An Angry History of Science Part 3: Astrology isn’t all horoscopes

In this series, I’m talking about big issues in how we think about our scientific past. This week I get into more detail about how  astronomy broke away from astrology. Read the first post in this series, on ancient Greece and math. Read the second post, on the Scientific Revolution.

In the previous post in this series, I talked about why astronomy is a difficult example of the significance of the Scientific Revolution, since it’s hard to separate the substantive advancements in the field from the big breaks in the intellectual framework of science at that time. Perhaps the biggest intellectual break in astronomy between the pre-modern and modern periods is when astronomy distinguished itself from astrology. If we look back today and tell a history of astronomy, it’s easy to ignore astronomy’s loopy cousin, astrology – we just look at anything that seems like real science and ignore everything related to the zodiac. But for thousands of years, astronomy and astrology were part of the same ideology and there was no distinguishing them – they were both parts of a system that explained what happens on Earth in terms of what happens in the sky. Since the invention of the telescope and the adoption of heliocentrism, astronomy has become much more a science for itself – it is the study of everything off of Earth. The split between astronomy and astrology points to exactly where science became Science, and stopped just being knowledge.

We can’t talk about astronomy today without talking about two other things: Space and physics. Space being Outer Space, and physics being the theories of how matter acts that help explain what objects in space are, how they form and die, and how they move. But before the sixteenth century, neither of these was included in discussions of astronomy. Instead, astronomy was the study of the Heavens and the heavenly bodies. This was a lot less religious than it sounds. The “heavens” really just means the part of the sky that is obviously far away – it doesn’t include clouds or birds, and for the most part it ignores the weather. The heavenly bodies is a specific set of objects in the sky. Before heliocentrism, scholars understood the universe to operate under a terracentric model, in which all of the other moving objects in the sky orbited Earth, and the stars were fixed to a globe surrounding the Earth that itself rotated. So, the heavenly bodies included the Sun, Moon, Mercury, Venus, Mars, and Jupiter. Saturn, Neptune, and Uranus (and Pluto) were not included because they aren’t visible to the naked eye.

Now, I want to take a second here to appreciate the incredible powers of observation of pre-modern peoples. Take a look in the sky tonight and point out Jupiter. It should be easy, right? Out of the four smallest regularly moving objects in the night sky, it’s the smallest. Or, if it helps, the planets don’t twinkle like stars.

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It turns out that astronomical observation is harder to do than to talk about. And yet knowledge of these tiny bright dots in the sky has been commonplace for the extent of recorded human history. Pre-modern peoples in Europe, Asia, and North Africa (remember? the hot zone? that’s never going to catch on) thought about the heavens and the heavenly bodies not primarily as objects moving at a distance from Earth, but rather as objects whose movements signaled major activities on Earth. The explanations for this in the Abrahamic religions (Judaism, Christianity, and Islam) as well as the major ancient religions of the Mediterranean and Middle East (Greek, Roman, Egyptian, and Persian polytheism, plus Zoroastrianism) ranged from the idea that the heavenly bodies were literally God’s (or gods’) way of communicating with humanity to the heavenly bodies operated within a contained system that was linked to Earth’s system and mirrored it. So, all observations of the heavens feed these theories, which means there was no divide between observations of the physical movements of the planets and observations about the personalities of individuals born when certain stars are visible in the sky.

The kinds of studies that would now fit under the heading of astrology have had a mixed reception since at least the start of Christianity. Practitioners of the occult sciences – that is, studies based in mysticism like astrology, alchemy, magic, divination, and dream interpretation – attracted plenty of suspicion and were often accused of being charlatans and hustlers, just as they are now. But, at the same time, these practices offered a lot of appeal, just as they do now. Frankly, it’s hard to draw the line between what the stars can and can’t indicate. Royalty often employed astrologers to make predictions about important events or to mark special occasions like the birth of a child. But at the same time, scholars wrote lengthy treatises angrily explaining why mystics like those shouldn’t be trusted. It’s easy to see why – one charlatan predicting romance based on the movements of Venus could ruin the reputation of every scholar mapping the stars.

The studies of astronomy and astrology were inherently linked in the premodern world. On a practical level, understanding the map of the heavens in astrological terms is a handy visualization. It’s difficult to point up at the sky at a particular star (in a sky both brighter and more full of stars than ours is now) and say “I’m talking about that one”. Instead, if you say “the one that looks like the end of the handle of a big ladle”, you have some kind of frame of reference. And if you just visually track the movement of the stars based on those arbitrary shapes, you’ll notice some incredible things about the sky, like the fact that the big dipper spins over the course of a few months. Breaking up the year into months based on what stars are most easily visible in the night sky (depending on your geographical location, of course) is also a much more reliable way to record the long-term passage of time than simply counting, especially in an era when pretty much no one outside religious institutions has access to a physical calendar.

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Studies of the history of astronomy will often casually assert that the main reason ancient and early medieval peoples looked into the sky was to follow their religious calendars or to know which direction to pray in. But I find those claims disrespectful of past peoples. Would you know that it’s 2:30pm on March 3rd just by walking outside? Do you know which direction Washington DC is in relative to your current position, especially without a compass or a map? Saying that these earlier eras of astronomical observation were “just” for these orienteering purposes is reductive, and simplifies huge organizational issues. Moreover, it ignores the vast amount of energy that ancient peoples put into exploring the hokier elements of star studies. Just because they were doing something that isn’t worth anything to us doesn’t mean they were doing nothing.


Astrology’s organizational systems are everywhere in astronomy prior to the Scientific Revolution. Star maps with constellations, calendars with the zodiac months, and the very weird pre-modern practice of associating the planets with metals. This is where astronomy and astrology welcome alchemy to the party, and suddenly the lines between different scientific disciplines become even more blurred. Physics at this time was, in a certain way, much more conceptually broad than it is now, because it didn’t have the grounding of observations of particles and hadn’t yet formalized ideas about gravity or other forces. Before physics offered the theory of the building blocks of matter, that was alchemy’s job. Alchemy began in ancient Egypt as the science of metals – studying what they are, how they exist naturally, and how they react to outside conditions like heat. In that study, alchemy formalized knowledge about what matter is, and this became the basis for explorations into how matter behaves. Because metals can be refined and undergo obvious visual changes, they were a great testing ground for these explorations. Alchemists applied what they learned from metals to other substances, and theorized that the properties in each known metal (gold, silver, copper, iron, tin, and lead) also appeared in corresponding planets, and would often refer to metals by the name of a certain planet – e.g. iron might be called Mars, but Mercury was actually not mercury, it was tin, because mercury was not yet considered a metal. As astronomy became more precise in its modeling and emphasized the greater importance of the heavenly bodies on their own terms, scholars still wondered about the makeup of these foreign objects, but they relied less on terrestrial objects to explain them. Simultaneously, alchemy became more concerned with the nature of life, rather than the nature of matter, and split off to theorize about how to make new life while chemistry took over the study of matter.

The fifteenth century also brought a new focus for astronomy in Europe that made astrology’s involvement obsolete: navigation. Throughout the Middle Ages, astronomers in the Arab and Persian world developed and used astronomical tools such as the sextant and astrolabe to precisely map the sky. These tools allow the user to position themselves in a kind of objective reference to both the horizon and a point in the sky. For astronomical study, they were typically used to track the movements of stars and planets, where the observer was the fixed point. In the tenth century, astronomers in Khurasan had such sophisticated knowledge of these mechanics that they even understood that when they worked from the sketchier observations made by ancient Greek astronomers they had to shift their frame of reference to account for a different longitude. The basic principle of triangulating your location with the horizon and a star is the essence of navigation. And while some ancient cultures seem to have been able to navigate entirely by this method, the ancient peoples of the Mediterranean could only used it as a guide, which is why they never left sight of the shore (and why Sicily was such an important place – because it’s a huge island, it allows you to bisect the Mediterranean rather than hugging the outer coast all the way around).

It wasn’t until Europeans got their hands on astrolabes and sextants in the fifteenth century that they figured out how to travel on open water and use these tools for full-on navigation. With this level of precision navigation and observation, astrology was no longer as useful a tool for mapping the sky. And with all its occult friends getting pushed to the margins, astrology too became less of a science and more of a superstition.

Alchemy and astrology getting the boot wasn’t because astronomy suddenly decided to grow up and do something with it’s life. Europeans were getting along fine without navigational astronomy, since they didn’t need it to navigate the Mediterranean or to reach Africa. It wasn’t even necessary to reach India, although farther afield in Asia needed some better navigational tools. And it’s a little circular to say that Europeans needed more precise navigation to get to America, because they didn’t even know what America was yet. The European urge to travel so broadly came as the result of two big territorial disputes on opposite ends of the Mediterranean in the fifteenth century. One was the Ottoman capture of Constantinople in 1453 and the other was the fall of Muslim Granada to Isabel and Ferdinand of Spain in 1492.

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As empires consolidated all around the Mediterranean, Europeans developing a new identity that was tied both to whiteness and Christianity we extremely anxious about the Ottoman Empire. Because of the particulars of Islamic tax laws, the Ottomans (the successors of the Seljuks seen in this map), had a fluid zone of exchange across the Middle East and North Africa, whImage result for age of empires gifich allowed them better and cheaper access to goods from the Indian Ocean. Christian merchants, however, who largely operated out of the northern Mediterranean and other parts of Europe, were charged a tariff on those same goods. When the Ottomans toppled Constantinople in 1453, ending the last vestiges of the Roman Empire (the Byzantines), they not only took control of the Black Sea trade into the Mediterranean, they also inherited a claim to all the lands of the former Roman Empire.

At the same time, the Christian Kingdom of Leon and Castille in what is now Spain was pushing out the Muslim administration in the small independent “Taifa States” throughout the region. In 1492, this new Kingdom of Spain defeated the last Muslim rulers in the region at the Battle of Granada, and created a centrally-controlled Spanish Empire. They developed a strict sense of Spanish-ness, which was inherently tied to Catholicism, and launched two major ventures to see out their vision: the Inquisiton, and the exploration of the New World. The Inquisition was about forcing consistency within Spain, by expelling or converting all non-Catholics, including Muslims, Jews, and members of other Christian sects. The expansion was about continuing the so-called Reconquista (or reconquest) effort in Spain to new places – now that Spain had decided what it was, it wanted to assert that identity as a new and unchallenged authority.

These two territorial disputes shut the doors on exchange at either end of the Mediterranean, making it more difficult for everyone to get access to goods from the Indian Ocean and Africa, and forcing upon each landmass a religious identity. This territorial clamming up, paired with new notions of regional identity, spurred Europeans to find alternate means of navigating around the Mediterranean, not because the price of goods would be cheaper, but because they would control more of the profits. Whereas medieval shipping had independent actors at every stage, early modern shipping was a vertical system, controlled by the crown (or, later, the company) at the top. With this now very roundabout way for Europeans to get to Africa and Asia, precision navigation became a new goal. And when the Spanish and Portuguese discovered that there was something worthwhile all the way to the west of them, every new European nation-state wanted that same kind of navigational access to the Atlantic. For the Islamic world, the navigational sextant was already in use in the Indian Ocean, a smooth transition from the astronomical instrument. Now that the precision tools were being used for navigation, astronomy had a model for applying its theories to practical mapping and exploration, creating a trajectory that would eventually lead very easily to space travel (even if that was not its goal at the time). So, it was a very purposeful goal, not a general curiosity or the gradual uncovering of universal truths, that pushed astronomy into the realm of science, and astrology in to the realm of nonsense.