The end of the geocentric universe
The old ideas
Before the European Renaissance, the teachings of the Greek philosopher Aristotle dominated European thought. Lost for centuries following the fall of Rome, Aristotle’s works were re-introduced to Europe in Arabic translation during the 13th century CE. The power of Aristotle’s intellect led to the efforts of Thomas Aquinas to reconcile Aristotelian thought with Catholic dogma, with astounding success. Thus from that point what Aristotle had taught about physics and astronomy was pretty much what the dominant church in Europe taught.
Aristotle’s concepts of astronomy and physics were intertwined with his philosophy and concepts of social and political order, but briefly they were as follows:
- There is order in the universe: the baser elements lie at the bottom and the nobler elements at the top.
- The earth is the basest of all objects in the universe, therefore it is at the bottom.
- The earth is composed of four elements: air, water, fire and earth. These elements always seek each other, thus, air and fire rise (levity) and water and earth fall (gravity). Since water and earth have nowhere else to go, the earth (meaning the planet) must be at the bottom.
- Levity and gravity are natural motions. Once the materials have regained their rightful place, their natural tendency is to remain motionless.
- Sideways motion is caused by violent motion. Eventually this violent motion runs out and the object either rises or falls according to its nature.
- Violent and natural motion pertain only to the four elements.
- Celestial bodies are made of a fifth element called quintessence. Its natural behavior is to move in circles indefinitely around the earth. Thus, all celestial objects revolve around the earth.
- For these reasons (and others), the earth does not move, either by spinning or by moving through the heavens.
- The heavens are eternal and unchanging, whereas the earth is subject to decay and change.
Now in his defense Aristotle managed to develop a theory of celestial motion that rejected supernatural influences. Thus, gods did not pull the moon and sun, for example, in their daily paths. He had keen powers of observation, and to a certain extent, his explanation of motion makes some kind of sense. Certainly the universe he describes has a commonsense appeal to it. The casual observer can in fact feel as if everything revolves around the earth, and can in some sense understand the idea that the earth is remote and quite different from the materials of the heavens. Much of his philosophy paralleled existing Catholic thought about the place of humans and the earth in God’s universe.
Aristotle had no concept of inertia, so his argument about the earth’s immobility depends on this lack of understanding. He argued that a rotating earth would create fearsome winds, making it impossible for birds to fly and trees to stand. Apples falling from trees, moreover, would not fall directly below their branches, but yards away, as the rotating earth left them behind. (There is some question whether Aristotle ever observed sailors dropping objects from crow’s nests on ships. The objects, even on moving ships, fall directly below the nest.)
The Greeks did know the earth was a sphere and had a pretty good handle on its size. They also had some concept of the size of and distance to the moon, and realized the sun was at once much larger and much farther away than the moon, Mercury and Venus. Based on the information then available, and on his own theory, Aristotle’s universe had the earth surrounded by celestial spheres, embedded in which were, in order, the moon, Venus, Mercury, the sun, Mars, Jupiter and Saturn. Surrounding those was a sphere containing many holes corresponding to the stars, and beyond that was the celestial fire.
The old ideas begin to shudder
While elegant in its construction and argument, Aristotle’s theoretical universe had some practical difficulties. Principally, direct observation contradicted his contention that the planets orbited the earth in perfect circles. Most astrologers, astronomers, navigators and others realized that the planets Mars, Jupiter and Saturn at times seemed to perform loop-the-loops as they traveled across the night sky. This retrograde motion is apparent only during sustained observation of planetary motion over weeks or months, so perhaps Aristotle never noticed it, or perhaps he did and ignored it to save appearances. In any event, in the 1st century CE, a Greek in Alexandria amended Aristotle’s circular orbits to bring the theory more in line with observation. The result was a tome regarded by many as the best ephemeris available to civilization for milennia.
Ptolemy, as Aristotle, was influenced by the Pythagorean school, which held that the circle was the most perfect shape, indeed almost divine in nature. This philosophy required Ptolemy to adhere to circular celestial motion, but he amended Aristotle’s model to include circles within circles to explain retrograde motion. To account for all observed celestial behaviors, the Ptolemaic model demanded 90 or so circles, making the calculation of planetary positions exceedingly difficult. Nevertheless, the results were for the day supremely accurate, enabling those who depended on celestial positions to perform their duties much better than before. The Arab scientists who translated Ptolemy’s work from Greek into Arabic entitled it, Al Magest, “the greatest.” Ptolemy’s work is still known by that title even today.
There were still problems with the Aristotelian/Ptolemaic theory, however. By the 1500s, the civil and ecclesiastical calendar was falling out of sync with the annual motion of the sun. Without correction, the calendar, as developed by the Roman emperor Julian, would have the vernal equinox, for example, occurring in February rather than in March. This error demanded correction, and so the pope instructed a fairly obscure Polish cleric, Nicolai Koppernigk, to repeat Ptolemy’s efforts in astronometrics to provide the Church with better celestial data.
The beginning of the end for geocentrism
Koppernigk, who used the latinized version of his name, Copernicus, undertook careful measurements of the positions of the planets and other celestial bodies. At some point in his study, he apparently realized that calculating planetary positions would be exponentially easier if one assumed the sun was in the center of the planetary orbits and the earth the third planet out. Additionally, his calculations of the planetary orbits brought ancient astronomical models in line with more modern measurements, increasing the size of the known universe a hundredfold. He explained retrograde motion elegantly by noting that a faster object, the earth, will at times pass a slower object, say Mars, as the two orbit the sun. As the earth overtakes the slower planet, it will appear for a while as if the slower planet is moving backwards until their relative positions once again restore the “forward” motion of the observed planet.
Copernicus’s theory still depended on circles-within-circles, as Ptolemy’s did, but Copernicus had simplified matters by reducing the number of circles and thus the calculations. Historians debate whether Copernicus understood the sun was actually at the center of the universe or whether, as it states in the preface to his work, he merely adopted the convention as a mathematical “trick” to simplify calculations. It is clear, however, that it would have been dangerous for Copernicus to defy Catholic dogma openly, since these were the times of the Reformation and the Holy Inquisition. In the end, however, Copernicus died at nearly the same time his book was published. Initially, his proposal caused little havoc, and it led to a more accurate calendar, the Gregorian, the civil calendar we still use today.
The three horsemen, Brahe, Galileo and Kepler
In time, Copernicus’ work became the subject of hot debate among scholars in Europe. In a striking parallel to today’s evolution debates, scholars sided either with Copernicus or against him. Three in particular finally put the nails in geocentrism’s coffin by the early 1600s.
Tycho Brahe was a Danish nobleman and an influential astronomer for the day. He determined that comets and novae were not atmospheric phenomena as Aristotle had insisted, but celestial events. Using a technique called parallax, Brahe concluded comets orbited the sun among the planets and novae were out among the stars. His conclusions incidentally chipped away at Aristotle’s contention that the heavens were eternal and unchanging.
Brahe was given an island on which to construct an observatory. Since he predated the invention of the telescope, Brahe’s observatory consisted of huge protractors and sights to measure the positions of the planets, sun, moon and stars to unprecedented precision. Brahe’s motivation was to prove his own theory and disprove Copernicus’s theory of the universe. His efforts continued for nearly two decades, when he hired an Austrian mathematician, Johannes Kepler, to analyze the data.
Kepler realized the data supported the Copernican model more than his boss’s oddball model, but Kepler never really had to worry about presenting the news to Brahe. His boss died suddenly – legend has it of a burst bladder – and Brahe’s heirs gave Kepler his marching orders. Kepler did leave for Austria, but he also took Brahe’s data with him, an early example of corporate espionage.
After several years’ effort, Kepler threw out the circular orbit idea in favor of elliptical orbits, in one brilliant stroke eliminating both the necessity of complex calculations and preserving the essence of Copernicus’s genius. Kepler derived two algebraic relations explaining the variation in a planet’s orbital speed as it orbits the sun and the connection between average orbital radius and orbital period. He also suggested there was a universal force, akin to magnetism, that determined the planet’s behavior. Like Aristotle, however, Kepler had no notion of inertia, and so he proposed the sun also exerted a force in the direction of the planet’s motion to keep them moving.
While Kepler calculated, Galileo observed. Attacking Aristotle on two fronts, Galileo, an Italian mathematics professor, disproved by experiment Aristotle’s concepts of motion and verified by observation Copernicus’s model of the universe. Galileo threw out the idea of materials moving according to their nature in favor of an algebraic, universal law that affected all materials equally. He developed the concept of inertia, explaining that objects do not require a constant force to remain in motion. He was thus able to counter arguments that a rotating, moving earth was impossible. Using the new invention, the telescope, Galileo found craters on the moon, four previously unknown moons of Jupiter, spots on the sun’s surface, and the phases of Venus. This evidence, Galileo argued, supported Copernicus without a doubt and shattered Aristotelian theory. His continued efforts to prove his points and disprove Aristotle’s ran him afoul of Catholic authority. Eventually, under threat of excommunication and imprisonment, Galileo was forced to recant his support of Copernicus. The Church subsequently added Copernicus’ book to the list of works banned to Catholics, where it stayed for nearly 200 years.
Heliocentrism wins the day
Sir Isaac Newton in the 1680s finally brought geocentrism down. Unifying the efforts of Galileo and Kepler, Newton demonstrated that the universe obeyed simple universal laws, which he codified into three laws of motion and the law of universal gravitation. Using Newton’s and Kepler’s laws, astronomers were able to verify that comets did indeed revolve around the sun in predictable orbits and were able to discover two new planets, Uranus and Neptune, by the beginning of the 19th century. It was now clear that Aristotle’s physics and astronomy were false. They had no power of prediction, despite their elegance. The Church removed Copernicus’ work from the list of banned literature, and Aristotle’s geocentric theory became a footnote in scientific history. It just took nearly 2500 years.