• Understand the observed motion of the planets-- as the Sun circles the ecliptic, the inner ones move back-and-forth across the position of the Sun, while the outer ones usually advance in one direction, but with occasional temporary reversals known as "retrograde motion. "

• Understand how Hipparchus and Ptolemy explained such motions.

• Understand that from the time of Ptolemy to Copernicus, for some 1400 years, astronomy (and other sciences) in Europe saw little progress .

• Learn about the system proposed by Copernicus, and the way it explained retrograde motion: all planets moved in circles around the Sun, but the ones closer to the Sun always moved faster.

• Learn about Galileo, the first astronomer to use a telescope, and of his discoveries: the moons of Jupiter, craters on the Moon, phases of Venus (like the ones of the Moon), stars of the Milky Way.

• Learn about Galileo's defense of the Copernican theory, and the price he paid for it.

Terms: Heliocentric theory, retrograde motion, (Opposite: prograde motion), Ptolemy's theory.

Stories and extras: The theories of Ptolemy and Copernicus are briefly described. Only part of Galileo's work are covered, primarily his pioneering observations through the telescope and a brief discussion of his persecution.

Last time we talked about the difference between a scientific prediction and a lucky guess. Today we continue and ask, what is a scientific theory?

   The ancient Greeks and Copernicus each had an explanation of sorts, for the way the planets appeared to be moving in the sky. But there was a big difference.

   On one hand, you had Ptolemy's theory: that is what it is called today, because it came to us through the works of Claudius Ptolemy, though it was actually Hipparchus who proposed it, nearly 300 years earlier. Hipparchus assumed all celestial objects revolved around Earth. After all, for one such object--the Moon--that motion could actually be proved. In hindsight, it was just too bad that it was the only object that did so!

   Some of these planets seemed to go around the Sun, but others moved in strange ways, in "epicycles" around points which went around the Earth, the way the Sun was supposed to do. It was an attempt to predict where the planets would be but not to explain the motion. There was no scale we could put on the solar system--the theory gave no idea what the solar system really looked like.

   Copernicus, presented a logical picture of what the solar system looked like. The claim that all planets revolved around the Sun, and that the Earth was just one of those planets, a sphere that revolved around its axis, gave a way of predicting where the planets would be at any time.

   The idea that the Earth was not the center of the universe was opposed by many religious authorities. Copernicus therefore had to claim that he was proposing "a simpler way of predicting the positions of the planets, " not necessarily a different world-system. But actually it was much more than a prediction method.

   We like our physical theories to give us a logical picture, not just a mathematical solution. Among other things, such a picture allows us to understand intuitively the processes that are taking place.

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The following is an optional extension, about how modern physics has gone beyond our simple imtuitive view of reality:

   In this course about astronomy and space, dealing with large objects moving in space, we will try to provide you, the students, with such intuitive pictures. One can however note that the 20th century brought some cases where one can predict, but cannot form a logical picture.

   That happened in quantum theory--the study of physics on the atomic scale, where space and time tend to be "grainy. " We can predict where an electron is likely to be observed, but cannot tell where it actually is.

   Physicists found such cases very unsettling, and some argued that an underlying reality of where the electron actually was remained a meaningful concept (some still do so). Einstein was among those, and said "I cannot believe that God is playing with dice. " Most physicists however (e.g. Richard Feynmann) believed that 'what we see is what we get' and that no "reality" existed beyond the probabilities which theory prescribes.

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--Then go over section #9c in "Stargazers". Review the material, using the questions below:

--Who was Ptolemy?

Around 150 AD, a Greek scholar named Claudius Ptolemy collected all the results of ancient Greek astronomy (including his own) in a series of books. His works were preserved by the Arabs, who combined them as the "Almagest. " It became a leading influence in European astronomy, especially after about the year 1200.

These planets are never seen far away from the Sun, but move back and forth from one side of the Sun to the other. The Greeks correctly imagined that they moved around the Sun (or some point very near the Sun) and traveled with it around the celestial sphere.

Those planets also traveled around the celestial sphere, following more or less the ecliptic, but now and then they would retrace their motion and move backwards for a while ("retrograde motion") before continuing forward.

Hipparchus and Ptolemy proposed that the outer planets followed "epicycles" around points that traveled around the celestial sphere and therefore, they thought, around the Earth. Their motion was like that of Venus and Mercury, except that for the inner planets, the center of rotation was visible, it was the Sun (or a point near the Sun), while the centers of the motion of the outer planets were not visible.

Hipparchus at first thought his theory predicted the motion, and believed that all such motions proceeded evenly around circles. As more accurate observations were made, all sorts of corrective motions had to be added.

"Copernicus" is the Latin version of Nicolaus Kopernik, (1473-1543). He was a Polish church official and an amateur astronomer, who proposed that the Earth was also a planet, and that all planets orbited the Sun in circles

Venus and Mercury were planets closer to the Sun, and therefore were only seen near the Sun, moving back and forth across it. Their orbital periods were shorter than the Earth's.

These planets had orbits were outside the Earth's, and they moved more slowly. The Earth's orbit, therefore, overtook them from time to time, and when it did, they appeared to be moving backwards in the sky.

Yes, but he was very cautious, because some important people opposed his ideas. He only published his work at the end of his life.

Galileo was an Italian scholar. His contributions were many, but he is mostly remembered as the first astronomer to build and use a telescope.

• The craters and "seas" of the Moon.

• The fact Venus changed shapes--crescent, etc.--like the Moon, confirming it was a sphere illuminated by sunlight.

• Four large Moons of Jupiter, forming a small "solar system" around that planet. This confirmed to Galileo the ideas of Copernicus.

• Sunspots, though others also observed them at about the same time.

• He discovered the rings of Saturn. However, Galileo's telescope was so crude that he was not sure what they were.

• The Milky Way, a whitish cloud stretching across the heaven: Galileo found it was composed of many faint stars. We now know that the Earth is part of the galaxy, a collection of at least 100 billion stars, forming a flat wheel-shaped cloud. When we look at the Milky Way, we are seeing that cloud edge-on and therefore observe many distant stars.
  [Optional discussion: Should one say "the galaxy" or "a galaxy"? Actually, "the galaxy" was the term used for many centuries for this collection of stars--it means the milky way, from "gala," milk in Greek (and "galax" is a plant with milky sap!).

  Then in 1923 the American astronomer Edwin Hubble found evidence that a cloud-like "nebula" long observed in the constellation of Andromeda was probably a distant galaxy like ours, an "island universe." In the years that followed Hubble found many other such "galaxies," together with evidence that the more distant ones were all moving away from us, suggesting an expanding universe. Nowadays, of course, the word "galaxy" is understood to be any such star-system ("Long, long ago, in a galaxy far far away... "); but it pays to remember that originally only one galaxy was known, namely ours. It is often refered to specifically as "the Milky Way galaxy"]

He aggressively promoted the Copernican theory, making enemies among powerful people, who claimed the Earth was the center of the universe and that all celestial objects revolved around it.

He had to proclaim that he was abandoning his Copernican views, and was forced to spend the rest of his life confined to a rural estate.