Astronomy:The Solar System and Beyond 5th edition Michael Seeds
The passions of astronomy are no less profound because they are not noisy. JOHN STEINBECK The Short Reign of Pippin IV Chapter 4
Astronomy had its beginnings in simple human curiosity about lights in the sky. As early civilizations grew and developed, great philosophers struggled to understand the movements of the sun, moon, and planets. Later, mathematical astronomers made precise measurements and computed detailed models in their attempts to describe the motions of the heavenly bodies. The passions of astronomy gripped some of the greatest minds in history and drove them to try to understand the sky.
As you study the history of astronomy, you will notice that two themes twist through the story. One theme is the struggle to understand the place of Earth. It seemed obvious to the ancients that Earth was the center of everything but, as you now know, that is not true. The debate over the place of Earth involved deep theological questions and eventually led Galileo before the Inquisition.
The second theme is the quest to understand planetary motion. Astronomers built elaborate mathematical models, but they could not predict precisely the motion of the visible planets along the ecliptic. That mystery was finally solved when Isaac Newton described gravity and orbital motion in the late 1600s.
Only a few centuries ago, astronomers were struggling to understand the sky and, in the course of their struggles, they invented a new way of understanding nature—a new way of knowing about the physical world. That new way of knowing was based on the comparison of theories and evidence. Today, that new way of knowing is called science.
The great philosophers of ancient Greece wrote about many different subjects, including what they saw in the sky. Those writings became the foundation upon which later astronomers built modern astronomy. Classical Astronomy
You have probably heard of the two greatest philosophers of ancient Greece. In addition to their other achievements, Plato and Aristotle both influenced the history of astronomy. The Aristotelian Universe
The Aristotelian Universe • Plato (427?–347 BC) wrote about moral responsibility, ethics, the nature of reality, and the ideals of civil government. • His student Aristotle (384–322 BC) wrote about almost every area of knowledge and eventually became so famous that he was known during the Middle Ages as ‘The Philosopher.’ • Plato and Aristotle established the first widely accepted ideas about the structure of the universe.
Science and its methods of investigation did not exist in ancient Greece. So, when Plato and Aristotle turned their minds to the problem of the structure of the universe, they made use of a process common to their time known as reasoning from first principles. The Aristotelian Universe
The Aristotelian Universe • A first principle is something that is obviously true. • Once a principle is recognized as true, whatever can be logically derived from it must also be true. • However, what was obviously true to the ancients is not necessarily so obvious to us today.
When you study ‘The Ancient Universe,’ you will notice three important ideas that show how first principles influenced early descriptions of the universe and its motions. First, ancient philosophers and astronomers accepted as first principles that Earth was located at the center of the universe and that the heavens moved in uniform circular motion. The Aristotelian Universe
Second, you will notice how the evidence—the observed motion of the planets—did not fit the ancient model very well. The retrograde loops the planets made were very difficult to explain using geocentrism and uniform circular motion. The Aristotelian Universe
Finally, you will notice how Claudius Ptolemy attempted to explain the motion of the planets by devising a small circle rotating along the edge of a larger circle that enclosed a slightly off-center Earth. He even allowed the speed of the planets to vary slightly as they circled Earth. In these ways, he weakened the principles of geocentrism and uniform circular motion but did not discard them entirely. The Aristotelian Universe
Ptolemy lived roughly five centuries after Aristotle in the Greek colony in Egypt and, although Ptolemy believed in the Aristotelian universe, he was interested in a different problem: the motion of the planets. He was a brilliant mathematician, and he used his talents to create a mathematical description of the motions he saw in the heavens. For him, first principles took second place to mathematical precision. The Aristotelian Universe
The Aristotelian Universe • Aristotle’s universe, as embodied in Ptolemy’s mathematical model, dominated ancient astronomy but it was wrong. • The universe is not geocentric, and the planets don’t follow circles at uniform speeds. • At first, the Ptolemaic system predicted the positions of the planets well but, as centuries passed, errors accumulated.
Islamic and later European astronomers tried to update the system, computing new constants and adjusting epicycles. In the middle of the 13th century, a team of astronomers, supported by King Alfonso X of Castile, studied the Almagest for 10 years. Although they did not revise the theory very much, they simplified the calculation of the positions of the planets using the Ptolemaic system. They then published the result as The Alfonsine Tables, the last great attempt to make the Ptolemaic system of practical use. The Aristotelian Universe
Why did classical astronomers conclude the heavens were made up of spheres? Classical astronomers did not use evidence and hypotheses the way modern scientists do. Rather, they argued from first principles, and Plato argued that the perfect geometrical figure was a sphere. Then, the heavens, which everyone agreed were perfect, must be made up of spheres. Building Scientific Arguments
The natural motion of a sphere is rotation, and the only perfect motion is uniform motion. So, the heavenly spheres were thought to move in uniform circular motion. Thus, classical philosophers argued that the daily motion of the heavens around Earth and the motions of the seven planets (the sun and moon were counted as planets) against the background of the stars, had to be produced by the combination of uniformly rotating spheres, carrying objects around in perfect circles. Building Scientific Arguments
Although ancient astronomers didn’t use evidence as modern scientists do, they did observe the world around them. What observations led them to conclude that Earth didn’t move? Building Scientific Arguments
Copernicus • You would not have expected Nicolaus Copernicus to trigger a revolution in astronomy and science. • He was born in 1473 to a merchant family in Poland. • Orphaned at the age of 10, he was raised by his uncle, an important bishop, who sent him to the University of Cracow and then to the best universities in Italy.
Copernicus • He studied law and medicine and pursued a lifelong career as an important administrator in the Church. • Nevertheless, he had a passion for astronomy.
If you had been in astronomy class with Copernicus, you would have studied the Ptolemaic universe. The central location of Earth was widely accepted, and everyone knew that the heavens moved by the combination of uniform circular motions. For most scholars, questioning these principles was not an option because, over the course of centuries, Aristotle’s proposed geometry of the heavens had become linked with the teachings of the Christian Church. The Copernican Model
The Copernican Model • According to the Aristotelian universe, the most perfect region was in the heavens and the most imperfect at Earth’s center. • This classical geocentric universe matched the commonly held Christian geometry of heaven and hell. • Anyone who criticized the Ptolemaic model was questioning Aristotle’s geometry and indirectly challenging belief in heaven and hell.
Copernicus studied the Ptolemaic universe and probably found it difficult at first to consider alternatives. Throughout his life, he was associated with the Church. His uncle was an important bishop in Poland and, through his uncle’s influence, Copernicus was appointed a canon at the cathedral in Frauenberg at the unusually young age of 24. This gave Copernicus an income, although he remained at the universities in Italy. The Copernican Model
The Copernican Model • When he left the universities, he joined his uncle and served as his secretary and personal physician until his uncle died in 1512. • At that point, Copernicus moved into quarters adjoining the cathedral in Frauenburg, where he served as canon for the rest of his life.
His close connection with the Church notwithstanding, Copernicus began to consider an alternative to the Ptolemaic universe, probably while he was still at university. Sometime before 1514, he wrote an essay proposing a heliocentric universe in which the sun, not Earth, was the center of the universe, and that Earth rotated on its axis and revolved around the sun. He distributed this commentary in handwritten form, without a title, and in some cases anonymously—to friends and astronomical correspondents. The Copernican Model
He may have been cautious out of modesty, out of respect for the Church, or out of fear that his revolutionary ideas would be attacked unfairly. Although his essay discusses every major aspect of his later work, it does not include observations and calculations to add support. His ideas needed further work, and he began gathering observations and making detailed calculations in order to publish a book that would demonstrate the truth of his revolutionary ideas. The Copernican Model
De Revolutionibus • Copernicus worked on his book, De Revolutionibus Orbium Coelestium, over a period of many years and was essentially finished by about 1530. • However, he hesitated to publish it, even though other astronomers knew of his theories.
One reason he hesitated was that the idea of a heliocentric universe was highly controversial. This was a time of rebellion in the Church. Martin Luther was speaking harshly about fundamental Church teachings and others—both scholars and scoundrels—were questioning the authority of the Church. Even matters as abstract as astronomy could stir controversy. Remember, Earth’s place in astronomical theory was linked to the geometry of heaven and hell. So, moving Earth from its central place was a controversial, and perhaps heretical, idea. De Revolutionibus
De Revolutionibus • Another reason he may have hesitated was that his work was incomplete. • His model could not accurately predict planetary positions.
Copernicus was clearly concerned about how his ideas would be received. However, in 1540, he allowed the visiting astronomer Joachim Rheticus (1514–1576) to publish an account of the Copernican universe in Rheticus’s book, Prima Narratio (First Narrative). In 1542, Copernicus sent the manuscript for De Revolutionibus off to be printed. However, he died in the spring of 1543—before the printing was completed. De Revolutionibus
De Revolutionibus • The most important idea in the book was the placement of the sun at the center of the universe. • That single innovation had an astonishing consequence: the retrograde motion of the planets was immediately explained in a straightforward way without the large epicycles that Ptolemy used.
De Revolutionibus • In the Copernican system, Earth moves faster along its orbit than the planets that lie farther from the sun. • Consequently, Earth periodically overtakes and passes these planets.
Imagine that you are in a race car, driving rapidly along the inside lane of a circular race track. As you pass slower cars driving in the outer lanes they fall behind, and if you did not know you were moving, it would seem that the cars in the outer lanes occasionally slowed to a stop and then backed up for a short interval. The same thing happens as Earth passes a planet such as Mars. De Revolutionibus
De Revolutionibus • Although Mars moves steadily along its orbit, as seen from Earth, it appears to slow to a stop and move westward (retrograde) as Earth passes it. • Because the planetary orbits do not lie in precisely the same plane, a planet does not resume its eastward motion inprecisely the same path itfollowed earlier. • Consequently, it describes a loop whose shape depends on the angle between the orbital planes.
De Revolutionibus • Copernicus could explain retrograde motion without epicycles, and that was impressive. • The Copernican system was elegant and simple compared with the whirling epicycles and off-center equants of the Ptolemaic system.
However, De Revolutionibus failed to disprove the geocentric model for one critical reason. The Copernican theory could not predict the positions of the planets any more accurately than the Ptolemaic system could. To understand why it failed this critical test, you must understand Copernicus and his world. De Revolutionibus
Although Copernicus proposed a revolutionary idea in making the planetary system heliocentric, he was a classical astronomer with tremendous respect for the old concept of uniform circular motion. In fact, he objected strongly to Ptolemy’s use of the equant. It seemed arbitrary to him, a direct violation of the elegance of Aristotle’s philosophy of the heavens. He called equants ‘monstrous’ in that they violated both geocentrism and uniform circular motion. De Revolutionibus
In devising his model, Copernicus returned to a strong belief in uniform circular motion. Although he did not need epicycles to explain retrograde motion, he discovered that the sun, moon, and planets suffered other smaller variations in their motions that he could not explain with uniform circular motion centered on the sun. De Revolutionibus
Today, astronomers recognize that those variations are typical of objects following elliptical orbits. However, Copernicus held firmly with uniform circular motion. So, he had to introduce small epicycles to reproduce these minor variations in the motions of the sun, moon, and planets. Because Copernicus imposed uniform circular motion on his model, it could not accurately predict the motions of the planets. De Revolutionibus
The Prutenic Tables (1551) were based on the Copernican model, and they were not significantly more accurate than the 13th century Alfonsine Tables that were based on Ptolemy’s model. Both could be in error by as much as 2°, which is four times the angular diameter of the full moon. De Revolutionibus
De Revolutionibus • The Copernican model is inaccurate. • It relies on uniform circular motion and thus does not precisely describe the motions of the planets.
De Revolutionibus • However, the Copernican hypothesis that the universe is heliocentric was correct, given how little astronomers of the time knew of other stars and galaxies. • The planets circle the sun, not Earth, so their universe was heliocentric.
Why that hypothesis gradually won acceptance, in spite of the inaccuracy of the epicycles and deferents, is a question historians still debate. There are probably a number of reasons, including the revolutionary temper of the times, but the most important factor may be the elegance of the idea. Placing the sun at the center of the universe produced a symmetry among the motions of the planets that is pleasing to the eye as well as to the intellect. De Revolutionibus
De Revolutionibus • In the Ptolemaic model, Mercury and Venus were treated differently from the rest of the planets. • Their epicycles had to remain centered on the Earth–sun line.
De Revolutionibus • In the Copernican model, all the planets were treated the same. • They all followed orbits that circled the sun at the center.
The most astonishing consequence of the Copernican hypothesis was not what it said about the sun, but what it said about Earth. By placing the sun at the center, Copernicus made Earth move along an orbit like the other planets. By making Earth a planet, Copernicus revolutionized humanity’s view of its place in the universe. This triggered a controversy that would eventually bring the astronomer Galileo Galilei before the Inquisition. That controversy, over the nature of scientific truth, continues even today. De Revolutionibus
Although astronomers throughout Europe read and admired De Revolutionibus, they did not immediately accept the Copernican hypothesis. The mathematics was elegant and the astronomical observations and calculations were of tremendous value. However, few astronomers believed, at first, that the sun actually was the center of the planetary system and that Earth moved. How the Copernican hypothesis was gradually recognized as correct has been called the Copernican revolution. It was not just the adoption of a new idea but a total change in the way astronomers think about the place of the Earth. De Revolutionibus
Why do we say the Copernican hypothesis was correct but the model was wrong? The Copernican hypothesis was that the sun, and not Earth, was the center of the universe. Given the limited knowledge of the Renaissance astronomers about distant stars and galaxies, that hypothesis was correct. The sun is at the center of our planetary system. That hypothesis produces an elegant description of the universe without large epicycles and equants and explains retrograde motion in a simple way. Building Scientific Arguments