Galileo Was Wrong

That is the idea behind an odd website that I found which promotes the theory of geocentrism or the idea that the Earth is the center of the Solar System and that the Sun revolves around the Earth. Geocentrism was, of course, the idea held by every astronomer and scientist up until 1543 when Nicolaus Copernicus proposed his heliocentric, or Sun centered, model of the Solar System. For about a century there was a fierce debate among scientists and philosophers over the true structure of the universe. Heliocentrism won out, of course, and no educated person of the twenty-first believes that the Earth is at the center of the universe. Because of this, those who historically had supported Copernicus’s model, such as Galileo are held to be on the right side of science and history, while those who clung to the older geocentrism, such as many officials of the Roman Catholic Church, seem to have been backwards and on the wrong side. This website contends that in the controversy between Galileo and the Catholic Church, the Church was, in fact, in the right and Galileo was in the wrong, hence the title. The strange thing is that the website is actually correct, in a funny sort of way. Galileo really was in the wrong and the Church was right to be skeptical of Copernicus’s theories.

The controversy between Galileo and the Church has often been depicted as part of the never ending battle between the light of science and religious ignorance. It is generally accepted by historians today that Galileo’s troubles had far less to do with an alleged anti-science position taken by the Catholic Church and more to do with contemporary Italian politics and Galileo’s own irascible personality. What is generally less well known is that the Church had good scientific reasons to oppose Galileo. Neither Copernicus or Galileo had any way to prove that the Earth moves around the Sun. With his telescope Galileo did discover the four largest moons of Jupiter and the fact that Venus shows phases, which indicates that it orbits the sun. These discoveries were certainly  suggestive in that they showed that not everything in the sky directly orbited the Earth, but it was possible that while Venus and the Galilean satellites orbited the Sun and Jupiter, the Sun and Jupiter revolved around the motionless Earth. In fact, there wouldn’t be any conclusive proof that the Earth moves until eighty years after Galileo’s death when the astronomer James Bradley discovered the phenomenon of the aberration of light caused by the Earth’s motion through space.  By this time, there was hardly any doubt about the Earth orbiting the Sun.

Why were astronomers so quick to discard the millenia-old and common sense idea that the Earth rests motionless at the center of the universe without any direct proof? The answer is that a heliocentric Solar System better accounted for the motion of the planets. In order to understand this, we will have to go back to the origins of the science of astronomy.

No one knows where or when people began to really observe the night sky and take note of the motions of the heavenly bodies. It must have seemed obvious that the Earth was a flat surface with the sky a dome enclosing it. The Sun, Moon and stars rose into the sky moved from East to West across the sky and then set beneath the Earth. At some point, these early observers noticed that not all of the objects in the sky moved along with the background of stars. These objects seemed to wander about the sky, so the Greeks referred to them as planetes, or wanderers. There were seven of these “planets”. the Sun and Moon, and five star like objects that were named Mercury, Venus, Mars, Jupiter, and Saturn. It may seem strange to refer to the Sun and Moon as planets but they like the other planets, moved across the sky against the background of the fixed stars.

The Greeks and the Romans knew that the Earth is round and the Greek philosophers such as Plato and Aristotle held that the circle was the perfect shape. Because they believed that the Heavens were perfect and unchanging, as opposed to our corrupt and changing Earth, they believed that the seven planets orbited the Earth in perfect circles. This was the model proposed by the ancient Greek astronomers, especially the last and greatest of the Hellenistic astronomers, Claudius Ptolemy, who lived in the second century AD. For this reason the geocentric model is often called the Ptolemaic model.

There was a major problem with the model proposed by the ancient Greeks, the planets do not move in perfect circles across the sky from West to East against the background of the stars. The planets moved in different paths across the sky and at different speeds. Sometimes they seemed to move backwards in what was called retrograde motion.

This image was created as part of the Philip G...

The retrograde motion of Mars(Photo credit: Wikipedia)

As Earth passes Mars, the latter planet will t...

As Earth passes Mars, the latter planet will temporarily appear to reverse its motion across the sky. (Photo credit: Wikipedia)

 

This apparent retrograde motion is is observed because the planets revolve at varying distances from the Sun and so orbit at varying velocities around the Sun. The Earth being closer to the Sun than Mars travels faster than Mars and so occasionally overtakes the other planet. Venus and Mercury travel faster than Earth and overtake us occasionally. You might be able to get an idea of how this works by considering a group of cars travelling on an Interstate. If I am driving at 60 miles per hour and pass a car that is going at 55 miles per hour, that other car will seem to be going backwards even though we are both going in the same direction. As a car travelling 65 miles per hour passes me, I will seem to be moving backwards to them.

 

In order to account for these dependencies between Aristotelian theory and astronomical observations, Greek astronomers hypothesized that while the planets move in perfect circles, they also moved in smaller circles within the circles. Thus, the heavens were full of wheels within wheels. It was Claudius Ptolemy who developed this system to the form that was used in Medieval astronomy.

epicycle-epicycle

It is easy to disparage Ptolemy for developing such a cumbersome system of concentric circles, but remember he did not have the telescope or many of the instruments invented to observe the motions of the planets that came into use in later years. He certainly cannot be blamed for assuming that the Earth is motionless. After, we cannot feel the Earth move and if we had to go by our own personal observations, we could only conclude the same. In fact, Ptolemy’s system was able to predict the motions of the planets with a high degree of accuracy and this was what the ancient and medieval astronomers were most concerned with.

There is much more to say about how later generations of Islamic and European astronomers refined and improved Ptolemy’s model as better astronomical instruments were invented and how Ptolemy came to be at last dethroned, but I am afraid that will have to wait for another post.

 

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