Ancient Greek Astronomy and Cosmology
As the stars move across the sky each night people of the world have looked up and wondered about their place in the universe. Throughout history civilizations have developed unique systems for ordering and understanding the heavens. Babylonian and Egyptian astronomers developed systems that became the basis for Greek astronomy, while societies in the Americas, China and India developed their own.
Ancient Greek astronomers’ work is richly documented in the collections of the Library of Congress largely because of the way the Greek tradition of inquiry was continued by the work of Islamic astronomers and then into early modern European astronomy. This section offers a tour of some of the astronomical ideas and models from ancient Greece as illustrated in items from the Library of Congress collections.
The Sphere of the World
By the 5th century B.C., it was widely accepted that the Earth is a sphere. This is a critical point, as there is a widespread misconception that ancient peoples thought the Earth was flat. This was simply not the case.
In the 5th century B.C., Empedocles and Anaxagoras offered arguments for the spherical nature of the Earth. During a lunar eclipse, when the Earth is between the sun and the moon, they identified the shadow of the Earth on the moon. As the shadow moves across the moon it is clearly round. This would suggest that the Earth is a sphere.
Experiencing the Sphere of the Earth
Given that opportunities for observations of a lunar eclipse do not come along that often, there was also evidence of the roundness of the earth in the experiences of sailors.
When a ship appears on the horizon it’s the top of the ship that is visible first. A wide range of astronomy texts over time use this as a way to illustrate the roundness of the Earth. As the image suggests this is exactly what one would expect on a spherical Earth. If the Earth were flat, it would be expected that you would be able to see the entire ship as soon as it became visible.
Measuring the Size of the Earth
Lunar eclipses also allowed for another key understanding about our home here on Earth. In 3rd Century B.C., Aristarchus of Samos reasoned he could figure out the size of the Earth based on information available during a lunar eclipse. The diagram at the right illustrates a translation of his work. The large circle is the sun, the medium circle is the Earth and the smallest circle is the moon. When the Earth is in-between the sun and the moon it causes a lunar eclipse and measuring the size of the Earth’s shadow on the moon provided part of the information he needed to calculate its size.
Eratosthenes estimated Earth’s circumference around 240 B.C. He used a different approach, measuring the shadows cast in Alexandria and Syene to calculate their angle relative to the Sun. There is some dispute on the accuracy of his calculations as we don’t know exactly how long the units of measure were. The measurement however was relatively close to the actual size of the Earth. The Greeks were applying mathematics to theorize about the nature of their world. They held a range of beliefs about nature and the world but they were, in many cases, working to ground those beliefs in an empirical exploration of what they could reason from evidence.
Aristotle’s Elements and Cosmology
In the tradition of Plato and Empedocles before him, Aristotle argued that there were four fundamental elements, fire, air, water and earth. It is difficult for us to fully understand what this meant as today we think about matter in very different terms. In Aristotle’s system there was no such thing as void space. All space was filled with some combination of these elements.
Aristotle asserted that you could further reduce these elements into two pairs of qualities, hot and cold and wet and dry. The combination of each of these qualities resulted in the elements. These qualities can be replaced by their opposites, which in this system become how change happens on Earth. For example, when heated, water seemingly turns steam which looks like air.
The Elements in Aristotle’s Cosmic Model
In Aristotle’s Cosmology, each of these four elements (earth, water, fire and air) had a weight. Earth was the heaviest, water less so, and air and fire the lightest. According to Aristotle the lighter substances moved away from the center of the universe and the heaver elements settled into the center. While these elements attempted to sort themselves out, to achieve this order, most of experience involved mixed entities.
While we have seen earth, fire, air and water, everything else in the world in this system was understood as a mixture of these elements. In this perspective, transition and change in our world resulted from the mixing of the elements. For Aristotle the terrestrial is a place of birth and death, based in these elements. The heavens are a separate realm governed by their own rules.
The Wandering and Fixed Stars in the Celestial Region
In contrast to the terrestrial, the celestial region of the heavens had a fundamentally different nature. Looking at the night sky the ancient Greeks found two primary kinds of celestial objects; the fixed stars and the wandering stars. Think of the night’s sky. Most of the visible objects appear to move at exactly the same speed and present themselves in exactly the same arrangement night after night. These are the fixed stars. They appear to move all together. Aside from these were a set of nine objects that behaved differently, the moon, the sun and the planets Mercury, Venus, Mars, Saturn and Jupiter each moved according to a different system. For the Greeks these were the wandering stars.
In this system the entire universe was part of a great sphere. This sphere was split into two sections, an outer celestial realm and an inner terrestrial one. The dividing line between the two was the orbit of the moon. While the earth was a place of transition and flux, the heavens were unchanging. Aristotle posited that there was a fifth substance, the quintessence, that was what the heavens were made of, and that the heavens were a place of perfect spherical motion.
The Unchanging Celestial Region
In Aristotle’s words, “In the whole range of time past, so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts.” It’s important to keep in mind that in Aristotle’s time there simply were not extensive collections of observational evidence. Things that looked like they were moving in the heavens, like comets, were not problematic in this model because they could be explained as occurring in the terrestrial realm.
This model of the heavens came with an underlying explanation. The celestial spheres were governed by a set of movers responsible for the motion of the wandering stars. Each of these wandering stars was thought to have an “unmoved mover” the entity that makes it move through the heavens. For many of the Greeks this mover could be understood as the god corresponding to any given entity in the heavens.
Ptolemy’s Circles on Circles
Claudius Ptolemy (90-168) created a wealth of astronomical knowledge from his home in Alexandria, Egypt. Benefiting from hundreds of years of observation from the time of Hipparchus and Eudoxus, as well as a set of astronomical data collected by the Babylonians, Ptolemy developed a system for predicting the motion of the stars that was published in his primary astronomical work, Almagest. Ptolemy’s success at synthesizing and refining ideas and improvements in astronomy helped make his Almagest so popular that earlier works fell out of circulation. Translated into Arabic and Latin the Almagest became the primary astronomy text for the next thousand years.
The Almagest is filled with tables. In this sense the book is a tool one can use to predict the locations of the stars Compared to earlier astronomy the book is much more focused on serving as a useful tool than as presenting a system for describing the nature of the heavens. Trying to accurately predict the place of the stars over time resulted in creating a much more complicated model.
The Ptolemaic Model
By the time of Ptolemy Greek astronomers had proposed adding circles on the circular orbits of the wandering stars (the planets, the moon and the sun) to explain their motion. These circles on circles are called epicycles. In the Greek tradition, the heavens were a place of perfect circular motion, so the way to account for perfection was with the addition of circles. This resulted in disorienting illustrations.
To escape the complicated nature of this extensive number of circles, Ptolomy added a series of new concepts. To accurately describe planetary motion, he needed to use eccentric circles. With the eccentric circle the center of the planets orbit would not be Earth but would instead be some other point. Ptolemy then needed to put the epicycles on another set of circles called deferents. So the planets moved on circles that moved on circular orbits. Ptolomy also needed to introduce equants, a tool that enabled the planets to move at different speeds as they moved around these circles. The resulting model was complex, but it had extensive predictive power.
Ptolemy and Aristotle’s Cosmic Legacy
Ptolemy came to represent a mathematical tradition, one focused on developing mathematical models with predictive power. Aristotle came to be known for putting forward the physical model of the heavens. Ptolemy was also interested in deploying his model of the heavens to describe its physical reality. However, his most important work was the mathematical models and data he used for predicting the motion of heavenly bodies. For a long time his name was synonymous with the model of the heavens.
Astronomical Innovation in the Islamic World
Between the 8th and 15th centuries Islamic astronomers produced a wealth of sophisticated astronomical work. Largely through the Ptolemaic framework, they improved and refined the Ptolemaic system, compiled better tables and devised instruments that improved their ability to make observations. The extensive contributions of Islamic astronomy also exposed some weaknesses in the Ptolemaic and Aristotelian systems.
Expounding and Teaching
al-Farghani (died after 861), known in the west as Alfraganus, wrote Elements of Astronomy on the Celestial Motions around 833. This textbook provided a largely non-mathematical presentation of Ptolomy’s Almagest, updated with revised values from previous Islamic astronomers. The work circulated widely throughout the Islamic world and was translated into Latin during the 12th century. It became the primary resource that European scholars used to study Ptolemaic astronomy.
This book was largely responsible for the emergence of the Greek astronomical system of Ptolemy in the West. It circulated in several Latin editions and was widely studied in Europe between the 12th and 17th centuries.
In other cases, like the work of Ibn al-Hatheym’s Doubts on Ptolomey, went far beyond translating and transmitting knowledge to developing an extensive critique of Ptolomey, and turned the mathematical models into a physical representation of movement in the heavens.
Refining and Expanding on Ptolemy
In The Book of the Fixed Stars, Al- Sufi combined Ptolemy’s work of mapping constellations with Arabic astronomical traditions. Written around 964 the book contains extensive illustrations of each constellation from both the terrestrial perspective, looking up from Earth, and the inverse, as the constellation would look from outside the sphere of the fixed stars.
Al-Sufi’s drawings became the canonical representations of these constellations. In Europe his works were translated and widely circulated. Even today, we use many of the star names that he recorded in the book.
The Book of the Fixed Stars documented more constellations and more stars in those constellations than ever before. These include some of the first recordings of what we would later understand to be another galaxy. The star on the right side of the belt of Andromeda is not actually a star as Al-Sufi originally thought, but is instead one of only two galaxies visible to the naked eye. He had recorded an observation of what we would later come to know of as the Andromeda Galaxy.
The Source of Classical Astronomy in the West
The frontispiece of this copy of his most famous work shows the Islamic astrologer, Jafar Ibn Muhammad Abu Mashar al-Balkhi (805(?)-886), also known as Abu Mashar, holding an armillary sphere. It’s important to remember that our contemporary notions that associate astronomy with science and astrology with superstition are poor frames of reference to import into our understanding of the past. In Abu Mashar’s time, contemplating the meaning of stars’ movements and how they impacted future human events was a valid and empirical practice.
Aside from Abu Mashar’s work in astrology, his translations of Greek texts, in particular Aristotle’s works, played an essential role in disseminating Aristotle’s ideas in the Islamic world and later in Europe. His work was translated from Arabic into Latin in the 12th century and was held in great esteem by Medieval and Renaissance intellectuals.
Whose Revolution? Copernicus, Brahe, and Kepler
Copernicus is often described as a lone astronomer who defiantly argued that the sun, not the Earth was at the center of the cosmos. Copernicus’ contributions to astronomy are so significant that they warrant their own term: The Copernican Revolution.
The story of this revolution is problematic for several reasons. First, as much as Copernicius’ ideas broke with the past, his model of the cosmos has more in common with his contemporaries than it does with modern day astronomy and physics. Second, although Copernicus’ sun centered model was revolutionary it was part of a series of early modern and renaissance innovations. For example, Tycho Brahe collected observational data at an unprecedented scale, and developed his own competing model. Similarly, Johannes Kepler developed mathematical models for elliptical orbits that challenged some of the core assumptions of Aristotelian cosmology.
Looking back on these advances, exactly whose revolution was it? Or, given that each of these astronomers worked in ongoing traditions of modeling and understanding the heavens, was there a revolution at all?
By briefly reviewing the works of Copernicus, Brahe and Kepler this essay offers you the chance to develop your own answer to these questions.
Copernicus’s Quest for Deeper Harmony and Order
Copernicus anticipated his ideas would be controversial. Because of this, he waited more than 30 years to publish his book in 1543. De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres) puts the sun at the center of the universe and the Earth in motion across the heavens as one of the planets.
De Revolutionibus opens with a brief argument for the heliocentric universe and follows with an extensive technical set of mathematical proofs and astronomical tables. Copernicus was not trying to thumb his nose at the accepted wisdom of astronomers and religious thinkers; instead he sought to uncover a more elegant order for the universe.
It was a revolutionary idea. With that said, Copernicus’ ideas built on an existing line of thinking. The movement of Mercury and Venus had been perplexing for a long time. Plato and Eudoxus noted that these planets never strayed far from the sun. It was almost like they were tethered to the sun, they could move a bit ahead of it or lag behind. In the 5th century AD Martianus Capella had argued that Mercury and Venus orbited the sun, which in turn rotated around the Earth. This was not the first sun-centered system that was argued either. Aristarchus of Samos had proposed a heliocentric system and the Pythagoreans before him had argued that the sun was the “central fire”. Although not part of the mainstream these were all ideas that Copernicus built upon.
While Copernicus’ contributions to astronomy were revolutionary, they are fundamentally different from our conception of our solar system today. His model still required perfect circular motion in the heavens. This meant that, like Ptolemy, he needed to use circles on circles, called epicycles, to account for the movement of the planets. Copernicus’ circles were much smaller, but the model didn’t get rid of the need for them.
Brahe’s, Data Collection and Importance of Overlapping Circles
Copernicus had largely based his work on a body of existing observations of the heavens. Although he did some observational work, the bulk of his contribution was focused on re-evaluating existing data from a different perspective. However, Tycho Brahe had a different approach. Born in 1546, (three years after the publication of Copernicus’ De Revolutionibus) Brahe became a famous astronomer, well known for his unprecedented collection of astronomical data. Brahe’s contributions to astronomy had revolutionary impacts in their own right.
In 1563, at age 16, he observed Jupiter overtaking Saturn as the planets moved past each other. Even with his simple observations he saw that existing tables for predicting this conjunction were off by a month, and even Copernicus’s model was off by two days. In his work, he demonstrated that better data could help to create much more robust models.
New Stars and Interpretations of Comets
In November of 1572 Brahe observed a new star in the constellation of Cassiopeia. With a sextant and cross-staff he was able to measure the star’s position and became convinced that it was in the realm of the supposed unmoving fixed stars. This observation was inconsistent with the longstanding belief that the celestial realm was a place of perfect and unchanging fixed stars.
Alongside this development, the appearance of a comet in 1577 provided additional evidence that things did change and did move in the celestial sphere. Based on careful measurements, Brahe was able to identify that the comet was outside the sphere of the moon and he eventually suggested it was moving through the spheres of different planets.
Brahe’s Model of the Cosmos
As a result of these observations, Brahe put forward a new model for the cosmos. In Brahe’s model, all of the planets orbited the sun, and the sun and the moon orbited the Earth. Keeping with his observations of the new star and the comet, his model allowed the path of the planet Mars to cross through the path of the sun.
Many scientists have been critical of Brahe’s model as a backward step in the progress of science. However, it is critical to remember the value that Brahe’s system offered. This system had the advantage of resolving the problem of stellar parallax. One of the persistent critiques of Copernicus’s model (and even of Aristarchus model in ancient Greece) was that with a moving Earth one should expect to see parallax movement of the stars. As the Earth changes position in relationship to that of the stars, one would expect to see the stars change position relative to each other. Copernicus’ answer was that the stars had to be so distant that it wasn’t possible to detect parallax. Still, the distance required to make this work was so massive as to be a problem for the system.
This was not a problem for Brahe’s system because his model allowed for the circles in the heavens to intersect. Brahe’s model was not a step backward; but revolutionary in the sense that it was a competing way to make sense of the data the heavens provided.
Kepler’s Harmonies of the Heavens
Johannes Kepler, born in 1571, made major contributions to astronomy as his work mixed sophisticated mathematics and astronomy with mystical ideas about astrology. Because of this Kepler remains difficult for contemporary readers to understand. He was excited about the possibilities of developing new astrology that was grounded in the work he engaged in as an astronomer. Kepler worked for Tycho Brahe, publishing an extensive amount of Brahe’s data in Rudolphine Tables. Although he used much of that data for his own publications Kepler’s work would significantly depart from Brahe’s.
Kepler’s first major work, Mysterium Cosmographicum (The Cosmographic Mystery, 1596), and his later work Harmonice Mundi (Harmonies of the World, 1619) are both largely concerned with the order and geometry of the heavens. In these works, he explored how the different shapes of Platonic solids could be combined to explain a superstructure for the heavens, and how the movements and patterns of the heavens could be mapped on to scales. For Kepler, the heavens literally made harmonies through their movements. He was not afraid to attribute qualities to these harmonies and order that would strike us today as strange superstitions. He was as interested in bringing together geometry and physics as he was with bringing together alchemy and astrology.
Kepler’s Elliptical Orbits
Kepler’s quest to bring together geometry and physics led to a new shape of the planetary orbits. In Astronomie Nova (1609),Kepler presented extensive research on the orbit of Mars.
Using Tycho Brahe’s observational data, Kepler was able to fine tune the movements of the planets and demonstrate that the movement of Mars could be described as an ellipse. The diagram from Astronomia Nova shows the difference between the perfect circle and the more pinched or squished inner ellipse. It was generally taken for granted that motions in the heavens would involve only perfect circles. However, through innovations in mathematics, Kepler was able to mathematically describe ellipses that closely fit the paths the planets moved through in the heavens. The ellipse enabled the removal of the epicycles and could account for the path of the planets in a single shape. His commitment to order pushed him to recalculate and rework his research until he figured out how to represent the orbits of the planets. Alongside describing the elliptical nature of orbits, Astronomie Nova offered initial arguments for a force of attraction that could organize and hold this kind of system together. Kepler’s work foreshadowed the discovery of one of the fundamental forces of physics, the law of gravity.
So Whose Revolution Was It?
Tracking the work and research of Copernicus, Brahe and Kepler illustrates a much more intertwined and complicated story. The discontinuity usually ascribed to Copernicus turns out to be a misconception, as his revolutionary work was part of a long line of astronomers and philosophers whose ideas began to expose cracks in the Aristotelian model. Instead of a simple narrative of progress and resistance to progress we find a series of distinct advancements made in particular historical contexts. Copernicus offered an important new model and a revised set of observational data. Brahe left us a competing model and new observations of motion in the heavens. Kepler’s work on elliptical orbits played a key role in moving toward a different conception of the cosmos. In each case, these individuals were part of ongoing dialogs between astronomers, theologians and other scholars.
Without substantive use of the telescope, these stories illustrate how focused observation and exploration can result in important advances. At the same time, it’s best not to confuse their understanding of the world with our own. Copernicus remained sure in the perfect heavenly spheres, Brahe spent a lot of time working on alchemy and Kepler wrote a great deal about astrology. Their underlying interest in understanding the order and structure of the universe was consistent with their belief in alchemy and astronomy. This suggests the need to recognize that our understanding, like theirs, is contextualized in the world as we know it.
Galileo and the Telescope
The invention of the telescope played an important role in advancing our understanding of Earth’s place in the cosmos. While there is evidence that the principals of telescopes were known in the late 16th century, the first telescopes were created in the Netherlands in 1608. Spectacle makers Hans Lippershey & Zacharias Janssen and Jacob Metius independently created telescopes. The telescope emerged from a tradition of craftsmanship and technical innovation around spectacles and developments in the science of optics traced back through Roger Bacon and a series of Islamic scientists, in particular Al-Kindi (c. 801–873), Ibn Sahl (c. 940-1000) and Ibn al-Haytham (965–1040).
The story of Galileo’s telescopic observations illustrates how a tool for seeing and collecting evidence can dramatically change our understanding of the cosmos.
Early telescopes were primarily used for making Earth-bound observations, such as surveying and military tactics. Galileo Galilei (1564-1642) was part of a small group of astronomers who turned telescopes towards the heavens. After hearing about the “Danish perspective glass” in 1609, Galileo constructed his own telescope. He subsequently demonstrated the telescope in Venice. His demonstration of the telescope earned him a lifetime lectureship.
After his initial success, Galileo focused on refining the instrument. The initial telescope he created (and the Dutch ones it was based on) magnified objects three diameters. That is, it made things look three times larger than they did with the naked eye. Through refining the design of the telescope he developed an instrument that could magnify eight times, and eventually thirty times.
This increased magnification of heavenly objects had a significant and immediate impact. These new observations were by no means exclusive to Galileo. The story of Galileo and the telescope is a powerful example of the key role that technologies play in enabling advances in scientific knowledge. With that said, the telescope isn’t the only technology at play in this story. Galileo deftly used the printed book and the design of prints in his books to present his research to the learned community. This is not a story of a lone thinker theorizing and piecing together a new model of the cosmos. Quite the contrary, an array of individuals in the early 17th century took the newly created telescopes and pointed them toward the heavens. Unlike those other observers, however, Galileo rapidly published his findings. In some cases, Galileo understood the significance and importance of these observations more readily than his contemporaries. It was this understanding, and foresight to publish, that made Galileo’s ideas stand the test of time.
Starry Messenger, Galileo’s Rapidly Published Findings
Shortly after his first telescopic observations of the heavens, Galileo began sketching his observations. He wanted to get his findings out. His observations and interpretations of stars, the moon, Jupiter, the sun and the phases of the planet Venus, were critical in refining our understanding of the cosmos. In March of 1610, Galileo published the initial results of his telescopic observations in Starry Messenger (Sidereus Nuncius), this short astronomical treatise quickly traveled to the corners of learned society.
The Moon is not a Perfect Sphere
The engravings of the Moon, created from Galileo’s artfully drawn sketches, presented readers with a radically different perspective on the Moon. Due to Galileo’s training in Renaissance art and an understanding of chiaroscuro (a technique for shading light and dark) he quickly understood that the shadows he was seeing were actually mountains and craters. From his sketches, he made estimates of their heights and depths. These observations, only possible by the magnifying power of the telescope, clearly suggested that the Aristotelian idea of the Moon as a translucent perfect sphere (or as Dante had suggested an “eternal pearl”) were wrong. The Moon was no longer a perfect heavenly object; it now clearly had features and a topology similar in many ways to the Earth. The notion that the moon had a topology like the Earth led to speculation on what life might be like on the Moon.
It’s now understood that English astronomer Thomas Harriot, (1560-1621) made the first recorded observations of the Moon through a telescope, a month before Galileo in July of 1609. Moreover, the map Harriot created of the Moon in 1612 or 1613 is more detailed than Galileo’s. Harriot observed the Moon first, and the maps he created included more information, but he did not broadly distribute his work. However, over 500 copies of the Starry Messenger were printed and sold, solidifying Galileo’s legacy in astronomy.
Jupiter Has Its Own Moons
When Galileo turned his telescope to observe Jupiter, he saw what he initially thought to be three previously unobserved fixed stars. After continued observations it became clear that they were not fixed, and in a matter of days he had come to the conclusion that these new stars were in fact orbiting Jupiter. He had discovered three of the largest moons of Jupiter.
The implications of this discovery, of objects orbiting a planet, were part of what pushed Galileo to argue for a sun-centered cosmos. Jupiter’s moons countered a key argument against the Earth orbiting the sun. Critics of Copernicus’ sun-centered cosmos asked, how could the Earth drag the moon across the heavens? Remember, the idea of the underlying mechanism of gravity wouldn’t come until Newton’s Principia Mathematica in 1687, which makes this both a reasonable and important question. Since there was wide agreement that Jupiter was already in motion, the fact that Jupiter clearly had its own moons offered a clear refutation of an important critique of the heliocentric system.
In Mundus Jovialis (1614), Simon Marius claimed that he, not Galileo, had first discovered the moons of Jupiter. In his times, Marius was publicly condemned as a plagiarist. Galileo had published his results already in 1610 and was rather well known and powerful in renaissance court. Only in the 19th century, would historians return to examine the evidence. It turns out that Marius had not plagiarized Galileo. Clearly his observations were different; in fact he had more accurately charted the orbits of Jupiter’s moons. It’s now broadly understood that Marius was an independent observer of Jupiter’s moons.
A Spotted Rotating Sun
In observing the sun, Galileo saw a series of “imperfections”. He had discovered sunspots. Monitoring these spots on the sun demonstrated that the sun in fact rotated. Furthermore, later observations by Francesco Sizzi in 1612 suggested that the spots on the sun actually changed over time. It would seem that the Sun, like the Moon, was not the perfect sphere that learned Europeans thought of as a key feature of their universe.
These sunspots were also independently observed by the Jesuit priest and astronomer Christoph Scheiner (1575-1650). Scheiner observed sunspots in 1611 and published his results in 1612. Over the course of their careers Galileo and Schiener feuded over who should get credit for the discovery. Unbeknownst to either of them, Thomas Harriot had observed them in 1610 and the German theologian, David Fabricius and his son Johanes likely beat both Scheiner and Galileo to the publication of the discovery with their Apparente earum cum Sole Conversione Narratio in June of 1611. However, their publication was not widely circulated and thus remained obscure in its times. Outside the western tradition of science. Chinese astronomers have long observed sunspots, going back to at least 165 BC.
Physical Astronomy for the Mechanistic Universe
Aristotelian cosmology was still present in 17th century understanding of the cosmos. This section briefly explores the contributions of Rene Descartes and Isaac Newton to the development of a new mechanical model for describing the relationship between heavenly bodies. In continental Europe, Rene Descartes theory of vorticies served as a powerful conceptual tool for theorizing the nature of the heavens. In England, Isaac Newton developed a universal theory of gravitation that would provide an underlying mechanism for describing a wide range of celestial and terrestrial motions.
The overlapping circles in Tycho Brahe’s geocentric model of the cosmos created a significant problem for the Aristotelian notions of the heavenly spheres. If Brahe was right and the orbits of the planets crossed each other each other then they couldn’t be a set of solid.
Rene Descartes offered a solution to this problem in his 1644 Principia Philosophiae. In Descartes system, like Aristotle’s, the universe was full of matter, there was no such thing as empty space. To explain motion Descartes introduced the concept of vortices. The system consisted of different kinds of mater or elements rubbing up against each other. His model included three different kinds of elements: luminous, transparent, and opaque. Luminous was the smallest and was what the stars were made of. Earth and the planets were made up of the denser opaque. The space between the planets and the stars was made up of transparent He stated that Lumnious would settle at the center of these vortices and the transparent and opaque elements would keep shifting around each other. This shifting created the movement of objects in the heavens.
Popularity of Cartesian Vortices
Descartes theory of vortices solved a series of existing problems for astronomers and philosophers. It was becoming increasingly difficult to keep the Aristotelian notions of solid crystalline spheres intact. Descartes vortices offered a new mechanism for explaining the movement of the heavens and filled a need to establish an underlying theory to support the new astronomy. As a result, Descartes’ ideas about vortices were widely adopted as a way of thinking about the cosmos.
This way of describing the heavens worked as a theoretical philosophy, but in practice was not particularly good at explaining phenomena. In hindsight, Descartes’ vortices served an important cultural role as an underlying philosophy for a Copernican sun centred theory, but very little of the ideas in this work continue on. The model of vortices played an important role in advancing the idea that the stars themselves are suns, and that there may be a plurality of planets orbiting those suns.
Newton’s Principia and the Genesis of Universal Gravitation
Isaac Newton’s 1687 Principia Mathematica Philosophiae Naturalis (Mathematical Principles of Natural Philosophy) generally referred to as the Principia, is often cited as one of the most important books in the history of the physical sciences. Where Descartes had offered an explanation of how a sun centred heavens could work with his theory of vortices, Newton offered a mechanical model of the cosmos anchored in a set of mathematical laws.
In the 1660s Newton had explored Kepler’s laws of planetary motion. By working with the orbit of the moon he had roughly confirmed the idea that force acting on objects diminished inversely by the square of the distance between the objects. That is, the closer objects are to each other the more they pull on each other. The data however wasn’t particularly strong to confirm his ideas, and he did not publish on this issue until twenty years after he made his discovery.
In 1684 Newton asked astronomer John Flamsteed for data on the orbits of Jupiter’s moons. One of the things he wanted to know is if there was any evidence to suggest that Jupiter might be affecting the orbit of Saturn. The question pointed to Newton’s developing ideas about an underlying universal theory of gravitation. He wanted to know if he could see if one planet was pulling on another.
Uniting the Celestial and the Terrestrial
Newton’s universal law of gravitation bridged the terrestrial and celestial realms in a single set of laws. By positing that an object’s gravity pulled on other objects Newton simultaneously explained the movement of the planets, the comets, the moon, the earth, and the tides in the oceans. Principia provided a logic that explained the behavior that Kepler had documented in his descriptive work on the movement of the planets. At its time of publication, the book was controversial. In particular, the idea that objects could act on each other across a distance through empty space was unsatisfying to many. This action at a distance just violated conventional thinking about how forces work in the world. Descartes’ model of the cosmos as vortices filled with matter was continued to be broadly popular in Europe and also had adherents in England.
In hindsight, the Principia is often cited as a defining moment in the development of modern science. The mention of “natural philosophy” in its full title suggests its important connections to the past as well. Our notion of “science” is very different and much more modern than Newton’s. Aside from his work on gravitation and optics, he had a range of research interests that we would not associate with science today. For example, Newton spent time conducting research in alchemy and working on biblical chronology. In the context of his time science and religion were intertwined and alchemy, biblical chronology, astronomy and mathematics were all legitimate ways to understand the natural world.
Competing Cosmological Models
In 1543, Copernicus suggested the sun was at the center of the cosmos. However, it was centuries before a sun-centered model became widely accepted. The history of science is often thought of as a procession of discoveries and advances. This obscures the complex stories of how theories and models can compete and coexist over long periods of time. When the Copernican model eventually won out it had been so extensively refined that the name “Copernican” doesn’t tell the whole story.
As evidence mounted in favor of a sun centered model, it remained one of many competing models for describing and explaining the heavens. By looking at how Tycho Brahe’s model, the Copernican model and the Ptolemaic model were represented in the 17th and 18th centuries, we will see the long period of competition between these models.
Weighing Cosmological Models in 1651
When Jesuit astronomer, Giovanni Battista Riccioli published his Almagestrum Novum or “New Almagest” the title alone suggested the boldness of the project. This was to be a new and updated take on Ptolemy’s Almagest. The book offered new insight into the state of thought about the cosmos in 17th century Europe.
The frontispiece to Almagestrum Novum illustrates Riccioli’s evaluation of three models of the universe. Discarded at the bottom left of the image is the Ptolemaic model. In the center Urania, the muse of astronomy, weighs a variant of Tycho Brahe’s revised Earth-centered model against Copernicus sun-centered model. Brahe’s model, in which the planets orbit the sun and the sun orbits the Earth, beats out Copernicus model in this evaluation. From Riccioli’s evaluation, the Earth-centered model of the cosmos was still the best choice.
In the book, Riccioli presents problems with the Ptolemaic, Copernican and Tychonic models and then offers a variant of Tycho Brahe’s model which he believes slightly more correct. For him, it made the most sense to have Mercury, Venus and Mars orbit the sun but still have Jupiter and Saturn orbit the Earth.
One of Riccioli’s arguments in favor of the Tychonic model was that since everything was created for humanity it simply made more sense that Earth would be at the center of creation. The argument about our place in the cosmos was a major philosophical issue that scientists, philosophers and theologians debated for centuries.
Beyond the argument about the centrality of humanity and Earth in the universe Riccioli leans on the authority of a number of contemporary and historical thinkers. He lists 38 different astronomers and thinkers, such as Aristotle, Ptolemy and others who believe the Earth to be the center of the universe. He compares them to the 16 astronomers, including Copernicus, Kepler, and Descartes, who favor a sun centered model. The frontispiece and text of Almagestrum Novum show that in 1651 there was still a vibrant debate between the merits of competing models for the cosmos, and suggests that the Earth centered model was still favored.
Ongoing Model Competition and Refinement
Illustrations juxtaposing the Copernican, Tychonic and Ptolemaic models of the universe are found on maps and atlases well into the 18th century. René Descartes ideas about the universe as a series of vortices and Isaac Newton’s theory of universal gravitation had provided a robust framework in support of a sun centered solar system. However, as the illustrations in a range of sources show, competing models of the structure of the heavens were still being presented as viable explanations into the 18th century.
Reviewing illustrations of the cosmos over time reveals the gradual acceptance and refinement of celestial models. Many of these gradual shifts are evident in Johann Doppelmayr’s Systema Solare et Planetarium. Printed as part of an atlas in 1742, this presentation of a sun-centered system includes a wealth of information on understanding the cosmos at the time. For example, it includes 4 of Jupiter’s moons and 5 of Saturn’s.
Some of the most interesting details in this illustration are tucked away in the corners. In the upper right corner, among the clouds, are small representations of additional solar systems. Beyond the central diagram, the mapmaker shows the concept of the plurality of worlds. Each of these little sets of circles represents its own solar system with a star and planets. This image directly draws on the literary author, de Fontenelle, who building on the ideas of Newton and Descartes’, explored the significance of living in a universe with a plurality of worlds each orbiting their own stars. For comparison, see the frontispiece from de Fontenelle’s 1686 book Conversations on the Plurality of Worlds for similar visual representations.
In the bottom right corner (shown larger here) are the Ptolemaic, Tychonic and Copernican models of the cosmos. The models are arranged from left to right and upward, communicating progress. This illustration is in direct contrast to the frontispiece from Riccioli’s Almagestrum Novum, which showed the Tychonic model winning out over the Copernican when weighed on the scale.
The story of models of the solar system in the 17th and 18th centuries shows how competing explanations and theories can persist over considerable periods of time. The sun-centered model was gradually accepted and promoted, but only after a range of evidence and theory converged to support and substantially refine it.
Stars as Suns and the Plurality of Worlds
What does it mean for a planet to be a world? How did we come to understand that our sun is just another one of the stars? Many are familiar with the shift from an earth-centered cosmos to a sun centered one. In parallel to that story, there is a story of a plurality of worlds and the realization that each star in the sky is a sun like our own but incredibly far away.
The story touches on ideas of life on the moon, on clever advances in measuring distance, and the astonishing ability to figure out what things are made of based on the light they give off. In the later half of the 17th century, Descartes’ ideas of vortices and Newtonian mechanics begged a vexing question. If the fixed stars were not part of an outer shell around our solar system but instead were suns like our own, then what was our place in the cosmos? What exactly was the scale of this new universe?
An Infinite Universe Teaming with Worlds
What is a world? In part, spurred on by Copernicus’s ideas, the Dominican Friar Giordano Bruno published De l’infinito universon e mondi (On the Infinite Universe and Worlds), in 1584. As part of a suite of mystical, magical and heretical ideas, he suggested that Earth was one of many inhabited worlds in an infinite universe and that the stars were suns, which had their own worlds.
From the World to Worlds
As a result of shifting views of the universe the very idea of “world” (in Latin, Mundi) was changing. In the Aristotelian cosmos, the world was effectively synonymous with the Earth. The sphere of the world and the terrestrial realm were one in the same. Once Earth became one planet among many orbiting the sun, those planets became Earth like worlds. This new understanding of worlds is reflected in the title of The Discovery of a World in the Moon from the 1630’s. As it took a long time for the Copernican model of the cosmos to win out over competing models, it took a considerable bit of time for ideas similar to Bruno’s to come to fruition.
The increasing acceptance of Descartes theory of vortices in the later half of the 17th century brought with it the idea that the stars were like our sun and had their own planets orbiting around them. Bernard le Bovier de Fontenelle’s popular 1686 book Entretriens sur la pluralite des mondes (Conversations on the plurality of worlds) broadly disseminated this notion, in a range of editions and translations. You can read a full-digitized copy External of an 1803 English translation of Conversations on the Plurality of Worlds online from the Library of Congress collections.
The Popularity of the Plurality of Worlds
The book Conversations on the Plurality of Worlds is organized, as the title suggests, as a series of conversations. The book presents fictional discussions between a philosopher and his hostess, a marquise. As the two characters walk the grounds of her garden at night they discuss the stars above them. Their conversations touch on the features of the Copernican system, potential encounters with extraterrestrial life and the idea of the universe as a boundless expanse. Written in this accessible format, it found a broad audience. As the book was translated into a variety of languages and republished in new editions for hundreds of years, it presented both this cosmology and the idea of life on other worlds to a range of audiences.
The popularity of this idea, the plurality of worlds orbiting their own sun like stars, is evident in the extent to which visual representations in the frontispiece of Fontenelles’ book appear in other places. Similar depictions of other solar systems shrouded in clouds beyond our own appear in a range of maps and diagrams. For an example of one of these depictions of small cloud shrouded solar systems, see the upper right corner of the 1742 Systema Solare Et Planetarium.
Stars and Their Worlds as the Third System
Changing ideas about the structure of the universe are well illustrated in diagrams from William Derham’s 1715 book Astro-Theology. Derham, an English natural philosopher, astronomer and clergymen wrote a series of works exploring connections between natural history and theology.
Derham provides a diagram of three systems of the cosmos, explaining that figure 3 shows “the Fixt Stars with their Systemes (represented by little Circles about those Stars, which Circles signify the Orbits of their respective Planets) are placed without the limits of the Solar Systeme, and the Solar Systeme is set in the Center of the Universe, and figured as a more grand and magnificent part there of.” From his perspective, the shift to thinking about the plurality of worlds was significant enough that it should be set alongside the Copernican Revolution as one of the three major shifts in thinking about the nature of the universe.
Exactly How Far Away Are the Stars?
The move away from thinking about the stars as being affixed to an outer shell of the universe was tricky, in part because the stars really look to move altogether. The inability to detect any form of stellar parallax, any relative motion of the stars, had been an issue for astronomers. Simply put, it looks like the stars are fixed. This had been a long-standing argument for the idea of an Earth centered universe. The idea being that if the Earth was moving one should expect that the stars should change their position relative to us. As we orbit the sun we would expect to see their position in the sky change as we got closer and further away from different individual stars. This is where the idea of parallax becomes important.
Measuring Stellar Parallax
If you think of those two opposite sides of the earth’s orbit as setting up two different lines of sight then we should see the star move relative to the other stars in that movement. Throughout the 16th century astronomers attempted to measure stellar parallax, but telescopes weren’t advanced enough to be able to detect parallax. What is particularly important about measuring stellar parallax is that it gives one the ability to calculate the distance to a star. The distance between the two extremes of the Earth’s orbit was known so the change in the location of the star enables one to calculate the distance to it.
In the 1830s, advances in the design of telescopes enabled scientists to detect parallax which kicked off a race to be the first to detect it. Ultimately it was Friedrich Wilhelm Bessel in 1838 who won the race and discovered that 61 Cygni had a parallax of 0.314 arcseconds. An arcsecond is 1/3600 of a degree in a circle. That gives you a sense of just how tiny that movement is to detect. Given the diameter of the Earth’s orbit suggested that the star was 10.4 lightyears away. That translates into roughly 61,000,000,000,000 miles away. He narrowly beat Fredrich Georg Wilhelm Struve and Thomas Henderson who, in the same year, measured the parallaxes of Vega and Alpha Centauri.
Prisms and Star Stuff
In 1835, the French philosopher Auguste Comte had suggested that, given that investigations of the stars would always be visual, we would never be able to know what the stars were made of. His perspective on the limits of science makes sense; however, he would quickly be proven wrong.
Working with prisms, natural philosophers in the 18th century had learned that light was made of a series of different colors that can be broken apart with a prism. The resulting colors from the prism are the light spectrum. Studying the spectrum of light from different stellar sources would offer profound results.
In the first half of the 19th century, chemist William Wollaston noticed and wrote about the existence of black lines in the solar light spectrum. In 1814, Joseph Fraunhofer found the same kinds of lines. Through studying the spectrum of light, he found a consistent bright fixed line that appeared in the orange color of the spectrum. Given that the lines did not seem to mark the boundaries of one color to the next, he dismissed the idea that these were boundaries between different colors. When he studied the spectrum of light from the sun, he found 574 dark lines in the solar spectrum. These lines were named Fraunhofer lines after him. Interestingly, he found that the star Sirius and several other stars differed in their spectral lines from the Sun.
The Chemistry of the Heavens Visible in Light
Many researchers studied the spectrum of flames in their laboratories. Gustave Kirchhoff and Robert Bunsen made a series of critical advances. In 1860 they published on research that showed that different elements emitted different colors of light in the spectrum. Connecting the studies of elements spectra in the laboratory with the known spectrum of the sun and stars enabled researchers to identify the chemical composition of the stars. In short, the dark lines in the spectrum offered a kind of fingerprint for the elements that make up a star. Some of the lines in the solar spectrum change depending on the placement of the sun in the sky, indicating that some of the lines are the result of the interference of the atmosphere. Given this, Kirchhoff was able to study the spectrum of the sun and argue that iron, calcium, magnesium, sodium, nickel and chromium were present in outer layers.
Further advances were made in 1863 when Italian Astronomer, Father Pietro Angelo Secchi went on to collect spectrograms of over 4,000 stars. He found that he could distinguish a number of distinct kinds of stars that had different spectral patterns. The other stars were suns, but they weren’t exactly like our sun. In his classifications he identified blue and white stars that had spectra like Sirus, stars that had spectra similar to the sun, red stars with bands, and carbon stars. Not only had astronomers figured out how to study what the sun was made of, they had also discovered that there were a range of different kinds of stars.
We now know that our sun is a star, but through increasingly sophisticated telescopes and some basic trigonometry we were able to figure out how far away the stars are. Beyond that, by understanding the properties of light we were able to deduce what the stars are made of. From our tiny home here on Earth we have been able to learn an amazing amount about the stars. Knowing just how far away they are opens up the immensity of our cosmos. However, even at this enormous scale, we still hadn’t come to realize that most of those stars were just in a small corner of the universe we call the Milky Way galaxy.
The Milky Way: One of the Many Galaxies
The idea that each star is a sun, many with their own solar systems, is a powerful reminder of the immense scale of the cosmos. However, the distances to stars in our galaxy are tiny in comparison to distances to other galaxies.
Since antiquity, observers have noted the existence of nebulous stars; diffuse smudgy or cloudy looking stars. Some of them turned out to be what we now know as nebulae, the places where stars form. Many turned out to be something else entirely. It wasn’t until the 1920s when it was confirmed that many of these nebulous stars were in fact completely different galaxies, whole other sets of billions of stars like the Milky Way, far beyond our own.
We now know the Milky Way is but one of the billions of galaxies in the universe. Looking back at how astronomy developed this concept over time one can see how philosophers and scientists struggled with comprehending the nature of galaxies, and thus the enormity of our universe.
The Milky Way Resolves into More Stars
To the naked eye it is unclear exactly what the Milky Way is. In ancient Greece, the atomist philosopher Democritus had proposed that the bright band of light might consist of distant stars. The atomists’ views were eclipsed by Aristotle’s perspectives on the universe.
In Aristotelian Cosmology, the Milky Way was understood to be the point where the celestial spheres came into contact with the terrestrial spheres. One of the important observations Galileo noted in his 1610 Sidereus Nuncius was that, under the view of a telescope, parts of the Milky Way resolved into a cluster of many stars. Once again a weakness in Aristotelian Cosmology was found – the Milky Way wasn’t the result of interactions between the terrestrial and celestial spheres. Galileo’s observations demonstrated the Milky Way was a massive grouping of individual stars, planets and other nebulous elements.
Island Universes and External Creations
In 1750, English astronomer Thomas Wright, published An original theory or new hypothesis of the Universe. In this book, Wright speculated that the Milky Way was a flat layer of stars, a part of which which was our solar system.
Beyond this he suggested that many of the very faint nebulae “in all likelihood may be external creation, bordering upon the known one, too remote for even our telescopes to reach.” The idea that the faint nebulae could be their own “external creations” suggested the universe was much large than previously imagined. In 1755, philosopher Immanuel Kant elaborated on Wright’s ideas and referred to these faint nebulae as “island universes.” Both the notions of external creations and island universes struggled to capture the implications of this new larger scale of the universe. Beyond the fact that our sun was a star, could nebulae be their own universes or completely separate creations?
Surveying the Milky Way
In the 1780s William Herschel surveyed the stars in a range of different directions. He found that the stars were much denser on one side of the sky than those of the other side.
His son John Herschel conducted a similar study of the sky in the southern hemisphere and found the same pattern. What they were seeing was the core of the Milky Way galaxy, where there is a much greater density of stars.
Herschel had placed our sun nearly at the center of the Milky Way; it wouldn’t be until the 1920’s when Harlow Shapley’s demonstrated that our sun was far from the center of the Milky Way.
Andromeda and Other Nebulae
Nebulous stars have been observed for thousands of years. In 964 Islamic astronomer Al-Sufi had observed and recorded what he called “a small cloud” in an illustration of the constellation Andromeda. We now understand this description as the Andromeda galaxy. Only with the advent and refinement of the telescope was it possible to start to document different kinds of nebulous stars.
As already mentioned, Thomas Wright and Immanuel Kant had published their speculations that faint nebulous stars like this were in fact independent entities like the Milky Way. In the late 18th century Charles Messier compiled a catalog of the 109 brightest nebulae, which was followed by a William Herschel’s much larger catalog of over 5,000. Even while documenting all of these nebulae it remained unclear as to exactly what they were.
Finding and Interpreting Red Shift
Studying the light spectrum of nebulae like Andromeda would ultimately provide the information about what exactly these objects were. A range of astronomers worked on this issue in the early 20th century. In 1912 astronomer Vesto Slipher studied the light spectra of some of the brightest nebulae. He was interested in determining if they were made of the kinds of chemicals one would expect to find in a planetary system.
Slipher found something very interesting – it is possible to calculate the relative speed and distance of a star or nebulae is moving by examining the light spectrum it gives off and seeing how much the indicators for elements have shifted into the blue or red color spectrum. Objects shifted blue are moving closer to us and red shifted objects are moving away from us. In Slipher’s analysis, the spectrums for the nebula were shifted so far into the red that these nebulae must be moving away from the earth at speeds beyond the escape velocity of the Milky Way. Along with this evidence, in 1917 Herber Curtis observed a nova, the brightening of an exploding star, inside the Andromeda Nebula. Looking back over photographs of the Nebula he was able to document 11 more novae that were on average 10 times fainter than those of the Milky Way. The evidence was mounting to suggest that these nebulae were well outside the Milky Way.
In 1920, Harlow Shapley and Heber Curtis debated the nature of the Milky Way, nebulae and the scale of the universe. Using the 100 inch telescope at Mt. Wilson, Edwin Hubble was able to resolve the edges of some spiral nebulae to identify they were in fact collections of stars, some of which matched standard patterns that enable astronomers to calculate that the stars were too distant to be part of the Milky Way. Thus, the idea of the Milky Way as just one of many galaxies came to be the dominant scientific perspective.
Where the Earth was once understood to be the center of a relatively small universe we have come to understand it as one world orbiting one of the 300 billion stars in our galaxy which is itself just one of more than a hundred billion of galaxies in the observable universe. Even today it remains difficult to grasp just how tiny and small our planet is in the vastness of the observable universe.