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A Brief History of Astronomy Before the Telescope

An overview of astronomical thinking through Kepler

Greetings all. Thank you all for coming. This is the Brief History of Astronomy Before the Telescope panel. As a bit of an introduction I’m Jon Voisey and I graduated from the University of Kansas in 2008 with a BS in Astronomy.

I’ve also been interested in medieval re-creation as a member of the Society for Creative Anachronism since 1998. So, you may expect that I put these two passions together quite early. But in reality, I only started really studying the history of astronomy beginning around 2017. The reason it took me so long is that, for a long time, I thought, as I’m sure many of you do, that the history of medieval astronomy could be summed up by the phrase, “and nothing happened.”

This is because, if you pick up an introductory astronomy textbook, you’ll be lucky to get more than a few pages on the history of astronomy prior to the Renaissance. And the narrative generally goes something like this:

“Once upon a time there were the Greek astronomers who came up with the Geocentric model. It worked well enough that everyone believed it for 1500 years and then Copernicus, Brahe, Kepler, Galileo, Newton, and modern astronomy was born.”

Even major communicators of science tell the story. We see it in the original Cosmos series by Sagan in which he tells of the scientific inquisitiveness of the Ionian scientists who came up with ideas thousands of years ahead of their time, such as atoms and a sun centered cosmos. He then suggests this rational inquiry was snuffed out by “the mystics” as he calls them, not-so-subtly pointing at Christianity.

It’s a compelling narrative that persists even if you try to scratch the surface because, if you do, you are immediately hit with an inescapable fact. Ptolemy wrote the summary of the Greek geocentric model in the Almagest in the 2^nd century CE. 1400 years later when Copernicus and Kepler wrote their treatises on heliocentrism, both felt the need to respond to the prevailing model of the day in their introductions. That model was still the Ptolemaic geocentric model, virtually unchanged in the 1400 years since.

Astronomy had evidently not progressed.

(Pause)

But one of the cool things about being in the SCA is that sometimes, someone finds something you didn’t know you needed.

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Practical

Philosophical

Mathematical

Astrological

The first could be referred to as “practical” astronomy. This involves things like creating calendars and telling time, navigation - things that we’ve become so good at in modern day, that we scarcely recall their astronomical underpinnings. This type of astronomy doesn’t require any attempt at understanding why things are the way they are, but instead, put astronomical phenomena and the patterns of the sky to use.
The second form is what I refer to as “philosophical” or “cosmological” astronomy. This form of astronomy is rooted in natural philosophy and attempts to uncover the fundamental nature of the cosmos. This was the largest stumbling block of early astronomers as their base notions of the universe were fundamentally wrong. But, even after the invention of the telescope, it would still take quite some time to challenge all of these false assumptions.
With those philosophical assumptions about nature set, astronomers could then come to the third type of astronomy: “mathematical” astronomy which attempts to build mathematical models of a cinematic universe. This is the type of astronomy most congruent with our modern conceptualization of the field, but is extremely narrow. Furthermore, its challenging mathematical nature meant that it could only be approached by the most well trained astronomers.
Lastly, we can ask what it all means. Just as the stars could tell the seasons, the belief that they could also tell the fates of people and nations was widespread and commonly practiced. Today, we recognize astrology as pseudoscience. But to the pre-telescopic astronomer, the idea that the cosmos were communicating divine knowledge was taken as fact.

What we tend to think of as astronomy today most closely aligns with this third form. However, mathematical astronomy is quite challenging and, as a result, it’s actually relatively rare to encounter works that fully dive into it. That being said, it is what my primary area of focus is, so this class will have a serious bias towards this category.

I mention this because I don’t want to give the impression that, if you want to study astronomy in the SCA, it’s going to be endless calculations. It can be, if you’d like. But there’s many ways to engage with period astronomy. So, if you’d like to add astronomy to your persona but aren’t interested in math, it is certainly possible. And, if I don’t give enough clarity on how, feel free to reach out to me to discuss further.

Lastly, I should note that while I’ve drawn these tidy categories, their boundaries are not always clear and they, in fact, frequently overlap. Which we can immediately see by considering the most ancient astronomers we have any substantial detail on, the Babylonians.

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Babylonian Astronomers

Observed dates and position of sun, moon, planets, eclipses, comets
First records of periods of return for ecliptic longitude and anomalistic motion
Divided a circle into 360 degrees with each degree being 60 minutes and each minute being 60 seconds

The Babylonian records we have today are written on clay tablets which makes them quite durable, so a surprising number of them have survived to present day.

The most common type we have from the Babylonians is a sort of astrological handbook known as “Enuma Anu Enlil” which contains roughly 7,000 omens, most of them astronomical in nature (others are meteorological). They would say things like,

“If Jupiter stands in Pisces, [then] the Tigris and the Euphrates will be filled with silt.”

Perhaps, because of their astrological interest, the Babylonians were keen observers of the night sky and many of their observations were recorded in these clay tablets as well. They described the position of the sun, moon, and planets for various dates. They also recorded eclipses, comets, when the various bodies were in conjunction, opposition, entered or left retrograde, or other phenomenon, such as when one would appear in a lunar halo.

These “Astronomical Diaries” were regularly recorded from roughly the 7th or 8th century BCE through to the 1st century BCE, making this the longest continual observational program ever conducted.

But, in addition to the astronomical contents, these reports also included descriptions of the weather - perhaps to explain why, on some nights, the sky was not visible. These weather reports indicate that the weather was more frequently poor than it is today in this region, as this area has become a desert. But, due to the frequently poor weather, astronomers would need to know what the omens the sky announced were even if clouds prevented them from being seen.

To that end, Babylonian astronomers developed the first astronomical calculations, sometime around the 3rd century BCE. But, these weren’t based on a model that was supposed to represent the actual workings of the heavens. Rather, these calculations used the periods of the sky to loosely predict when a planet would begin its retrograde motion or other such phenomena as this was important to their astrology.

As a basis for these calculations, the Babylonians made the first known records of the periods and ratios of “returns” for the sun, moon, and planets. What this means is that they recorded how many years it would take an object to return to, not only the same point in the sky, but at the same apparent speed. For example, they recorded that Saturn would return to the same position at the same speed after a period of 59 years while completing 57 cycles of its change in speed (traceable through the retrograde motions it makes). These observations would form the basis for the Greek astronomical models which we’ll discuss shortly.

We also know, from another set of tablets known as MUL.APIN, that the Babylonians had a number of constellations, many of which are the direct predecessors to ours today.

The Babylonians also divided the circle of the sky into 360º with each degree being subdivided into 60 minutes and each minute being again divided into 60 seconds - a tradition astronomers still follow to this day, and which is why our system of measuring time does so all the way to the present.

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Greek Astronomy

348 BCE

Plato

Earth center of cosmos but spins on axis
Platonic solids associated with elements
Proposed complex planetary motions a result of combination of simple, uniform circular motions

337 BCE

Eudoxus of Cnidus

Wrote several astronomical treatises
Created models for planetary motion using concentric spheres

322 BCE

Aristotle

De Caelo
Spherical, stationary earth

290 BCE

Autolycus of Pitane

First text on spherical astronomy

230 BCE

Aristarchus of Samos

First proposal of heliocentrism

194 BCE

Eratosthenes

Measured the circumference of the earth

100 BCE

Theodosius of Bithynia

Invented sundials suitable for any location
Wrote book on spherical astronomy

Following the Babylonians, the Greeks had a rich astronomical tradition. And with the great Greeks philosophers, we begin seeing real thinking about that cosmological astronomy which considers the unanswerable philosophical questions (at least, unanswerable in their time) such as: What substance is the cosmos made of? What makes the planets move? What shape is the Earth? Does it move?

Many of these questions were wrestled with in Aristotle’s De Caelo (On the Heavens) which posed the questions and then used philosophy to attempt to answer them. These questions would then be re-evaluated and new answers devised as natural philosophy evolved for centuries to come.

Meanwhile, mathematical astronomers attempted to build models that can mathematically predict the positions of celestial objects in the past or future. But, while this is more the sort of hard science that earns respect in the modern world, it was often considered a lesser form of astronomy as it did not address the big questions like the cosmological natural philosophy.

Obviously these two fields did share a great deal of overlap. For example, the question of the center of the cosmos was in many ways a question for natural philosophy and cosmological thinking, but was inherently a starting point for any mathematical model

The lack of observational basis for the cosmological arguments led to many contrasting views. For example, the Greek philosophers had widely varying opinions on the base nature of matter. We also see disagreements on the arrangement and motions of the cosmos. Aristotle placed the earth at the center of the cosmos and argued that it did not rotate on its axis. Heraclides of Pontos, in the late 4th Century BCE is the earliest to claim that the earth did rotate which Plato agreed with in his dialogues “Timaeus and Critias.”

We also see disagreement on whether the earth or the sun is the center of the cosmos with the first mentions of a sun-centered, or “heliocentric” cosmos. This idea is generally attributed to Aristarchus of Samos. However, his only surviving work, “On the Sizes and Distances” (of the Sun and Moon) does not assume a sun centered cosmos. Rather, we know of Artistarchus’ heliocentric opinions from other authors, such as Archimedes, who criticized Aristarchus’ reasoning. As such, we don’t know how compelling of an argument Aristarchus may have made - whether his proposition was qualitative or quantitative in nature. Regardless, Artistarchus’ heliocentric proposition failed to stick.

Other ideas did stick around. In particular, Plato was highly interested in the concept of perfect forms such as cubes or spheres, which could not exist in this world as any object we created in those shapes would have some flaw, no matter how small. He suggested our reality was but an image or copy of the real world – a divine one in which perfect forms existed. This idea of perfect shapes would reverberate all the way through the end of the medieval period. In particular because Plato was aware of four polyhedrons for which all sides were the same: The tetrahedron (a pyramid shape), a cube, an octahedron, and the icosahedron. These, he associated with the four classic elements of fire, earth, air, and water respectively. However, a fifth regular solid would later be discovered, the dodecahedron, which would become associated with the stuff of the heavens, often called the aether or quintessence, whose form was perfect and unchanging.

During this period, other astronomers laid the foundations for the first fully fledged mathematical model of the solar system. And the most challenging thing they would need to explain was retrograde motion. The first astronomer we know of that attempted to model this sort of motion was Eudoxus of Cnidus (nigh-dus). In Eudoxus’ models, we see him adopting the Platonic and Aristotelian concepts of perfect motion which is to say that objects should move at a consistent speed in perfect circles. However, the motions that these models would need to explain were decidedly not uniform. For example, the sun moves faster with respect to the background stars in the winter than it does in the summer. Although this can’t be directly observed since the sun’s brightness washes out the stars during the day, it can be teased out by noting the intervals of time between successive solstices and equinoxes. Planets were even harder to describe as they could not only speed up and slow down, but sometimes would reverse direction completely the aforementioned retrograde motion.

[CLICK] Eudoxus explained these motions using concentric spheres, here illustrated as rings around their equators. The rings were joined together at an offset angle and rotated in opposite directions. Doing so, he could explain the back and forth motion exhibited by planets undergoing this retrograde motion. According to some sources, Eudoxus’ model relied on 27 spheres.

The results of this were evidently unsatisfactory as we know that later astronomers attempted to revise the arrangement and number of spheres. This included the previously mentioned Callippus, who increased the number of spheres to 34. Aristotle found this insufficient as well and brought the number to somewhere between 43 and 55 spheres.

Ultimately this type of model was abandoned, but the idea of nested spheres rotating about one another persisted. We find this in the work of two of the most important astronomers in the Greek tradition.

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Hipparchus (190 – 120 BCE)

Catalogued hundreds of stars with positions and brightnesses
Discovered precession of the equinoxes
Calculated length of the year, lunar month, and other cycles
Created geometric models of the solar system to account for the varying speeds of the sun, moon, and planets

The first is Hipparchus who lived in Rhodes in modern day Greece. He compiled a catalog with positions and brightnesses for hundreds of stars numbering their brightnesses from 0 (the brightest) to 7 (the faintest). Although the definitions have been tweaked, this system still forms the basis for how astronomers define the brightnesses of objects to the present day.

He is also credited with discovering a phenomenon known as the precession of the equinoxes. This means that the position of the sun on the date of the equinoxes slowly changes its place along the zodiac. During his time, the sun was at the beginning of the constellation of Aries on the spring equinox. Today, it lies near the beginning of Pisces. It also has the effect of changing the location of the north celestial pole. Today it lies very near the star Polaris, but this was not true 2000 years ago when it lay between the constellations of Ursa Minor and Draco and there was no “north star.”

Hipparchus also made careful observations of the solar and lunar cycles, leading to the best estimates at that time of the length of the year and lunar month. He used this, along with other observations from Babylonians to work out an earth-centered, or “geocentric”, mathematical model of the solar system which dates could be put in to predict the positions of objects in the sky to a reasonable degree of accuracy for the time.

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Ptolemy (100 – 170 CE)

Almagest

Comprehensive mathematical astronomy textbook describing motion of celestial objects
Geocentric model
Objects move series of spheres rotating with uniform circular motion (deferents and epicycles)
Some spheres off center (eccentric)

Other books on Geography, Astrology, Music, Optics…

The second is Claudius Ptolemy writing from Alexandria in modern day Egypt. He is known for taking the work of all his predecessors (especially Hipparchus), building on it, and putting it all into a comprehensive series of 13 books collectively known as “Mathematike Syntaxis”, but more commonly known by its Arabic title, the “Almagest,” which simply means “The Greatest”. This work was so influential, all the previous Greek astronomers I mentioned have no known copies of their primary works, and what knowledge we have of them comes almost exclusively from commentaries by other authors.

In this masterwork, Ptolemy lays out the geocentric model in full mathematical detail. This cosmos he describes has the earth at the center, both unmoving and not rotating. Ptolemy details his reasoning for this in the opening chapters of the “Almagest”. He had no understanding of gravity as we do today, in which the mass of objects themselves is the source of attraction. Rather, he believed that all objects tended towards the center of the cosmos, with denser ones being attracted more strongly and more rarified ones less so. Thus, if the earth were somehow moved from the center of the cosmos, it would naturally return to the center.

Ptolemy also argues against the notion that the earth could orbit the sun (implicitly nodding to Aristarchus), noting that such a motion would bring the earth closer to different points on the sphere of the stars throughout the course of the year. This should mean that less than half of all stars are visible at any given moment which we don’t observe. Ptolemy’s reasoning is actually largely correct. What he failed to understand is that the size of earth’s orbit is infinitesimal compared to the distance to the stars, even though he understood this to be true for the size of the earth itself. The cosmos are so large that detecting the earth’s motion on the stars wouldn’t occur until the early 19th Century.

In his model, the earth is surrounded by concentric spheres (echoing Eudoxus), on which the sun, moon, and planets are all carried. To explain the varying speed of the celestial objects, Ptolemy used three methods, two of which are thought to have been created by Hipparchus.

The first was that of epicycles. In this, the celestial body was carried around on a second smaller sphere whose center was on the larger sphere. When the body was being carried on the smaller sphere in the same direction the larger one rotated, it would appear to speed up. When rotating in the opposite direction, it would appear to slow down. If the smaller sphere rotated fast enough, the object would sometimes appear to move in reverse.

The second was that of an eccentre. Here, the center of the main sphere that carried the celestial object was simply moved away from the center of the earth. Then, when the object was closer to the earth, it would appear to move faster. Conversely, when further away, it would appear to move slower while still moving on its sphere with a uniform circular motion.

The third was a concept known as an equant. With this, Ptolemy splits the components of the eccentre (the motion and the distance) into two points. The eccentre remains the point about which the planet’s orbit has a fixed distance, but the equant is a new point about which the rotation is constant. By combining these explanations and fine tuning the speeds of the spheres, Ptolemy was able to reproduce the complex motions of celestial objects.

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Islamic Golden Age

550 CE

Aryabhata

Determined length of a sidereal day to within one hundredth of a second, length of a year to within 3 minutes and 20 seconds

850 CE

al-Khwarizimi

Produced astronomical tables using Ptolemy’s methods

870 CE

Al-Farghani

Wrote conceptual astronomical text – On the Science of Stars
Wrote treatise on the Astrolabe
Wrote treatise on spherical geometry

885

Abu Ma’shar

Wrote the Greater Introduction to Astrology

901 CE

Thabit ibn Qurra

Reformed Ptolemaic system proposing cyclical precession of the equinoxes

928 CE

al-Battani

Refined Ptolemaic model
Produced astronomical tables
Determined length of a year to within 2 minutes 22 seconds

986 CE

al-Sufi

Compiled star catalog adding star color
First written identification of Andromeda galaxy and Magellanic clouds

1000 CE

Abu-Mahmud Khojandi

Built mural quadrant with radius of 20 meters
Determined obliquity of ecliptic to be lower than Ptolemy and inferred it was not constant

1009 CE

Ibn Yuus

Refined Ptolemaic rate of precession from 1º/100 years to 1º/70 years

1037 CE

Ibn Sina

Wrote commentary on Ptolemy
Distinguished between astronomy and astrology

1040 CE

al-Haytham

Wrote works critical of Ptolemy
On the Configuration of the World

1048

al-Biruni

Observed eclipses and reviewed other astronomers works

1087

al-Zarqali

Helped compose astronomical Toledo tables

1274 CE

al-Tusi

Created idea of spheres rolling inside each other as alternative to epicycles
Founder of trigonometry

1375 CE

al-Shatir

Revised Ptolemaic model making additional use of al-Tusi’s concepts

1449

Ulugh Beg

Built mural sextant with radius of ~36 meters
Compiled catalog of 994 stars
Determined length of year to within 58 seconds

From Egypt, Ptolemy’s mathematical model of the motion of objects moved East and found its way to the Islamic Middle East. This region had been primed to have an interest in astronomy in the 8th century when an unknown astronomer from India visited the Abbasid Caliphate. His knowledge impressed the Caliph, al-Mansur who ordered that the astronomical handbook of his writing be translated into Arabic. This was done by Ibrahim al Farzi and is known as the “Zij as-Sindhind.”

This sparked an interest in astronomy and a search for further knowledge, in no small part because determining direction from the sky was useful in determining the direction of Mecca to which they were required to pray. Islamic astronomers were then introduced to the Greek tradition by Christians in Khuzestan in modern day Iraq. They then translated the works of Aristotle, Archimedes, Euclid, Apollonius, Ptolemy, and other mathematicians into Arabic leading to an Islamic golden age in astronomy. Here, Islamic astronomers adopted Ptolemy’s “Almagest” and were fully capable of embracing the advanced mathematical models.

Initially, they simply followed his methodologies to reproduce the tables needed for calculations with values adjusted in latitude and longitude for their native cities. We see this with astronomers such as al-Khwarizimi in the 9th century. Similarly, al-Sufi in the 10th century reproduced Ptolemy’s star catalog, updating it for the precession of the equinoxes, and adding star colors.

But, other Islamic astronomers realized the discrepancies with the Ptolemaic predictions and became critical of Ptolemy’s work. However, they did not reject the model in concept.

Rather, they revised key figures or parameters. In effect, they turned the mathematical model off and then back on again, resetting it for their present day, but using improved parameters for things like the length of a year, or the rates of precession to see if that would help.

For example, Aryabhata in the 6th Century improved the accuracy of a year from Ptolemy’s 6.5 minutes off, to only 3 minutes and 20 seconds off. This was further refined to be only 2 minutes and 22 seconds off by al-Battani in the 10th century.

The 9th Century astronomer al-Farghani made an ill-fated attempt to measure the size of the earth. His conclusion was that it was 6,500 miles in diameter which is a significant underestimate, and a large step backwards from the 2nd Century BCE estimate of Erotasthenes who put it at 7,850 miles - just 50 miles short of the true diameter.

Other astronomers, such as Ibn Yunus in the 11th century, attempted to improve the value for the precession of the equinoxes. Ptolemy had declared that the rate of precession was exactly 1 degree per century. Ibn Yunus calculated it at 1 degree every 70 years. The true value is 1 degree per 72 years.

The 10th century astronomer, Khojandi, worked on the obliquity of the ecliptic, which is the angle between the celestial equator and the sun’s path through the sky. Ptolemy’s value was 23.85º. Khojandi determined it to be 23.53º, which is much closer to the true value of 23.57º.

Unfortunately, Khojandi incorrectly trusted the accuracy of Ptolemy’s values and concluded that this value was decreasing over time. Still, it was an impressive correction - one which al-Biruni critiqued less than a century later, noting that Khojandi likely slightly underestimated his own value due to his massive astronomical instrument settling into the ground over time, which is most likely true.

These improvements on the parameters came, in part, from having a longer baseline over which to determine averages. Ptolemy only had a handful of centuries between the Babylonian astronomers and his own time. The Islamic astronomers had several more centuries than Ptolemy did.

Other improvements came from new instrumentation, some of staggering size. Ulugh Beg, for example, created a mural quadrant with a radius of 36 meters!

However, as time passed, more and more astronomers became disillusioned with Ptolemy’s model as, even with improved parameters, it still drifted over the centuries.

In response, some Islamic astronomers adjusted the arrangements of epicycles and eccentrics, but were unable to improve the accuracy, so these changes fell by the wayside.

Others revived previously discarded ideas. The first of these was a concept known as “trepidation” which came from the 4th Century mathematician and philosopher, Theon of Alexandria. In his commentary on another of Ptolemy’s works (“The Handy Tables”), he mentioned that some believed that the precession of the equinoxes was not constant in one direction along the ecliptic, but instead, oscillated back and forth over a span of 8º and a period of 80 years. Although Theon did not endorse this theory, it was adopted by the 9th century Islamic astronomer Thābit ibn Qurra.

Another example comes from Ibn Bajja, Ibn Tufail, and al-Bitruji who, in the 12th Century, attempted to revise the models of Eudoxus, Callippus, and Aristotle to see if they offered insights.

We also see some new innovations, such as one from al-Tusi who introduced the concept of spheres rolling within spheres (as opposed to spheres on spheres) as a variation on the explanation for the non-uniform apparent motion.

While these astronomers failed to fundamentally change the geocentric view, much of the work they did would be instrumental for the western astronomers who would ultimately break from the geocentric tradition.

Now, let’s jump back a bit and take a look at what was going on in the Christian areas of Western Europe.

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Computus

Determining the date for easter based on lunar cycles and the equinox

There is a popular belief that the rise of Christianity led to a suppression of science in its domain. I don’t want to get into that too much here as it’s quite a deep topic, and I have an entirely separate class on it,

What I will say is that the relationship between Christianity and astronomy is complicated. There certainly are attacks on astronomical thought that come from the Church and its figureheads, but so too do we find promotion of astronomical thought and practice. In particular, astronomy is fundamental to the telling of time and the setting of calendars. This is very important if you need to know when to have prayers or particular holidays. The Christian church had an entire methodology known as “Computus” dedicated to calculating the “correct” date of Easter. This calculation was heavily dependent on the knowledge of the previously discussed Metonic cycle.

Additionally, it was the Church that organized the first universities. And amongst their upper division studies, known as the Quadrivium, was astronomy.

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Carolingian Renaissance

Charlemagne tutored in astronomy by Alcuin of York, the abbot of Marmoutier
Also wrote to Irish Monk Drugal on astronomy
Familiar with eclipse prediction
Not familiar with planetary positioning

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The Aratea

However, interest in astronomy at this time was evidently strong. This was, perhaps, driven by Greek astronomical texts being rediscovered and translated into Latin. This includes a text by Aratus of Soli, which was a poetic version of a text written by the previously mentioned Eudoxus. This text, known as the “Aratus Latinus” gave a tour of the night sky, laying out the placement of the constellations relative to one another. Additionally, it described where various constellations were when the zodiac constellations were rising and setting.

This text was translated into Latin in the second quarter of the 8th Century although not particularly well. Modern scholars have called the translation “barbarous.” However, a revised translation followed soon after.

The popularity of these texts spawned an entire series of astronomical works with a focus on the constellations. Some would mix the constellation descriptions with the mythology on the constellation's origins. However, these works were all very conceptual in nature and lacked the mathematical models of Ptolemy.

Instead, the understanding of planetary astronomy came from less comprehensive sources. Specifically, texts from the 3rd and 4th century CE writers Martianus Capella and Macrobius which presented qualitative models as opposed to mathematical ones. While these models were still geocentric in nature, they differed from Ptolemy’s model by having Mercury and Venus orbit the Sun as depicted here.

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Western Expansion: Translation

1125

Hugh of Santalla

Translated al-Muthanna’s methods for calculating astronomical tables

1133

John of Seville

Translated Abu Ma’Shar’s Greater Introduction

1140’s

Hermann of Carinthia

Translated Abu Ma’Shar’s Greater Introduction
Beganplans to translate Almagest with Robert of Chester

1160’s

Unknown Translator

Almagest translated into Latin. Does not receive wide circulation

1160’s

Daniel of Morely

Spent time in Toledo and returned with many astronomical texts which drew ire of his bishop

1135

John of Seville

Translated al-Farghani’s On the Science of Stars

1175

Gerard of Cremona

Translated Almagest

Ptolemy’s models would begin trickling their way into the Christian regions of Europe beginning in the 12th Century by way of the Muslim territories in modern day Spain - largely in the cities of Castille and Toledo. This process came in a few distinct stages. It begins with texts being translated. This includes a translation of al-Muthana’s methods for calculating astronomical tables in 1125 by Hugh of Santalla, abu Ma’Shar’s Greater Introduction to Astrology in 1133 by John of Seville who also translated al-Farghani’s On the Science of Stars in 1135, the Almagest in the 1140s by Hermann of Carinthia as well as a second translation by an unknown translator in the 1160s.

This first wave of new material invigorates those interested in astronomy, but without the proper mathematical background, readers obviously struggle with the texts and do not produce any original works.

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Western Expansion: Digestion

1215

Robert Grosseteste

Wrote De Sphaera

1223

Robert Grosseteste

Wrote On Comets

1230

John of Sacrobosco

Wrote De Sphaera

1252

Alfonso X of Castile

Commissions Alfonsine tables

1264

Campanus of Novara

Wrote Theorica Planetarum

1344

Levi ben Gershon

Rejected Ptolemaic model and created new one from scratch

By the 13th Century, Western astronomers began digesting the translated works and started writing new works of their own beginning with Robert Grosseteste writing one of many introductory texts on astronomy that would be called “De Sphaera”. While it was conceptual and largely based on Ptolemy’s Almagest, this work emphasized the need to understand lines, angles, and figures. The work also deviated from Ptolemy by adopting some of the Islamic innovations.

In 1223, Grosseteste wrote another text, “De Cometis” (“On the Comet”), which was prompted by an appearance of Halley’s comet the previous year. In it, he harkened back to the celestial spheres and classical elements suggesting that the comet must be part of the terrestrial sphere since it changed its appearance. However, he still assigned it an astrological significance.

Shortly thereafter, around 1230, John of Sacrobosco wrote another work entitled “De Sphera.” This one was so popular, it is the most common astronomical text to have extant copies of today, even though its editio princeps (its first printing) wasn’t until 1472 - nearly 250 years after it was written. And even after it did make its way to print, it continued to be regularly republished for several hundred more years, having over 80 editions by the end of the 17th Century.

In 1252, King Alfonso X of Castile commissioned a new set of astronomical tables, which bear his name, and were used to predict planetary positions. The calculations associated with these tables made use of the incorrect theory of trepidation mentioned previously.

Things quieted down in the 14th Century, but what little we do see indicates that astronomers were starting to come to terms with the mathematical concepts required to understand Ptolemy’s models.

One, Levi ben Gershom, a Jewish astronomer in France, completely rejected the Ptolemaic model and created a new one from scratch. It still involved epicycles and eccentrics and was still geocentric in nature, but it offered notably improved results, especially for the moon. However, he wrote in Hebrew, and his work saw very little attention.

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Western Expansion: Comprehension

Johannes von Gmunden

Began lecturing on mathematical astronomy at Vienna University

1420

Georg von Peuerbach

Wrote Theoricae Novae Planetarum
Calculated the Tabulae Eclipsium

1450’s

George of Trebizond

Translated Almagest into Latin from Greek manuscript

1450’s

Regiomontanus

Wrote an abridged Almagest

1460’s

Albert of Brudzewo

Wrote Little Commentaries on Peuerbach’s work

1482

Peter Apian

Published Astronomicum Caesarean

1580

By the 15th Century, this mathematical astronomy started to make its way into universities and new books were written on the subject, although many more conceptual and philosophical works continued to be created as well. Towards the middle of the 15th century, George of Trebizond authored a new translation of the “Almagest” into Latin from a Greek manuscript.

Georg von Peuerbach wrote a popular new book, entitled “Theoricae Novae Planetarum” (“New Theories on the Planets”). This book was one in a line of books that went beyond the model presented in Ptolemy’s Almagest. In that work, Ptolemy had only determined the size of the solar system in arbitrary and undefined units that weren’t even consistent between planets. However, Ptolemy also had another book, entitled the Planetary Hypothesis in which he attempted to work out the true size of the solar system by assuming that the maximum distance that one planet could achieve was the minimum distance the next could have. This reasoning was also passed down through the Arabic tradition before being brought to the Western public’s attention by Peuerbach.

Then, after observing an eclipse that started a mere eight minutes later than predicted from the Alfonsine tables, Peuerbach created an entirely new set of tables specifically for eclipse predictions, known as the “Tabulae Eclipsium” (“The Eclipse Tables”). However, they wouldn’t see publication until 1514.

Perubach’s work received a review by Albert of Brudzewo in the 1480s. In it, we start to see some questioning of the ancient idea of celestial spheres. Albert pondered whether they are actual physical spheres or simply convenient mathematical devices representing the motion, but not physical reality. Ultimately, he concludes that they were physical objects.

Puerbach died early - at the age of 37. At the time of his death, he was working on yet another translation of the “Almagest”. He had asked his teacher Regiomontanus to complete it - which Regiomontanus did sometime in the 1460’s - although it would not see publication until 1496.

A few more noteworthy texts are published through the end of the medieval period including “Astronomicum Caesarean” (“Astronomy of the Caesars”), by Peter Apian. While this not mathematical text in nature, the book was notable for extremely lavish and colored diagrams which had rotating paper disks known as “volvelles” to help illustrate the motions of the heavens and concepts.

But the 16th Century is when the big names started appearing that would change the course of astronomical history.

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Nicolaus Copernicus

Revived Heliocentric model
Still assumed perfect circles, used epicycles and eccentrics
Relied on concepts from Islamic astronomers especially the rolling spheres of al-Tusi
Was reluctant to publish
Widely read but not widely commentated publicly

The first was Nicolaus Copernicus, who revived the heliocentric model of Aristarchus. However, he was not fully able to break with tradition, as his model still assumed that there must be perfect, uniform circular motion. Thus, he again turned to epicycles and eccentres to be able to explain the varying speeds of celestial bodies. In addition, he adopted ideas from the Islamic astronomers, especially the idea of rolling spheres from al-Tusi (for which he did not give credit).

But, being that his work was such a departure from the standard of the day, he was understandably reluctant to publish. However, his pupil, Georg Joachim Rheticus, pushed him to do so, going so far as to write “Narratio Prima”, (the “First Account”) which served as an abstract, announcing Copernicus’ upcoming work.

Copernicus ultimately relented and agreed for this work to be published, if Rheticus would lead the effort. However, when Rheticus was called away from overseeing the printing to take a position at the University of Leipzig, he was replaced by Andreas Osiander.

Osiander was a theologian and, perhaps sensing the religious friction this would cause a century later, he added an unauthorized preface insisting that this work was not to be taken as a literal model of how the solar system worked, but merely a mathematical convenience that was coincidentally able to make reasonably accurate predictions.

Whether it was for that reason or others, we do not find commentaries being written on Copernicus’ work as frequently as for others. However, studies of the many extant copies of the publication indicate it was widely read, and the notes in the margins indicate it was given a good deal of consideration.

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Tycho Brahe

Patron of King Frederick II
Observed an eclipse occurring a day later than predicted
Set up a private castle observatory on the island of Hven
Made numerous instruments to obtain more precise observations
Rejected Copernican model and instead advocated for a geo-heliocentric model
Observed a comet and determined its distance was greater than the moon’s

The next big name in our history was Tycho Brahe. Due to his father having rescued King Frederick II of Denmark and Norway from drowning, the Brahe family fared well in the good graces of the king.

Although Tycho was the first-born son, he was largely uninterested in overseeing lands and instead pursued his passion for astronomy. This passion had been stoked by the observation of an eclipse occurring a day after it had been predicted.

Like astronomers centuries before, Brahe had realized that the models that made the predictions relied on key parameters, and so he set about building ever more accurate instruments to obtain the most precise observations ever recorded in an effort to refine them further, as well as to provide the key observations needed to calibrate the models. A review of some of his instruments determined they were accurate to nearly 1/100 of a degree.

Brahe agreed with neither the geocentric model of Ptolemy nor the heliocentric one of Copernicus. Instead, he proposed a fusion of the two in which the earth was at the center, and the moon and sun went around the earth, but all the planets would go around the sun. However, he was never able to get the math to work better than either of the other models.

In 1577, Tycho observed a comet. He attempted to measure its distance through parallax (the shift in apparent position caused by viewing an object from two points of view). However, the comet had no discernable parallax. Thus, Brahe concluded that it must be beyond the lunar sphere since the moon does show an observable parallax. This defied the ancient notion that the stuff of the heavens (the “aether” or “quintessence” as we referred to it before) was eternal and unchanging. In addition, the comet's path that Brahe was able to work out, would indicate that it should cross through several of the spheres that carried other celestial objects. This disproved the notion that the celestial spheres could be actual physical objects.

Ultimately, his expertise and observations led him to become a renowned figure in astronomy. The king granted Brahe use of the island of Hven for him to build an observatory where he, his wife, and his sister did their work. The only problem was that the island was already inhabited and the residents, while technically already subjects of the Crown, did not take kindly to having a lord rule over them so directly. Especially because Tycho would be entitled to some of their labor to construct and maintain his estates.

After Frederick’s death, his son, Christian IV replaced him and was eager to make a name for himself militarily. And the funny thing about wars is that they’re very expensive. Christian used the protests of Tycho’s vassals as well as Tycho’s marriage to a common woman to cut his royal stipend.

Upset over this treatment, Brahe sought refuge in Germany where he became the Imperial astronomer and continued his work before his death in 1601.

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Johannes Kepler

Originally studied to become a minister
Taught mathematics at a Protestant school in Graz, Austria
Firmly supported the Copernican heliocentric model
Worked for Brahe to try to prove Brahe’s model
Used Brahe’s data after his death to work out a correct model of the solar system using ellipses instead of perfect circles on which planets moved faster when closer to the sun

The last major pre-telescopic astronomer was Johannes Kepler. He studied at the University of Tubigen to become a minister, but math and science were part of the formal education. His teacher Michael Maestlin instructed him in both the geocentric and heliocentric models. Kepler became a staunch defender of Copernicus’ heliocentric model.

Ultimately, his proficiency in mathematics earned him a position as a math teacher in Graz, Austria. While teaching there, he was struck with a self-described epiphany noticing that, if he inscribed an equilateral triangle within a circle, and then another circle within that triangle, the ratio of the diameters of the circles bore a striking similarity to the ratio of the orbit of Jupiter to the orbit of Saturn.

Now, a triangle is a two-dimensional form of a tetrahedron, one of the five Platonic solids. This gave Kepler the idea that perhaps these Platonic solids were the hidden foundations for the spacings of the planets and explained why there were only five planets – each was tied to one of the Platonic solids. He assigned the cube to Saturn, the tetrahedron to Jupiter, the dodecahedron to Mars, the icosahedron to Venus, and the octahedron to Mercury. Kepler published these thoughts in “Mysterium Cosmographium” (“The Cosmographic Mystery”) in 1596 and while no one took it too seriously, it did earn him a reputation as an excellent mathematician and astronomer.

In 1600, Kepler met Brahe, and ultimately agreed to come work with him to help prove Brahe’s geo-heliocentric model. Kepler was never able to get the model to work, despite promising to do so to Brahe on Brahe’s death bed. Rather, Kepler used the data that Brahe had recorded and worked out a new version of the heliocentric model – the correct one.

In his 1609 book, “Astronomia Nova” (“The New Astronomy”), he correctly asserted that planets travel in ellipses instead of perfect circles, and that their speed is not uniform – it varies, increasing speed when closer to the sun and slowing when further away.

This new model fit the data better than any others he considered. In particular, in his introduction, Kepler pointed out that the previous models all had an error of just under 5º (about 10x the width of the full moon) for the position of Mars in the Autumn of 1593. Kepler’s new model made this error disappear.

But, it would still take several more centuries to be fully accepted, as it had to challenge two millennia of tradition and ultimately fell foul of the Church. In Kepler’s work we see the full arc of two thousand years of astronomical thinking in which the echoes of Greek thought – with Platonic solids, perfect spheres, and uniform motion – ultimately met with the observation age of astronomy.

Let us now return to our original question of why astronomical progress seemingly stalled for 1,400 years.

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Thank you for attending

JonVoisey.net

@AstronomyBeforeTheTelescope

To my mind, it is largely due to the fact that we fail to recognize the diversity of how astronomy was practiced prior to the telescope. Today, we think of astronomy as a rigorous and mathematical discipline based on hard evidence, this is a relatively modern conceptualization of the field.

However, as we’ve seen in this video, there were a number of other ways astronomy was engaged in prior to the invention of the telescope. What we should now understand is that astronomical progress never actually stalled. Rather, much of what was happening in this period were simply types of work that we don’t tend to think of as astronomy today.

Even within that narrow mathematical field, astronomical progress continued between the time of Ptolemy and Copernicus. Islamic astronomers spent centuries trying to save Ptolemy’s model by refining the parameters. When they were unable to salvage it, they began trying to revise it. Later, astronomers even started from scratch. Ultimately, they failed to make any improvements substantial enough that Kepler felt them necessary to discuss. But, it certainly wasn’t for lack of effort.

In this video, I’ve largely concentrated on mathematical astronomy, as it’s what I’m both most well trained and most interested in. But, I hope that, by offering some glimpses into these other areas, you can come to appreciate the exciting diversity of the field as it was practiced before the telescope.

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