``A missionary of the Medieval Ages tells that he had found the point where the sky and the Earth touched...'' An illustration from Camille Flammarion's book, L'atmosphère: météorologie populaire (The atmosphere: Popular Meteorology).

It happens very often to those who claim to study astronomy: newly acquinted people came up to you and ask for their fortune told, revealing that their zodiacal sign is so-and-so. Most are just jesting to annoy the astronomer, but sometimes there are serious ones, thinking that astronomy is a field of study to understand the ways to predict the future based on the position of celestial objects.

Jokes and seriousness aside, interchanging (modern) astronomy with astrology is an easy and sure way to destroy my mood. The credo of astrology, that the position and movement of certain celestial objects have influences on human live, is now no longer corroborated by newer observational and experimental facts that natural laws are universal and do not have special preferences to humanity.

The Waluku constellation (The Waluku means the plough, known in the west as the Orion constellation) has an important role in the Javanese Pranoto Mongso (the traditional Javanese craft of timekeeping). The rising of Orion at dawn is an indication for farmers to start ploughing the field and planting. Photo by Emmanuel Sungging Mumpuni.

Having said that, I do not deny the fact that up to three and a half century ago, astronomy and astrology are more or less the same thing. The ability to know the position and movement of celestial objects are skills that everybody need to know. This relates mostly to the usage of celestial objects—The Sun and The Moon for example—as well as using them to determines our position on Earth. Farmers need astronomy to know when is the time to plough the land or to harvest, sailors need astronomy to determine their orientations, architects to determine the cardinal directions, and hunters to know their orientations.

Compare it with modern life. How many among us who can still recognize the constellations and determine directions and time based on the positions of celestial objects? Do we know where is the pole star and/or the Southern Cross? Is that a star or a planet? In modern times, time has been determined with the aid of mechanical clocks and standardized time zones, orientations are calculated by GPS or a compass, and days are determined by calendars. To a modern human, astronomy is something detached from daily life, but for people from one millenium ago, astronomy is a part of daily life.

Because of the closeness of celestial objects to daily lives of olden times, it is natural to think that celestial objects influence our life. Under this paradigm was Aristotelian cosmology developed. Aristotle said that the immobile Earth is the center of the universe and the sky (defined to be whatever that is above the Moon, including the Sun, the planets, and the so-called fixed stars) has its own laws different from natural laws on Earth (also called the sublunar realm). The sublunar realm composed of four elementary elements (water, fire, earth, and air) constantly transmutating into each other while the celestial realm composed of the eternal aether.

An astronomer from pre-telescopic times is measuring the position of a star with a cross-staff. We can see around him other measuring aid, notably the two orreries on his sides and an astrolabe lying on the floor. An illustration by George Billerger.

The paradigm of Aristotelian cosmology dominates the worldview of educated European for two milleniums. According to Aristoteles, all that is happening in the sublunar realms are caused and determined by things occuring in the sky. Under this paradigm was astrology found its place in the consciousness of people of old times.

Interpreting the position and directions of movement of celestial objects to foretell the future becomes an important business, especially for those in power. Employing a royal astrologer is a necessity. Astronomy as the craft to accurately measure the position of stars and planets can prosperously grew was also because of this need. The position of stars and planets must be measured accurately, the more accurate the better so that the interpretation is better. Their position in the future must be accurately predicted, so much the better to make better foretelling. The Ptolemaic geocentric model can predict accurately the positions of celestial bodies at any time, able to produces tables of planetary positions at any given time. The Almagest is an important asset for those who can get their hands on it and understand it, since this allows a royal astrologer to interpret the positions and movement of celestial objects and predict the future.

Because of this important position astronomty hold, in a medieval university curriculum, astronomy is a subject students has to master. In the 12th and 13th centuries, in the University of Paris for example (as well as other universities in Europe), a student first has to pass the Trivium—the three subjects of grammar, logics, and rhetorics—before continuing with the Quadrivium, the four subjects of Arithmetics, Geometry, Music, and Astronomy. Of course in medieval times higher education is a privilege enjoyed by the upper levels of society.

Why astronomy is considered important it has to be a required study in universities, is also because of the paradigm that celestial objects have influences on humanity. Everybody must know how to determine and interpret the positions of celestial objects. Even medical knowledge of those times relies on astronomy to know the causes and cure of diseases. Can we imagine our general practitioner consults a sky map and tables of planetary positions while diagnosing our illness? For a modern human this might seem absurd but those where the answers of the professors of the faculty medicine at the University of Paris when they were asked by the King of France about the cause of the Great Plague of 1348–9

[The pestilence is due to] an important conjunction of the three higher planets in the sign of Aquarius, which, with other conjunctions and eclipses, is the cause of the pernicious corruption of the surrounding air, as well as a sign of mortality, famine and other catastrophes.

Further they also explains that

The conjunction of Saturn and Jupiter brings about the death of peoples and the depopulation of kingdoms… The conjunction of Mars and Jupiter causes great pestilence in the air.

A medieval physician is examining the urine of his patient. Your physician might not directly examines you, but will diagnose your illness based on your urine and the positions of the planets. Source: Bibliothèque nationale de France, (BNF, FR 135, fol. 223)

These alignments of planets are tought to lead to the increased concentration of noxious vapours and fetid air (miasma), which will be blown by the wind enters the human body though breathing and the skin’s pores. This in turn will disturb the balance of the “humors.” The health of individuals and society are thought to be influenced by planetary conjunctions and lunar phase. The concepts of bacteria, infections, and disease contractions are alien concepts in those times and would not be developed until five centuries later. The common practice of bloodletting can be beneficiary on certain lunar phases but not on other times. When will a patient recover is determined by knowing the positions of certain planets at the time the patient fell ill. Older medical texts tell that the movement of planets control the workings of different organs: Mercury determines the workings of the brain, Jupiter the liver, and so on.

The human anatomy and the zodiac signs that controls the human organs. In medieval times, physicians thought that the workings of a human body is regulated by the position of the zodiacal constellations and the planets. This is an illustration from one of the most important illuminated manuscript of the 15th century, Très Riches Heures du Duc de Berry.

Medical texbooks of medieval ages are full with such instructions, diagnoses based on astrology, numerology, misconceptions about the workings of the human body, direct experiences, and hearsay. A cynical person might say that medieval people are ignorant about medicine, but the description above about the practice of medicine based on astrology in fact reveals the contrary: Medieval physicians are knowledgeable about medicine, only that their knowledge is very different from our own, the modern people. Their medical knowledge operates on different paradigms than ours. Medieval physicians and surgeons not only command a very high salary but also have broad and many knowledge and experiences, not so many difference with modern physicians and surgeons. The difference is, unfortunately, that the knowledge and experiences of a medieval physician will not help us much in our recovery. Some of these knowledges are downright dangerous if not lethal.

The description above is one picture about the important role of astrology in medieval society. In these times astronomy and astrology are more or less interchangeable. A medieval physician must be knowledgeable also in astronomy if he still wants to be employed, and so do people with higher education. If you live in medieval times and you get acquinted with somebody who knows astronomy, I guess it would be acceptable to seek his advice on the future (especially if you have the legitimacy for that, e.g. you are the King of England).

Nicolaus Copernicus, a Polish priest, thinks that the Sun should be the center of the universe and not the Earth. This photo is the face of 70 years old Copernicus, based on forensic reconstuction of the skull of Copernicus, found by archeologists.

The shift from Aristotelian paradigm started to take place when Nicolaus Copernicus published his book, De revolutionibus orbium coelestium (On the revolutions of celestial spheres). In this book Copernicus offers a new paradigm in which the Sun is the centre of the universe instead of the Earth, and the methods to calculate the positions of celestial objects with this paradigm. Copernicus died right when the book is out of the printing press, but the book is read by the educated of those times even though Copernicus’ views are considered only as a matter of discourse. The next generation, Galileo Galilei, did series of experiments which results contradict Aristotelian physics. However Galileo’s efforts to promote Copernican paradigm ends with his trials by the Inquisition Office in the Vatican and his house arrest until the end of his life. The failure of Galileo happened not only because of the unwillingness of the Vatican to change their paradigm but also because Galileo only provides analogies as his argumentations and not quantitative arguments that can support Copernican paradigm.

Meanwhile, in more or less the same time, in Northern and Central Europe live Johannes Kepler and Tycho Brahe. As mathematician, Kepler believes that can be mathematically described and that the orbit of celestial objects can be described by a circular orbit with a radius determined by the shape of a regular polygon that circumscribe it. Kepler’s model fail to predict the position of the planets, but in front of astronomical data, the old Kepler is different from the his younger self. The old Kepler throws away his younger self’s conception, and—based on Tycho Brahe’s accurate astronomical data—formulates the three laws of planetary motion known later as the Kepler’s Laws.

Johannes Kepler worked as court astrologer and published many planetary tables predicting the positions of the planets. However the predictions based on the Ptolemaic model is getting different from the reality that he observes. This painting is made in 1610, during Kepler's lifetimes, by an unknown painter.

But before all that, Kepler also worked as a court astrologer for General Wallenstein, a Bohemian warlord. The man that will employ him later, Tycho Brahe, a Danish nobleman, considered as the best astronomical observer in pre-telescopic times, also practice astrology to his acquaintances. However both men, Kepler and Tycho, are made restless by contemporary astrological techniques. Both men still hold to the Aristotelian paradigm that celestial objects had influences on humanity, but they were not sure on the exact details of how this works. The geocentric Ptolemaic models were also getting inaccurate as they acculumate data during their lifetimes. The geocentric model started to show it defects: it is no longer accurate to predict the positions of celestial objects. Using Tycho’s observational data, Kepler managed to ditch the Ptolemaic model, put the Sun in the center of things, use elliptical orbit instead of circular, and derive the three laws of planetary motions that stars to change people’s perspectives on the workings of the universe.

By formulating his Laws of Motion, Isaac Newton found mechanics and unites the natural laws on Earth and on the Heavens. The illustration symbolically depict Newton's thought in describing the world, by the English artist William Blake.

The effort to unite the natural laws on Earth and on the Heavens was done succesfully by Isaac Newton. After formulating his famous Laws of Motion in Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), Newton set out to apply his three laws of motion to celestial motions. Assuming the existence of a universal force of gravitation, he managed to derive the laws of planetary motion with exactly the same form as Kepler’s. Not only that, gravitation also explains why objects fall to the Earth and how fast the object is falling. Newton’s Law has a very strong predictive power and can explain, in the same frame paradigm, the phenomenon of the Earth in orbit around the Sun as well as the phenomenon of falling objects. By performing experiments on Earth and confronting Newtonian Law with data from astronomical observations, people proved that Newton’s Law is valid everywhere, on the Heavens and on Earth. Using the Newtonian Law, for example, astronomer Edmond Halley can calculate the orbit of a comet that he predicts to reach the closest distance to the Sun every 76 years, a prediction proven to be correct and the comet was later named Halley’s Comet.

With the formulation of the Newtonian Laws, the paradigm shift from Aristotelian Physics to Newtonian Mechanics was largely complete. Kepler’s observation and the theory of Newtonian Mechanics show that the natural laws on Earth are the same as the natural laws on the Heavens, i.e. natural laws are universal. Astronomi as one of a branch of science has found its new stepping stone, the theories of Physics. With this new stepping stone it moves away from astrology. The measurement and observations of celestial objects is no longer conducted to foretell the fate of people, but to understand how the universe works.


  • Anthony Aveni, People and the Sky: Our Ancestors and the Cosmos (2008)
  • Arthur Koestler, The Sleepwalkers: A History of Man’s Changing Vision of the Universe (1959)
  • Ian Mortimer, The Time Traveller’s Guide to Medieval England: A Handbook for Visitors to the Fourteenth Century (2009)

Versi Bahasa Indonesia di sini.

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