This page is intended as a relatively brief introduction to the very large subject of astrology.

horoscope-2Today few people think of astrology as a science. For many of us, astrology is just that column in the back of the newspaper with rather cryptic and occasionally amusing advice. Here’s my horoscope from August 20, 2014, from the “Real Astrology” column by Rob Brezsny in the Oklahoma City Gazette (I’m a Libra):

 Desire can conquer fear.  Love trumps cowardice.  The power that your tenderness affords you may not completely dissolve your doubt and worry, but it will quiet them down so much that they will lose their ability to paralyze you.  These truths are always good to keep in mind, of course, but they are especially useful to you right now. No obstacle will faze you, no shadow will intimidate you, as long as you feed your holy longing and unshakable compassion.

This is a birth horoscope, the kind most of us are familiar with. I’m a Libra because the sun was in the sign of Libra when I was born (Recall that the sun moves through the zodiac over the course of the year. Look back at the page on Greek Cosmology if you don’t understand what that means.) It’s not particularly individualized, since billions of people all over the world share this star sign. There were (and are) considerably more complex forms of birth horoscopes that take account of the positions of ALL the celestial bodies at the time of a person’s birth.

Some people still lend great credence to astrology, and there are still trained astrologers. For example, President Ronald Reagan and his wife Nancy Reagan regularly consulted an astrologer, a fact that caused a certain amount of shock and consternation when it was revealed.  But the Reagans aside, astrology is not a subject taught in high schools or universities, and I’m not aware of any physicists or astronomers who consider making astrological predictions to be a part of their scientific work.

This was not true in the past. Almost all of the astronomers we will learn about in this class were also astrologers. Indeed, many medieval and early modern historians have started using the term “science of the stars” to denote the combined fields of astronomy and astrology and to acknowledge that the two fields were very closely linked in the past. (Several of these historians contribute to a blog called Science of the Stars. Another prominent historian of science who uses this term is Robert Westman, who has written an important new book on the Copernican Revolution.) In its broadest sense, astrology could mean any practical application of knowledge of the heavens. Practical applications included using observations of celestial bodies to tell time, to make calendars and to navigate at sea. A major practical application of astronomical knowledge was making predictions about the course of human affairs by observing and by calculating the positions of the celestial bodies.

The idea that the sun, moon, stars, and planets exert an influence on the live of people on earth emerged very early in the development of Greek philosophy. Note that astrology is part of Greek natural philosophy. It is not part of Greek religion. Although the planets were named after gods and goddesses, they were not themselves gods and goddesses. They were natural objects, not supernatural beings, and their influence on human life was purely natural. The ancient Greeks (and medieval and early modern people) saw compelling empirical evidence for celestial influences. The changes in the seasons were clearly related to the movement of the sun around the earth. The tides were related to the motions of the moon. (Of course, the Greeks were correct in both these observations, although we today would explain these phenomena very differently).

Moon Over Twin Rocks by Micheal Mattingley

Moon Over Twin Rocks by Micheal Mattingley

Further, the Greeks believed the cosmos was an orderly whole, with each part interconnected. Thus the heavens were connected to the earth, although they were separate realms. Many Christians in medieval and early modern Europe believed that God had created the entire cosmos for the use and benefit of human beings. They found it eminently plausible that the Divine Creator would have arranged the heavens in such a way that they influenced human life. (Not all Christians believed this, as we shall see.)

Greek astronomers/astrologers began casting birth horoscopes in around the second century B.C. These birth horoscopes were far more complex than our notion of “star signs.” As I noted above, when I say I’m a Libra, it means the sun was on the ecliptic in the sign of Libra when I was born. But birth horoscopes from the Greeks through the eighteenth century (and even today for people who practice astrology) charted the positions of the moon and five visible planets, as well as the sun, at the time of a person’s birth. Ideally, they should be able to determine the exact position of each celestial body at the moment of a person’s birth. Some proponents of astrology (like Hildegard of Bingen) thought it would be more accurate to know the positions of all the celestial bodies at the time of a person’s conception, but obviously this could be a bit trickier to determine! The configuration of the heavens at the time of a person’s birth was supposed to determine their character and the kind of life they would have – would they be a great leader? a criminal? a scholar? a warrior? Each planet, the sun and the moon had a specific kind of influence on a human life, and this influence was strengthened or weakened depending on where in the zodiac it was.

The heavenly bodies could also have effects on very large groups of people. Rare and striking configurations of the planets, sun and moon could cause great disasters like war, famine, and epidemic disease. For example, many astrologers explained the outbreak of plague in 1348 (the first outbreak in Europe) as the result of the conjunction of Saturn, Mars, and Jupiter on March 20, 1345. A “conjunction” is when two or more planets appear together in the same zodiacal sign. Two is unusual; three is really quite extraordinary.

Celestial Conjunction at Paranal" by ESO/Y. Wikimedia Commons

Conjunction of moon, Venus and Jupiter at Paranal Observatory. Wikimedia Commons

The position of the sun, moon and planets could also determine what days were good or bad to travel, conduct business, get married, etc. For example, one piece of astrological advice reads: “If Saturn is in the neighborhood of [the star] Argo, it indicates shipwreck.” This example comes from the most influential ancient textbook of astrology, Ptolemy’s Tetrabiblos. We will be discussing Ptolemy’s Almagest, his great work of mathematical astronomy soon. In the Almagest Ptolemy offered mathematical models that allowed him (and his readers) accurately to predict the motions of the sun, moon and planets. One of the major uses of these models was for making astrological predictions. It is all very well to assert that heavenly bodies exert influence over life on earth, but unless you can predict the positions of sun, moon and planets in relation to stars, this information can’t be turned to practical uses. You have to be able to calculate the future positions of celestial bodies. To return to the example of Saturn and Argos, you need to be able to predict when Saturn will be near the star Argos if you are planning to travel by sea. Further, astrologers also had to be able to calculate where the celestial bodies had been in the past. Many (probably most) birth horoscopes were actually cast when the person was older. Occasionally people cast horoscopes for a newborn, but even in these cases, astrologers would need mathematical models to calculate where some of the celestial bodies were, since not all are visible on any given night. In the Tetrabiblos, Ptolemy asserted that the heavens influenced human life, but he acknowledged that astrological prognostications were not always certain. Different astrologers might interpret a horoscope differently. For Ptolemy, mathematical astronomy was an exact science; astrology was not. This doesn’t mean it had no validity. Astrology was in many ways analogous to medicine. Even today, two physicians presented with the same patient with the same set of symptoms might arrive at different diagnoses or different recommendations for treatment. And even experienced physicians sometimes miss signs that later turn out to be crucial. This is not to denigrate medicine or doctors! It is just to point out that there are so many variables when dealing with human lives that we do not consider medicine to be an exact science. But that doesn’t mean it isn’t valid, or that we don’t grant it a high degree of respect. This may also help to answer the question that many students have about astrology: Didn’t people see that it didn’t work? Part of the answer is that there are many examples of astrologers being right. And the other part is that failure was often attributed to lack of skill on the part of an individual practitioner rather than taken as an indictment of the subject as a whole. Many of us have heard stories of medical mishaps, but few of us completely reject the tenets of modern western biomedicine.


Depiction of Ptolemy employing a quadrant, from Giordano Ziletti’s Principles of astrology and geography according to Ptolemy, 1564. Wikimedia Commons.

Greek astrology was readily adopted and built upon by scholars in both the Islamic world and in Christian Europe. Some early Christians criticized astrology on grounds that it negated the concept of free will. If the stars determine our destinies, what role does human choice have? Do we choose to do good or evil, or do we just do what the stars have predetermined? However, in the end, many Christians ended up accepting astrology, with the proviso that the heavens influenced but did not fully determine the course of human lives.

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