The Copernican Revolution

FAS Astronomers Blog, Volume 31, Number 1.

Back in the old days, people would look up at the sky. They noticed that the Sun and the stars would rise in the east and set in the west. From month to month the stars would follow a similar patter as they rose earlier and set earlier each night. Over the course of a year, they would notice that the Sun would slowly shift its position against the background of stars moving through one constellation after another along a path called the ecliptic.

The planets, on the other hand, would wander against the background of stars. Mars completed a cycle over a period of months. Two others, Jupiter and Saturn, took years to finish one trip across the sky. From time to time, some of them would reverse course and exhibit a retrograde motion. The inferior planets (Mercury and Venus) had an even more complex cycle, sometimes appearing in the east before sunrise and sometimes in the west before sunset.

As far as the ancients could tell, they were standing on a firm and unmovable Earth while everything in the heavens moved about them. For centuries this was the prevailing view. Although, there was one “crazy guy” named Aristarchus of Samos. He had a notion that it was the Earth that was moving while the Sun and stars remained fixed. But no one paid much attention, so his “outlandish idea” was soon forgotten.

Ptolemy and The Almagest

The concept of what we now call a geocentric (earth-centered) universe was formalized with the work of Claudius Ptolemy around 150 A.D. He collected all the astronomical information of the day and compiled it into a single work depicting the prevailing view of the cosmos. The book, through some translations from Greek to Arabic to Latin, is known as the Almagest. It not only described Ptolemy’s geocentric view, but it was also mathematical in nature allowing for the prediction of the movement and position of heavenly bodies including the planets.

Ptolemy’s biggest challenge was to explain the retrograde movement of the planets. To make it all fit, he modeled the motion of each planet using a large circular orbit called a deferent and a small circle called an epicycle. A planet moves around an epicycle that in turn moves around a deferent with the Earth at the center. It was the epicycle that allowed the planets to move both west to east and east to west relative to the Earth.

However, the math didn’t quite work, and this model didn’t correctly predict the observed movement of the planets. Ptolemy had to go a bit further and add an eccentric, which is at center of the deferent and at a constant distance from the center of a planet’s epicycle. On one side of the eccentric is the Earth and on the other side is the equant, which is an empty point around which the planet’s epicycle moves at a constant speed. It was complex, but, at the time, the Almagest provided the best representation of how the cosmos worked and how the planets appeared to move.

Over the centuries to follow, there were a few fleeting thoughts about a non-geocentric cosmos, but Ptolemy’s view and the Almagest became almost universally accepted.

The Copernican Revolution

Some 1,400 years after Ptolemy, Nicholas Copernicus took a close look at the Almagest. He didn’t like something in it. It wasn’t the geocentric view of everything, rather it was Ptolemy’s use of the equant. Both Copernicus and Ptolemy thought that planets should move in perfect circular motions. He thought Ptolemy had violated this principle by allowing planets to move at a constant speed around the equant rather than the center of the deferent. So, Copernicus set out to fix the problem.

Copernicus soon began to question Ptolemy’s view of the heavens. He realized that placing the Earth at the center wasn’t going to work. He collected his initial thoughts in 1514 and compiled a small handwritten manuscript entitled The Little Commentary (Commentariolus). In it he introduced seven axioms about the motion of the planets.

1. The heavenly bodies do not revolve around the same center.
2. The Earth is not the center of the universe.
3. The Sun is the center of the planetary system.
4. The Earth is much closer to the Sun than to the stars.
5. The daily revolution of the heavens is due to the rotation of the Earth.
6. The annual motion of the Sun is due to the Earth revolving around the Sun.
7. The retrograde motion of the planets is due to the Earth’s motion.

Based on these axioms, Copernicus constructed the first fully heliocentric model of solar system (at least since Aristarchus) with the Sun at the center and the Earth along with the other planets revolving about it. Although he sent it out to several colleagues, it is possible that he published it anonymously and not everyone knew he was the author.

The Little Commentary was a short work with some background, but not the full mathematical treatment he wanted. Copernicus planned to expand it into a much more definitive thesis. He labored on it for many years, but just didn’t get around to publishing it. Maybe he was afraid of the reaction others would have to this “crazy idea” of a sun-centered cosmos. However, some sources suggest that he was afraid that he was wrong, or that he had made mistakes in his calculations and analysis. So, he kept it all to himself.

In 1538, Georg Joachim Rheticus, intrigued by Copernicus’ ideas, paid a visit. He planned on staying for a couple of weeks but remained with Copernicus for two years. It was a bit risky for Rheticus, who was a lecturer in mathematics at the University of Wittenberg. At the time, Wittenberg was the center of the Protestant revolution of Martin Luther. Copernicus, on the other hand, was a Catholic canon for the church in Frombork, Poland. Rheticus went anyway, traveling hundreds of miles and arriving on Copernicus’ door unannounced. Despite their different backgrounds they hit it off and began a long collaboration that lasted from 1539 to 1541.

Rheticus worked with Copernicus, and over time, he became more and more enamored with what Copernicus had created. After absorbing much of it, including the complex mathematics, he summarized it all in early 1540 as The First Report (Narratio prima). Here he laid out the groundwork for the more complete work to follow. In 1542, he also published the trigonometric sections of Copernicus’ work as On the Sides and Angles of Triangles (De lateribus et angulis triangulorum).

As Copernicus reviewed and revised his manuscript, Rheticus would gather it up and copy it. In doing so, he created a cleaner version, which would be available for publication. Rheticus left Copernicus in September 1541 with the finished manuscript and returned to Wittenberg. In May 1542, he delivered the manuscript to the printing house of Petreius in Nuremberg for the final printing and publication. However, during this time, Rheticus had taken up a new post as a full professor of mathematics at Wittenberg and later moved on to a post at the University of Leipzig. Unable to oversee the printing, the task was turned over to Andreas Osiander, who had previously corresponded with Copernicus and was known to both Rheticus and the printer.

Finally, in 1543, as Copernicus lay near death, his heliocentric model was published as On the Revolutions of the Heavenly Spheres (De revolutionibus orbium coelestium). Although, Copernicus had placed the Sun at the center, he still believed that planets moved in perfect circles. Therefore, his model of the solar system included the epicycles of Ptolemy, which were still needed to make the math work. Despite this, it was a revolution in the way the cosmos was viewed. The Earth no longer held the privileged position at the center, and it became just one of several planets orbiting the Sun.

Copernicus was probably worried that his “new cosmology” would upset the Catholic Church. First, he dedicated On the Revolutions to Pope Paul III. He also included a letter from Nicholas Schonberg, Cardinal of Capua, in which Schonberg specifically referred to the new cosmology and encouraged Copernicus to publish his work. There was also the anonymous forward (see below). It all worked out. Initially, the Catholic Church had little to say. On the Revolutions wasn’t added to the list of forbidden books until 1616, and then it wasn’t banned, but was “suspending pending corrections.”

The publication of On the Revolutions was not without controversy. A forward entitled “To the Reader, concerning the hypotheses of this work” had been added. It stated that “Since he cannot in any way attain to the true causes … for these hypotheses need not be true or even probable.” The forward was published anonymously, and, at the time, it was not clear if Copernicus wrote it himself or it was penned by someone else. Eventually, it was discovered that Osiander, rather than Copernicus, was the author. Osiander’s intent was to soften the potential antagonism of Copernicus’ opponents. However, according to Owen Gingerich (The Book Nobody Read, page 180), when Rheticus received copies of the book in 1543; he was so outraged that he proceeded to mark out the introduction with a red crayon. And, to make matters worse, the acknowledgments included in the final printing completely ignored Rheticus and his contributions.

Post Copernicus

Not everyone immediately accepted Copernicus’ ideas about the heavens. Owen Gingerich (The Book Nobody Read) notes that many of the astronomers of the day focused more on Copernicus’ mathematical analysis rather than on his sun-centered view of the cosmos.

Although the Catholic Church didn’t react much, the Protestants of Martin Luther had much more of a negative reaction. Their approach is referred to by Robert Westman as the “Wittenberg Interpretation“, which focused on the technical/mathematical aspects and not the underlying cosmological model. However, it was this interpretation that may have spread the word about Copernicus and On the Revolutions across Europe.

• Phillip Melanchthon published the 1549 textbook Doctrines of Physics (Initia doctrinae physicae) in which he utilized the mathematics. However, he rejected Copernicus’ underlying interpretation.
• Erasmus Reinhold was one of the most influential astronomers of his time and colleague of Rheticus at the University of Wittenberg. He compiled the Prussian Tables (Prutenic Tables) in 1551. These astronomical tables were also based on Copernicus’ method of calculating the position of astronomical objects. Reinhold, however, avoided the cosmological implications.
• Reinhold’s student Casper Peucer shared a similar view and approach.

Thomas Digges became one of Copernicus’ early supporters and, in 1576, compiled an English translation of three chapters and included a diagram of Copernicus’ model, but with the stars stretching out to infinity.

In 1582, Michael Maestlin published a popular textbook on astronomy, Epitome of Astronomy (Epitome astronomiae), based on Ptolemy’s model. However, he later taught classes at the University of Tübingen based on Copernicus’ model. He also added an appendix on the Copernican system to Johannes Kepler’s first book The Cosmic Mystery (Mysterium Cosmographicum) in 1596.

Several astronomers of the day, including Paul Wittich, Tycho Brahe, and Raymers Baer, took smaller steps and introduced geoheliocentric models with some of the planets orbiting the Sun, but the Sun still orbiting the Earth.

Paul Wittich’s view of the cosmos had Mercury and Venus orbiting the Sun with the rest of the planets orbiting the Earth. Rather than publish his work, it is documented as annotations in copies of Copernicus’ On the Revolutions, including the copy that eventually end up in the Vatican Library. Initially, it was thought that others had made the notations. However, Gingerich and Westman (The Wittich Connection) discovered that they were in fact made by Wittich himself.

Tycho Brahe is best known for his extensive catalog of stars and planets. However, he couldn’t quite accept Copernicus’s model, so he developed his Tychonic system, with the Sun and Moon revolving around the Earth, the Earth stationary, and the other planets revolving around the Sun. Tycho published this model in 1588 as the eighth chapter of his work Second Book About Recent Phenomena in the Celestial World (De mundi aetherei recentioribus phaenomenis liber secundus). It was this model of the solar system that Tycho thought he would be most remembered for, rather than for his catalog.

In 1588, Ursus (aka Reymers Baer) published his Fundaments of Astronomy (Fundamentum astronomicum) in which he described a planetary model very similar to Tycho’s. Ursus had visited Tycho at his observatory on the island of Hveen in 1584 and his work was published before Tycho’s. As such, Tycho was fully convinced that Ursus had stolen his ideas. He subsequently did everything he could to discredit Ursus including initiating a lawsuit that resulted in the destruction of many copies of Ursus’ Fundaments of Astronomy.

Copernicus Rules

In 1610, Galileo first pointed his telescope up toward the night sky. Over the span of a few weeks, he observed the four large moons of Jupiter as they orbited around the planet. Later that year, he observed the phases of Venus. With a few simple observations that we all do today the Copernican view of the heavens was vindicated.

It was Galileo who finally, in 1632, put the argument between the geocentric and heliocentric views in a definitive publication that addressed both sides with Dialogue Concerning the Two Chief Systems of the World – Ptolemaic and Copernican (Dialogo sopra i due massimi sistemi del mondo). The book is a discussion between Salviati, a supporter of the Copernican (heliocentric) system, and Simplicio an Aristotelian (geocentric) philosopher. A third character, Sagredo acts as a somewhat biased neutral observer.

For various reasons, Galileo soon found himself in front of the inquisition and was forced to recant his belief in the Copernican system. Galileo is alleged to have muttered “and yet it moves” (eppur si muove – nevertheless it moves) even as he publicly rejected the notion of the Earth revolving around the Sun. However, most think that this statement was apocryphal. He was then place under house arrest and remained there for the rest of his life.

Prior to Galileo’s 1610 observations and subsequent publications, Johannes Kepler had actually put the Copernican view on a firm theoretical foundation. Kepler seems to have embraced the Copernican view with the publication of The Cosmic Mystery (Mysterium Cosmographicum) in 1596. Later Kepler found his way into the employment of Tycho Brahe in 1600. Tycho assigned him the problem of determining the orbit of Mars. After a long period of trial and error, he came upon the answer that Mars (and every other planet) moves in an ellipse. In a series of publications, Kepler described the movements of the planets and introduced his three laws of planetary motion. With this, the epicycles of Ptolemy and Copernicus were banished forever.

The final nail in the coffin of the geocentric cosmos came with Isaac Newton, who developed a firm mathematical foundation for Kepler’s planetary laws and the heliocentric model. Newton published his theory of gravity in 1687 as Mathematical Principles of Natural Philosophy (Philosophiae Naturalis Principia Mathematica) or Principia. In it he articulated his three laws of motion and his law of gravity, which explained how everything moved including the planets.

Conclusion

Finally, it all made sense, and everything moved like clockwork. That is until Albert Einstein and a few others came along in the 20^th century, but that’s another story.