that that process was led by astrology. The Abbasid Caliphate was centred on what had been the Persian Empire, and courts had employed an astrologer as a political advisor. As part of their campaign to bring the Persians onto their side, the Abbasids adopted this practice, and the first Greek texts that they rendered into Arabic were astrological ones. This was followed by the translation of astronomical, mathematical and philosophical treatises to underpin the astrological ones.

This process was mirrored in the High Middle Ages when Europeans began to translate Arabic scientific texts into Latin. Amongst the very first texts to be translated were the Arabic zījs, tables of planetary movements and positions used by astrologers to cast horoscopes. And at this point in time there existed the possibility of a break in the duality of astrology and astronomy.

The central intellectual authority in Europe was the Catholic Church. The Greek astrology reintroduced into Europe by the Arabs was deterministic, that is it presupposed that the course of an individual’s life was already fixed at birth. This clashed with the Church’s belief in the concept of free will: If an individual’s life was predetermined at birth there could be no moral responsibility. However, instead of simply throwing out astrology, the Church redefined it. Astrology was no longer deterministic but only indicative; the individual or God was capable of changing the prognosticated future. And so astronomer and astrology continued to march through history arm in arm.

Astrological Medicine, Mathematics and the Renaissance University

Astrology reached a high point in its existence during the humanist Renaissance through the rise of astrological medicine (or iatromathematics as it was called), to become the dominant form of school medicine. According to this theory, the cause of an ailment, its course and its cure could all be determined by casting and interpreting a horoscope. With this, astrology entered the Renaissance university. In the early fifteenth century, chairs for mathematics were established in the Northern Italian humanist Renaissance universities and at the university of Krakow. These chairs existed to teach astrology and the casting of horoscopes to students of medicine. To do astrology one needed to do astronomy, and to do astronomy one needed mathematics, therefore chairs for mathematics. In about 1470 Germany got its first chair for mathematics at the University of Ingolstadt, established for the same purpose. And so the teaching of astronomy and astrology spread through the European universities.

In the Reformation, the Lutheran Protestant Church needed to set up its own education system, as the existing schools and universities were controlled by the Catholic Church. Philipp Melanchthon was entrusted with this task. Melanchthon had studied for at time at the University of Tübingen under Johannes Stöffler, the professor of mathematics, who was a graduate of Ingolstadt. Stöffler was one of the leading astrologers of the age and Melanchthon became a glowing advocate of astrology. Starting in 1525, Melanchthon established chairs of mathematics in all the Protestant schools and universities in order to ensure a steady supply of future astrologers.

Starting in the thirteenth century, European ruling courts followed the Arabic and Persian custom to employ court astrologers as advisors. This began in Sicily, which had close contacts with Arabic culture, and spread from there throughout Europe. By the fifteenth century almost every court in Europe, including the Vatican, either employed a full-time astrologer or maintained a university mathematician on a retainer as court astrologer. These astrologers were almost all qualified mathematicians, who along with their astrological activities of producing yearly prognostica and medical calendars (of which more in a minute) as well as often giving medical advice, were expected to fulfil other mathematical activities. These included designing and producing sundials and other astronomical instruments, or working as architects or hydraulic engineers designing water features. The professional astrologer/astronomer had become a fixture of Renaissance Europe.

Astrological medicine also set out that there were good days and bad days for the application of the most used medical treatments—bloodletting, cupping, purging—, which were determined by the phases of the moon, eclipses and other astronomical phenomena.  In order that doctors and barber-surgeons, who carried out these treatments, knew these good and bad days, towns employed calendar makers, whose job it was to publish a yearly calendar containing all the astronomical and astrological data required for medical treatment. Many prominent mathematicians and astronomers worked as civic calendar makers, an occupation that provided a nice subsidiary source of income.

With the invention of movable type-printing, medical calendar production became big business. Johannes Gutenberg printed a medical wall calendar to earn money before he printed his better-known Bible. Medical calendars existed both as wall calendars and pocket calendars; the latter developed into the pocket diary when inventive Renaissance printers inserted empty pages to enable the owners to make notes.

From Peuerbach to Galileo: Astronomers were Practising Astrologers

The list of prominent Renaissance and Early Modern astronomers who were practicing astrologers is very long, and the list of those who weren’t is very short. Peuerbach, Regiomontanus, Peter Apian, Johannes Schöner, Gerolamo Cardano, Copernicus, Tycho Brahe, Johannes Kepler, Galileo and many others were all practising astrologers, who contrary to a popular claim didn’t just do it for the money but genuinely believed in the influence of the stars. Most notably, the Renaissance reform of astronomy, which would eventually lead to the so-called astronomical revolution and the adoption of heliocentricity, was driven by astrology.

Regiomontanus, who was at the time court librarian and astrologer in Budapest, was asked why astrological prognostications were so often inaccurate. He answered that because the astronomy on which it was based was so inaccurate. He moved from Budapest to Nürnberg in 1470 to start a major programme to reform astronomy in order to improve astrology. In Nürnberg he set up the world’s first scientific publishing house and his most successful publication was his ephemerides, planetary tables for the determination of the position of the planets. These were the very first printed ephemerides and noted for their level of accuracy. Although they could also be used for cartography and navigation, Regiomontanus explicitly mentioned their intended use in astrology in his introduction. Regiomontanus died too soon to come very far with his programme, but we find the same motivation in the work of other astronomers such as Apian, Brahe and even Kepler. In the Early Modern period, astrology was often the mistress and astronomy the handmaiden. The terms mathematicus, astronomus and astrologus were synonyms and designated one profession – that of the mathematical, astronomical astrologer.

The Siamese Twins are Separated

Throughout the Renaissance and early modern period, astrology was regarded as a science – as a system of knowledge on an equal footing with other academic systems of knowledge. In the late seventeenth century, the generation of astronomers beginning with Isaac Newton no longer believed in or practised astrology, although Newton, an astronomical autodidact, learnt his astronomy from the books of Vincent Wing and Thomas Streete, both of whom were practising astrologers.

So what led to the separation of our scientific Siamese twins? You can find some sources that claim it was the introduction of heliocentric astronomy that led to astrology’s expulsion from the halls of academia, but this is not true. Robert Westman famously pointed out that there were only ten true Copernicans between 1543 (the year of the publication of De Revolutionibus by Copernicus) and 1600. It should, however, be noted that all ten were practising astrologers. In fact, horoscopes are calculated according to the position of the planets along the ecliptic, the apparent yearly path of the sun through the heavens. And these remain the same whether your astronomical system is geocentric or heliocentric.

What led to the separation of astronomy and astrology was the change in the philosophical basis of scientific knowledge in the seventeenth century. Astrology took its legitimation from the Greek philosophical theory, also accepted by Aristotle, ‘as above so below’. This says that which occurs in the heavens is mirrored here on earth. In the Renaissance this was also know as the macro-cosmos/micro-cosmos theory, particularly in astrological medicine, where the movements of the heavens are mirrored in the human body. The fall of Aristotelian scholastic philosophy and its replacement with what we now call modern philosophy led to abandonment of the macro-cosmos/micro-cosmos theory, and with it astrology.

There is however a small coda to the story of astrology in western science. Although they believed in astrology, the Renaissance astronomers and mathematicians were well aware of the very shaky empirical foundations on which astrology stood and tried to improve those foundations. Astrology is not just about natal horoscopes but has several different branches, one of which was astro-meteorology. It seemed logical that since weather came from the heavens it must also be controlled by the heavens. During the Renaissance, several astronomer-astrologers began keeping accurate detailed weather diaries in which they recorded both the weather and the daily horoscope to try and determine the controlling pattern. They of course failed to find that pattern and thus added another nail to astrology’s coffin, but with their daily records of the weather they laid the foundations of modern scientific meteorology.

Another attempt to provide an empirical base for astrology was to create collections of biographies of prominent people and compare to real life biographies with the predicted astrological ones. Again, like the efforts in meteorology, these efforts proved more detrimental than supportive to astrology. However, these biographical efforts were one of the steppingstones in the emergence of modern history, which took place during the sixteenth and seventeenth centuries. Also, these biographical collections are an important contemporary source for modern historians researching the period. The most well known example of such a collection is John Aubrey’s Brief Lives.

Although it ceased to be regarded as a science or as an academic discipline in the late seventeenth century, astrology refuses to die and large numbers of often well educated and intelligent people continue to believe that the course of their lives are influenced or determined by the course of the planets along the ecliptic. I personally know at least three respected, academic historians of astrology, who are also practising astrologers. I also know a fourth, a successful university historian of science, who, whilst not really admitting to believing in astrology, was pleased to have one of the three cast and interpret her horoscope.

© Thony Christie

Recommended Readings
(Disclaimer: Purchases made on this site might generate small commissions, at no extra cost for you.)

Azzolini, Monica. The Duke and the Stars: Astrology and Politics in Renaissance Milan. Cambridge, MA: Harvard University Press, 2013 (review by Thony Christie) [Buy on Amazon] [Search on Abebooks].

Barton, Tamsyn. Ancient Astrology. London: Routledge, 1994 [Buy on Amazon] [Search on Abebooks].

Campion, Nicholas. A History of Western Astrology. Volume I. The Ancient World. London: Bloomsbury, 2008 [Buy on Amazon] [Search on Abebooks].

Campion, Nicholas. A History of Western Astrology. Volume II. The Medieval and Modern Worlds. London: Bloomsbury, 2008 [Buy on Amazon] [Search on Abebooks].

Heilbron, John L. Galileo. Oxford: Oxford University Press, 2010 [Buy on Amazon] [Search on Abebooks].

Kusukawa, Sachiko, and Liba Taub. Eds. The Starry Messenger. Whipple Museum and Department of History and Philosophy of Science, University of Cambridge (online presence), 1999-2000.

North, John. Cosmos: An Illustrated History of Astronomy and Cosmology. Chicago, IL: University of Chicago Press, 2008 [Buy on Amazon] [Search on Abebooks].

Tester, Jim. A History of Western Astrology. Woodbridge: Boydell Press, 1987 [Buy on Amazon] [Search on Abebooks].

Thorndike, Lynn. A History of Magic and Experimental Science. 8 vols. New York: Macmillan (vols. 1-2) and Columbia University Press (vols. 3-8), 1923-1958 [Buy on Amazon] [Search on Abebooks].

Westman, Robert S. The Copernican Question. Prognostication, Skepticism, and Celestial Order. Berkeley, CA: University of California Press, 2011 [Buy on Amazon] [Search on Abebooks].

Westman, Robert S. Copernicus and the Astrologers. Washington, DC: Smithsonian Libraries, 2016 (open access PDF).