Astronomy and the Cosmos

Wilbur Applebaum. The Scientific Revolution and the Foundations of Modern Science. Greenwood Press, 2005.

For the ancient Greeks, the traditional function of astronomy was to observe the celestial bodies systematically in order to develop geometrical techniques for the creation of tables that could be used to predict the positions of the Sun, Moon, and planets at different times of the year, as well as certain astronomical phenomena such as eclipses. The causes and explanation of celestial motions and appearances was the province of the natural philosopher or physicist, rather than the astronomer. This distinction would be eliminated in the course of the seventeenth century, and astronomers began to deal with both the prediction of the positions of objects in the heavens and larger cosmological issues.

The conception of the nature and structure of the universe as taught by Aristotle was dominant until the sixteenth century, as were the types of geometrical models employed by Claudius Ptolemy some centuries later. The work of Copernicus, however, set a new agenda for astronomers. It was followed by increased questioning of Aristotelian and Ptolemaic conceptions and practices, based on more precise observations than in the past, the revelations of the newly invented telescope, and new theories about the nature and causes of planetary motions.

The Universe of Aristotle and Ptolemy

The Aristotelian universe was finite, spherical, and eternal; medieval Aristotelians, however, both Muslim and Christian, conceived the universe as having been created. The Earth was at its center, and the Moon, Sun, planets, and the sphere of stars all revolved around it. The celestial bodies were each moved about the Earth by a number of uniformly rotating homocentric spheres. Unlike motions on the Earth or below the sphere of the Moon, the heavens were composed of a perfect and unchanging substance—the aether, or quintessence, in which all motions were circular and uniform.

Observations of the night sky had been undertaken by the ancient Greeks before Aristotle, and with much greater attention to detail after him. It was clear that the motions of the objects in the sky as observed did not entirely conform to Aristotelian principles. It was obvious, for example, that the notion of uniformly rotating homocentric spheres could not account for the varied lengths of the seasons and the apparent changes in planetary distances from the Earth. Changes at different times in the brightness of the planets and the apparent sizes of the Sun and Moon contradicted the idea that they were equidistant from the Earth at all times. Aristotle’s principles, nevertheless, were held to be true. In the practice of astronomy, however, some Aristotelian principles were ignored in order to predict, as effectively as possible, the positions of celestial objects. The greatest achievements in the ancient world in that effort were those of Claudius Ptolemy, who built upon and improved the work of his astronomical predecessors.

Ptolemaic Astronomy

The variation in length of the seasons and the apparent changes in the distances of the planets from the Earth at different times were accounted for geometrically by having the Sun and planets revolve uniformly about points a slight distance from the center of the Earth. Seen from the Earth, the celestial bodies, moving in their eccentric circles, therefore appear to move at different speeds at different times of the year. In addition, all the planets are seen periodically to reverse the directions in which they had been moving and then to resume their usual motions. These retrograde motions and the apparent changing distances of the planets and the Moon from the Earth were geometrically described by having them move uniformly on epicycles—circles whose centers were carried about on a deferent—a uniformly revolving eccentric circle.

To these techniques, Ptolemy, to further improve the accuracy of his geometrical calculating devices, added an equant, a point not at the center of an orbit, about which a planet moved uniformly.

These were all described in Ptolemy’s chief work on astronomy, which came to be known in the West by its Arabic title, Almagest. It remained the chief authority in astronomy for several centuries.

Ptolemaic Astrology

Ptolemy was also the author of the principal text in astrology, Tetrabiblos (Four Books). The determination of future events through observation of the orientation of the celestial bodies to one another had a lengthy history. Considered a branch of astronomy, astrology required the ability to determine the positions of the planets in the past or to forecast them. Natural astrology was concerned with forecasting the weather, the fates of kingdoms, and catastrophic events. Judicial astrology was concerned with the future of individuals, or their medical prospects, and required the plotting of horoscopes based on the moment of birth of the individual, or of his medical condition. Astrology became part of university curricula in the course of the Renaissance.

Efforts to Improve Ptolemaic Astronomy

In the Middle Ages and the Early Modern period astronomers frequently tinkered with the Ptolemaic devices to improve the accuracy of their predictions. As the states of Western Europe began increasingly to engage in commercial sea voyages, the accuracy of astronomical tables became more important because of their use in navigation. In the thirteenth century King Alfonso X of Castile commissioned the creation of more accurate tables than those in use, and the Alfonsine Tables, named for him, were widely used until the later sixteenth century. In the fifteenth century European astronomers began to pay greater attention to the details of Ptolemy’s Almagest. It became available in its original Greek, and a new translation, along with a manual explaining its use, was published. Astronomical observation, which had seriously lagged in the Middle Ages, was resumed, and useful data collected.

The geometrical procedures employing eccentrics, epicycles, and equants, however useful for the prediction of planetary positions, if taken to represent the real motions of the planets, contradicted the Aristotelian concept of the nature of the universe. The apparent loops in their orbits made by the planets in their retrograde motions and their apparent changing distances from the Earth were inconsistent with the principle of planetary motion from the center of the Earth at all times. Eccentrics and epicycles, however, did maintain the principle of uniform circular motion as characteristic of celestial events. The equant, in contrast, came to be seen as violating the principle of uniform motion because its planet’s uniform motion about a point not at the center of an orbit meant that the planet did not move uniformly with respect to the Earth.

The relationship between the useful geometrical calculating devices of the astronomers and the physical principles of the physicists remained a puzzling problem throughout the Renaissance. Although astronomy as an observational and predictive science was based on an assumption of certain physical principles—namely, the Earth at the center of the universe, and uniform circular motion in the heavens—it came to be thought of as a kind of applied mathematics. The real nature of the universe and of celestial events was seen as the province of the natural philosophers. It is not entirely clear, however, that this distinction was universally accepted. This distinction was an issue for some Muslim astronomers, and it would be an important one for Copernicus.

The Reform of Astronomy

Copernican Astronomy

Nicolaus Copernicus learned Aristotelian philosophy as a university student and also made himself a master of Ptolemaic astronomy, but elements of his proposed new astronomy violated some of the principles of both of his ancient predecessors. Among the aims in his goal of reforming astronomy were the elimination of the equant and the creation of a unified system to replace the individual independent calculating devices for each of the planets. In Ptolemaic astronomy a change in the parameters of one planet had no necessary effect on those of the others. Copernicus’ system, however, was a unified one where the relations among the planets constituted a whole. Moreover, Copernicus wished to show how certain coincidences resulting from Ptolemy’s procedures could be seen as consequences of his own system. He nevertheless continued to employ eccentrics, epicycles, and uniform circular motion in his heliocentric system. Copernicus asserted that he would be satisfied if his system were able to produce results as accurate as Ptolemy’s. In his elimination of the equant, Copernicus employed a geometrical device earlier used by a Muslim astronomer; it is unknown how he became acquainted with it. He also continued to have the planets riding on spheres. The earliest version of his ideas was in an anonymous manuscript entitled Commentariolus (A Little Commentary), which was circulated among a few individuals.

Copernicus ascribed three motions to the Earth, now conceived as revolving about the motionless Sun. His system was not truly heliocentric, for the planets along with planet Earth revolved about the center of Earth’s orbit, which was slightly distant from the Sun. The Earth also rotated; this accounted for day and night and the apparent daily motion of the Sun across the sky. Because the Pole Star retained its position in the northern part of the sky in the course of the annual revolution of the Earth, which rested on the surface of a sphere, it was necessary for the North Pole of the rotating Earth also to rotate in a small circle in the course of a year in order to keep pointing to the Pole Star.

The advantages of Copernican astronomy were several. In Ptolemaic astronomy, the positions of the Sun, Mercury, and Venus relative to the Earth were indeterminate; it was unclear whether the orbit of the Sun was above or below those of the two planets. In the Copernican system their distribution was unambiguous. While the outer planets could be seen in conjunction with the Sun, or at any angle up to 180° from it, Mercury was never seen farther from the Sun than at a maximum of about 28° and Venus at 48°. It was now apparent, with Earth as the third planet revolving about the Sun, why this was so. Further, the retrograde motions of the planets could now be explained as caused by Earth moving more rapidly than the inner planets and overtaking in its annual motion the slower-moving Mars, Jupiter, and Saturn. In Copernicus’ system there were six planets, not only the five observable with the naked eye. The nature of the planetary retrograde motions, as well as the sizes and times taken to complete the orbits of the planetary epicycles, could now be seen as reflecting the orbit of the Earth. Epicycles were therefore no longer necessary to fulfill the functions for which they had been invented. Copernicus retained them, however, solely for the purpose of the better prediction of planetary position. Finally, the Copernican system made it possible to determine the relative distances of the planets as proportions of the distance of Earth from the Sun.

Publication and Initial Reception of Copernicus’ Work

In 1539 Copernicus was visited by Georg Joachim Rheticus (1514-1574), a young astronomer from the University of Wittenberg who had heard rumors about Copernicus’ ideas. He became an early convert and urged Copernicus to publish. Concerned about the response to his radical ideas, Copernicus agreed to have Rheticus publish a preliminary account, which Rheticus did in 1540 in a work titled Narratio prima (A First Account). Its reception was favorable enough for Copernicus to agree to publish his full theory. Rheticus oversaw the printing of the first part of the manuscript, but the remainder of the task of supervising its publication was left to Andreas Osiander (1498-1552), a Lutheran minister. In an anonymous foreword to Copernicus’ De revolutionibus orbium caelestium (On the Revolutions of the Celestial Orbs), Osiander said that its geometrical techniques were an improvement over Ptolemy’s, but that the arrangement of the heavenly bodies in the work was not to be taken as reflecting reality. This argument was not the opinion of Copernicus or Rheticus. The book was published in 1543 and reached Copernicus as he lay dying. In 1551 Erasmus Reinhold (1511-1553), using Copernican models, published his Prutenic Tables, which were increasingly adopted as an improvement over the Alphonsine Tables.

Very few readers of the De revolutionibus during the sixteenth century disagreed with Osiander’s opinion. Yet the work was read by most astronomers, and many of them noted their comments on various parts of the text in their copies. It was reprinted in 1566, and it continued to receive detailed analysis throughout the second half of the sixteenth century. Astronomers at the University of Wittenberg, the chief institution of Lutheran higher education, gave it considerable attention. Although Copernicus’ heliocentrism was not accepted by those astronomers, his lunar theory, his elimination of the equant, and the tables derived from his work were considered an improvement over Ptolemy.

Problems of the Copernican System

Objections to its heliocentrism were both physical and religious. It clearly conflicted with a literal interpretation of passages in the Bible, but religious objections played no significant part in its reception until the early seventeenth century. Objections on physical grounds, however, were important. If the Earth rotates, why aren’t we and objects on its surface thrown off? Why don’t objects thrown aloft land to the west of the point from which they were thrown? And if the Earth revolves around the Sun, a star should be seen at slightly different angles when observed six months apart.

Such changes could not be observed, and they would not be until the nineteenth century. If Venus indeed circles the Sun, its brightness should vary considerably between its farthest and closest distances from the Earth; but it does not. Since, according to Aristotle, heavy bodies tend to fall to the center of the universe, which is also the center of the Earth, how do we explain falling bodies on an Earth revolving around the Sun?

To these objections Copernicus answered that objects on the Earth and those thrown aloft participate in the rotating motion of the Earth, giving them the ability to remain on the Earth or to return to the spot from which they had been thrown upward. The different angles at which a star is observed at six-month intervals are unobservable because the stars are much farther away than had been thought. The angles are too small to be observed by our instruments. Changes in the brightness of Venus do not show considerable variation because Venus has phases unobservable by us, and these phases are reduced in size as the planet approaches the Earth. Bodies fall to the center of a revolving Earth, according to Copernicus, because of a tendency for bodies separated from a globe to return to that globe when permitted to fall.

Tycho Brahe and Geoheliocentrism

A few individuals thought Copernicus’ hypothesis worth considering, but converts were rare. The most important astronomer after Copernicus, Tycho Brahe (1546-1601), was not among them. He devoted his life to the reformation of astronomy and held the most important consideration in that enterprise to be the creation of a series of new and precise observations of celestial bodies. Brahe’s observations of a nova that suddenly appeared in 1572 and of a comet in 1577 clearly showed that they were beyond the sphere of the Moon. Similar conclusions by other astronomers were important examples of a challenge to the traditional separation in function between the physicist and the astronomer; observations by astronomers had challenged a long-established principle of celestial physics. These observations seemed to contradict Aristotle’s ideas that the region beyond the Moon was changeless and that comets were sub-lunar phenomena. They eventually resulted in the abandonment of the concept of celestial spheres.

With the grant of an island just off Copenhagen by Frederick II, the King of Denmark, Brahe built an observatory, Uraniborg (Castle of Astronomy), which he staffed with assistants, a printing press, and instruments capable of the most precise measurements.

In the course of about two decades Brahe made a series of the most detailed and continuous observations of the celestial bodies ever carried out. Among Tycho’s astronomical achievements was an improvement in the ability to predict the positions of the Moon. With the accession to the throne of a new Danish king, Brahe was forced to leave Uraniborg; in 1599, and under the patronage of the Holy Roman Emperor, Rudolph II, he moved to a castle near Prague.

Many who rejected the Copernican theory, Tycho Brahe among them, nevertheless recognized that its advantages over Ptolemaic astronomy required a rethinking of the structure of the cosmos. Tycho could not accept the substitution of the Sun for the position of the Earth at the center of the universe, and he proposed a theory, known as geoheliocentrism, in which all the planets revolve around the Sun, which, in turn, revolves around the motionless Earth. This and a few similar geoheliocentric theories were seen for a few decades as a suitable alternative to the Ptolemaic and Copernican theories.

The Early Copernicans

Copernicus’ ideas impressed a few contemporaries, including some officials of the Catholic Church, but of those competent in astronomy, Rheticus must be seen as Copernicus’ first disciple. In the latter half of the sixteenth century, Thomas Digges (c. 1546-1595), was one of the few individuals open to the Copernican theory. He had carefully observed the nova of 1572 and concluded that it was beyond the sphere of the Moon. His A Perfit Description of the Caelestiall Orbes of 1576 included an English translation of parts of Book I of Copernicus’ De revolutionibus. It also had a diagram showing the stars infinitely extended and at varying distances from the Sun.

It may have been during a visit to England in the mid-1580s that Giordano Bruno (1548-1600), a Dominican monk who had begun to question certain Catholic theological opinions, first encountered the Copernican theory and the idea of an infinite universe. He published several works, the best known of which was La cena de le ceneri (The Ash-Wednesday Supper, 1584), giving his theological interpretations of the ideas of Copernicus, of an infinite universe and a plurality of worlds. After being imprisoned for several years and having been sentenced by the Inquisition, Bruno was burned at the stake in 1600 as a heretic, but not, it appears, for his Copernican beliefs.

Simon Stevin (1548-1620), who made important contributions to engineering, mechanics, mathematics, hydrostatics, and many other areas, was the first Copernican in the Low Countries. He published a book, De hemelloop (On Astronomy), in 1608, in which he showed the superiority of Copernicus’ heliocentric theory over Ptolemy’s geocentrism and made some slight improvements in the former. Stevin was but one of a small minority of heliocentrists in the first few decades after the publication of Copernicus’ path-breaking work.

Copernicanism Revised and Improved

Johannes Kepler, Planetary Orbits, and Their Causes

Among the few who accepted Copernicanism in the late sixteenth century were Michael Mästlin (1550-1631), professor of mathematics at the University of Tübingen, and his student Johannes Kepler (1571-1630). Kepler went beyond his teacher’s Copernicanism in insisting that part of the astronomer’s job was to try to account for not only how the planets moved but also why they did so. For Kepler the chief means to account for the motions of the planets lay in the nature of the Sun and in harmonic principles. His first book, Mysterium cosmographicum (The Cosmographic Mystery), published in 1596, held that the six planets and their distances from the Sun were related to the five perfect solids, and that the Sun played a role in the movements of the planets. After Tycho Brahe left Uraniborg and moved to Prague, Kepler joined him as an assistant. Upon Tycho’s death, Kepler acquired his observations, which proved decisive for his reformation of astronomy.

Kepler made significant changes and modifications in Copernicus’ theory. He published Astronomia nova … seu physica coelestis (A New Astronomy … or Celestial Physics) in 1609, based on his analysis of the orbit of Mars. In subsequent works, he applied his discoveries to the rest of the planets. The Sun played a special role in Kepler’s astronomy. Kepler discovered that each of the orbital planes of the planets intersected the Sun, and that the Sun lay in one of the foci of each of the elliptical orbits in which all the planets moved. Furthermore, an imaginary line from the Sun to a planet swept out equal areas in equal times. Planetary motion was therefore not uniform, but more rapid when close to the Sun, and slower when distant from it.

The Sun was also the cause of planetary motion. Kepler hypothesized that the Sun rotated and emitted quasi-magnetic forces that swept the planets around, alternately attracting and repelling them, thereby accounting for their elliptical orbits. Kepler also held that the tides are caused by the attraction of the Moon. He later discovered a mathematical law uniting all the planets of the solar system: that the cubes of each of the planetary distances were proportional to the squares of the times taken to complete their orbits. Here again, as in his earliest work, harmonics and harmonic proportions were influential in Kepler’s thinking.

Reception of Keplerian Astronomy

As had been the case with Copernican theory, Keplerian astronomy was slow to be accepted. Copernicanism was still rejected by most astronomers in the early years of the seventeenth century. Kepler’s area rule of planetary motion—that a line from an orbiting planet to the Sun sweeps out equal areas in equal times—could not easily be applied geometrically; it required the use of approximation, a method thought to be inappropriate in astronomy. Furthermore, Kepler’s speculations on the cause of planetary motion was also thought be an inappropriate part of the science of astronomy.

An important consideration in the subsequent acceptance of Kepler’s astronomy were the empirical successes of his Rudolphine Tables, named for his patron, derived from his theory, and published in 1627. The tables predicted planetary positions and celestial events such as eclipses and the passage of Mercury across the face of the Sun more accurately than any other tables then in use. By the middle of the seven-teenth century, many astronomers had been won over to planetary ellipses, non-uniform motion, and a role for the Sun in moving the planets. Kepler’s elimination of epicycles and the geometrical complexities employed by earlier astronomers were also noted as praiseworthy.

Galileo, the Telescope, and a New Mechanics

Another early Copernican, Galileo Galilei (1564-1642), however, was not convinced either of elliptical orbits or of the attractive powers of the Sun or Moon. His construction and use of a telescope, after his having heard that one had recently been invented, played a very important role in the further weakening of Aristotelian natural philosophy and the promotion of Copernicanism. Turning his telescope to the heavens in 1609, Galileo saw that the planets were more than points of light, that the Moon had mountains rather than a smooth surface, that Jupiter had moons, and that there were innumerably more stars than could be seen with the naked eye, raising the possibility that the stars were much farther than had long been thought. In the next few years it was discovered that Venus had phases, an observation discrediting the astronomy of Ptolemy, but not of Tycho Brahe. These findings were published in 1610 in a short treatise called Sidereus nuncius (The Starry Messenger). Galileo and others also discovered spots on the Sun. The observed movements of sunspots across the face of the Sun in the course of about a month, and the movements of the planets in their orbits in the same direction as the sunspots, led Galileo to hint at a role for the Sun in moving the planets; unlike Kepler, however, he refused to speculate further.

Galileo attempted to prove the motion of the Earth by showing that the tides could best be explained by the Earth’s daily rotation and annual revolution. He was unsuccessful in this endeavor, but the publication in 1632 in Italian of his Dialogue on the Two Great World Systems—Ptolemaic and Copernican indirectly made a strong case for the Copernican theory.

The publication of this work led to Galileo’s trial by the Inquisition for heresy; he was accused of having violated an earlier injunction forbidding the teaching of the Copernican theory. Galileo was forced to deny the Copernican theory and was confined for the rest of his life to his home near Florence. His Dialogue and Copernicus’ and Kepler’s works were placed on the Index of Prohibited Books; the Church forbade Catholics to read them without special permission. Galileo wrote nothing more on astronomy and cosmology, but he published his discoveries in the science of mechanics in his Dialogues on Two New Sciences (1638). In this work Galileo answered a number of objections to the idea of the Earth’s motion and demonstrated how the Earth could rotate without flinging off objects on its surface.

New Thoughts About Celestial Motions

On the Causes of Planetary Motion

During Galileo’s lifetime a few others promoted the Copernican theory or its Keplerian version with their own work. Ismaël Boulliau (1605-1694) accepted Kepler’s elliptical orbits because of the improved accuracy shown by the tables based upon them but rejected his area rule and proposed cause of planetary motion. Jeremiah Horrocks (1618-1641), a committed Keplerian, through his corrections to Kepler’s tables based on his own observations, was the first to predict and observe in 1639 the rare astronomical event of a transit of Venus across the Sun. His theory of the Moon was the most effective of its time, and he proposed a mechanical explanation instead of Kepler’s magnetic hypothesis of the cause of planetary motion. Another figure important in the transmission of the new astronomy was Giovanni Alfonso Borelli (1608-1679). Familiar with the work of both Galileo and Kepler, Borelli proposed that the planets tend to fly off from their orbits, but are attracted by the Sun, and are thus kept in orbital motion.

René Descartes (1596-1650), a Copernican who was familiar with Kepler’s and Galileo’s ideas, rejected the idea of attraction and forces. He held that the solar system, completely filled with imperceptible matter, was analogous to a whirlpool in which the planets, like something fallen into a whirlpool, are swirled around. Descartes also developed the idea of inertia—that all bodies in motion continue to move uniformly in a straight line unless struck or impeded by another body, a concept that would prove important in later theories of planetary motion.

Astronomers who accepted elliptical orbits had trouble dealing with Kepler’s area rule. To carry out the necessary calculations for the production of astronomical tables, a number of them adopted the practice of assuming that a planet moved uniformly about the empty focus of its elliptical orbit; others employed epicycles to generate elliptical orbits. Nonetheless, by the 1670s it was fairly well accepted that astronomers must concern themselves with the means of behind planetary movements as well as the ability to predict them.

Newton, Universal Laws of Motion, and Universal Gravitation

The causes of planetary motion were beginning to receive a good deal of attention by natural philosophers in the 1660s and 1670s. Robert Hooke (1635-1703) proposed that the planets were moved by both the principle of inertia and a centripetal force, a tendency to be attracted to the Sun. A few years later he suggested that the attractive force from the Sun operated according to the inverse-square of its distance. Edmond Halley (c. 1656-1743) showed that a comet becoming visible in 1680, moved in a parabolic orbit, was a periodic phenomenon and would return in about seventy years. His interest in the causes of celestial motions led him in 1684 to suggest to Isaac Newton that he try to prove mathematically how elliptical orbits could be derived from an inverse-square law of attraction and the principle of inertia. Newton undertook the task suggested by Halley, and the results were published in 1687 in his Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). In it Newton put forward what have become known as his three laws of motion and the law of universal gravitation, that all bodies in the universe attract all others with a force depending on their masses and the inverse-square of the distance between them. Even the Sun was no longer seen as motionless, since it both attracted the planets and was attracted by them. The tendency of bodies revolving about the Sun, when unconstrained, to fly off in straight lines with uniform motion, according to Newton’s first law—the principle of inertia—was counteracted by their attraction by the Sun. The work demonstrated that under different conditions, all celestial bodies could be seen as moving in various curvilinear paths, including ellipses and parabolas. Space, time, force, and mass became fundamental components in efforts to explain motions in both the heavens and the Earth.


The transformation of concepts about the nature and structure of the universe began with increased attention to the relationship between ideas about the causes of the celestial motions and the geometrical calculating techniques used for predicting celestial events. These concerns were supplemented by a significant increase in observational opportunities and precision that challenged long-held beliefs. It took over a century for the heliocentric theory to be accepted by astronomers and several decades for elliptical orbits and the role of the Sun in planetary motion to be accepted. The ancient axioms of uniform circular motion of the Moon and planets and of the unchanging perfection of the heavens were discarded, replaced by ellipses, earthlike mountains on the Moon, planetary satellites, and an uncountable number of stars. By the end of the seventeenth century, a single set of universal mathematical laws was seen to apply throughout an infinite universe. In subsequent generations these laws and continual improvements in the power of telescopes led to increasingly accurate predictions and new discoveries. The decisive event for the further progress of astronomy came with the publication of Newton’s Principia in 1687.