Focusing on the learning objectives will help you to study and understand the important concepts. Compare the objectives with the study questions for the lesson to be sure that you have the concepts under control. Before we're done with this lesson we will have seen how Aristotle's paradigm influenced the Greek culture in the city of Alexandria and eventually became forged into the Ptolemaic system through the contributions of the Hellenistic scientists with names like Aristarchus, Eratosthenes, Euclid, Hipparchus, and Ptolemy.
We'll see how the Ptolemaic system became the astronomy of choice, and how it differed from the cosmology of Aristotle. With the death of Alexander the Macedonian Empire fell apart. The Greek city states had lost the magic and no longer held the allure of the intellectual. With their decline, a new center of civilization began in the city of Alexandria, built by Alexander in honor of himself.
Who can really explain the details of social conditions that cause the decline of one region while another flourishes. History is full of such happenings, and it is not our purpose to trace their detailed demises and developments. It is important for this the purposes of this course to see how the ancient ideas adapted to the various values of these different cultures which modified them. Only then can we understand the importance of the great revolution in science, the topic of the seven lessons in section two.
The death of a leader of Alexander's strength, and the political glue and a philosopher of Aristotle's stature, who provides the philosophical strength of an empire can have devastating effects, especially when their lives were so closely linked and ended almost at the same time. Social unrest, dissent, threat of revolution, ethnic issues all arose. Think of the same effect some years ago, when an invading army of Macedonians accomplished what three generations of war with Sparta had not been able to do.
Ravaged by war, and torn by social dissent, the Greek culture was undermined by the new dominion of Phillip. There was little left of it by the time Alexander died, and without his leadership, which wasn't that good to begin with, it never made a comeback. The center of Greek culture shifted to Alexandria, a seaport on the Nile delta, on the opposite side from where the city of Cairo is today.
Greek mythology, grew from Alexander's efforts to spread the Greek culture as he conquered the world. After his death dynasties were established that brought back political disunity in the empire, while promoting the Greek unity of trade and learning. A new culture developed in art, letters, and science, in Alexandria and in other cities in Alexander's former empire.
In Alexandria the standard of living was high, especially for the well to do who were generally well educated. Literature was abundant and the concept of learning, as distinct from knowledge and philosophy, came into being. The Alexandrians built libraries, compiled anthologies, developed the art of sculpture and studied mathematics and science.
When life is good, there is more time for curiosity, just like in the early years of the golden age. So successful were they in spreading that Greek culture that the triumph of Rome was due in part for their ability to absorb rather than eradicate the Hellenistic influence. Two of the most famous scientists in Alexandria were Archimedes, whose understanding of buoyancy saved his king a fortune, and Euclid, whose geometry was so precise that it is still taught today almost exactly as Euclid presented it.
The mechanical arts such as painting and sculpture became popular as the disapproval of manual work disappeared. The making of instruments and maps had advanced to a very high level of sophistication by the first century A. A new interest in understanding the world and in doing experiments and taking measurements began to grow. The Library of Alexandria was the greatest repository of knowledge anywhere in the world, then and until the nineteenth century.
It was a huge and elegant building in the classical Greek style with large columns, you know the kind you see in Washington, D. Among other works, the library contained most if not all of Aristotle's writings. Some of them were original and only copies.
There were just too many of them to make enough copies for everyone. They have copying machines and faxes and other things hard as it might be for us to imagine a world without electronics. The library was destroyed in a series of raids over years. The first came when it was burned by Septemia Zenobia in A. Septemia Zenobia.
I love that name, it sounds like someone you would meet in the Starlight Cafe, or in a chat room on the intergalactic internet. I wonder what a little research on her would reveal. Who was she? The library was sacked in A. They burned tens of thousands of documents, Aristotle's writings no doubt among them.
We have no way of knowing what things were destroyed. To finish the job the library was completely destroyed in an invasion by Moslems from the east in A. Many of the documents were salvaged by the troops and made their way eastward where the new center of civilization was developing.
Aristarchus was one of the earliest of the Hellenistic scientists. His main contribution was his questioning of Aristotle's geocentric system. Aristarchus considered the possibility of a rotating earth revolving around the sun. He asked what kinds of effects we would observe. It was a wonderfully unbiased account, although he still considered only circular motion to be the only and true motion for the heavens. He did not collect data, he just considered what the universe would appear to do if the sun was the center of heavenly motion for earth and the planets.
More than anything else, Aristarchus started a debate which led to the rejection of the heliocentric theory and a strengthening of the geocentric theory of Aristotle.
It was a qualitative system. There were no calculations of planetary paths, and he offered no way to calculate them. It's like saying "on a merry-go-round it appears as if the earth is spinning around you, but you might really be spinning instead". On a rotating platform like a merry-go-round there are other clues that tell us we are moving and not the world around us.
We will explore this idea later. For now let's just note that the Greeks also looked for some signs of evidence for one theory over the other, but really couldn't find it. For one thing, there was no parallax. Parallax is what happens when you point at something with one eye closed then switch eyes.
Come on, try it. It's OK to point if it's for science. If you're family or friends think it's weird, just tell them it's for science, then they'll understand.
So what happens is that the finger you were pointing with can't line up with both eyes and the same object at the same time. Pointing with one eye closed keep your hand still and move your head from side to side. See, the finger doesn't point at the same place when you move because you are seeing the two objects, the finger and whatever it is lined up with from a different perspective. If the earth is moving in a circular orbit, then the same thing should happen with the background of stars when earth is in different parts of its orbit.
This is important because we know today that the stars are so far from us and so far apart that the parallax is too small to be seen without sensitive telescopes and tracking. In fact it was not observed until the nineteenth century despite the fact that the geocentric model had been discarded two hundred years earlier.
One type of parallax is alignment parallax. This is where two objects line up at one time and not at another as circular motion occurs. To see this, close one eye, hold up your index fingers on both hands, one in front of the other, like this demonstrate.
Now move your head from side to side and watch how the two fingers appear to move in relation to one another. The greater the distance apart try it the greater the apparent movement.
Angular parallax is similar, and a little harder to explain how to demonstrate it. I'll let you figure this out if you want to try it. Angular parallax does not require that the two objects ever line up. Whether or not they are lined up, the apparent angle between them will appear to change even if they are the same distance from us.
Besides that the heliocentric paradigm would not fit in with Aristotle's System of the World, which required homocentric and geocentric spheres to explain the motion of the planets. The equant is used to explain the observed speed change in different stages of the planetary orbit. This planetary concept allowed Ptolemy to keep the theory of uniform circular motion alive by stating that the path of heavenly bodies was uniform around one point and circular around another point.
So, Ptolemy appears superior on astronomical grounds, but is inferior philosophically. If Ptolemy is accepted, then Aristotelian physics is wrong. Defending the theory that vision is due to a flow emanating from the eye, Ptolemy analyzed the reflection of light on flat and spherical mirrors , and its refraction when it crosses the surface between two transparent media.
Brahe's Model of the Cosmos In Brahe's model, all of the planets orbited the sun, and the sun and the moon orbited the Earth. Keeping with his observations of the new star and the comet, his model allowed the path of the planet Mars to cross through the path of the sun. Capital-G Geocentrism is the belief that geocentrism is the only frame, the real one. Those who use relativity say that geocentrism can be right and is just as valid as heliocentrism or any other centrism.
That's correct! But the problem is that using relativity by definition means that there is no One True Frame. One problem with the geocentric model is that some planets seem to move backwards in retrograde instead of in their usual forward motion around Earth. Around A. The most highly developed geocentric model was that of Ptolemy of Alexandria 2nd century ce. It was generally accepted until the 16th century, after which it was superseded by heliocentric models such as that of Nicolaus Copernicus.
Galileo discovered evidence to support Copernicus' heliocentric theory when he observed four moons in orbit around Jupiter. Galileo's discoveries were met with opposition within the Catholic Church, and in the Inquisition declared heliocentrism to be "formally heretical. Copernicus and Galileo changed the knowledge of the world.
Further discovery showed that the sun is only at the center of our solar system , not the center of the universe as the Copernican theory postulated and is merely one of millions of stars. Since then scientists have discovered more than one galaxy.
The German astronomer Johannes Kepler provided a daring solution to the problem of planetary motions and demonstrated the validity of the heliocentric theory of Copernicus, directly associating the Sun with the physical cause of planetary motions.
At issue for Kepler was a mere 8 ft discrepancy between theory and observation for the position of the planet Mars. This degree of accuracy would have delighted Ptolemy or Copernicus, but it was unacceptable in light of the observations of the Danish astronomer Tycho Brahe, made from Uraniborg Observatory with a variety of newly constructed sextants and quadrants and accurate to within 1 ft to 4 ft. This new scale of accuracy revolutionized astronomy, for in his Astronomia nova New Astronomy, , Kepler announced that Mars and the other planets must move in elliptical orbits, readily predictable by the laws of planetary motion that he proceeded to expound in this work and in the Harmonices mundi Harmonies of the World, Only by abandoning the circle could the heavens be reduced to an order comparable to the most accurate observations.
Kepler's laws and the Copernican theory reached their ultimate verification with Sir Isaac Newton's enunciation of the laws of universal gravitation in the Principia In these laws, the Sun was assigned as the physical cause of planetary motion.
The laws also served as the theoretical basis for deriving Kepler's laws. During the 18th century, the implications of gravitational astronomy were recognized and analyzed by able mathematicians, notably Jean d' Alembert, Alexis Clairaut, Leonhard Euler, Joseph Lagrange, and Pierre Laplace. The science of celestial mechanics was born and the goal of accurate prediction was finally realized. During all of this discussion the stars had been regarded as fixed. While working on his catalog of stars, however, Hipparchus had already recognized the phenomenon known as the precession of the equinoxes, an apparent slight change in the positions of stars over a period of hundreds of years caused by a wobble in the Earth's motion.
In the 18th century, Edmond Halley, determined that the stars had their own motion, known as proper motion, that was detectable even over a period of a few years. The observations of stellar positions, made with transit instruments through the monumental labors of such scientists as John Flamsteed, laid the groundwork for solving a cosmological problem of another era: the distribution of the stars and the structure of the universe.
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