(9b) The Planets
The student will learn about the heliocentric theory of Copernicus, the ideas behind it and the explanations which it displaced. A discussion of retrograde motion of planets helps the students understand the logic underlying the ideas of both Ptolemy and Copernicus.
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements:
"The Planets," section #9b: on disk Splanets.htm, on the web http://www.phy6.org/stargaze/Splanets.htm
"The Discovery of the Solar System," section #9c: on disk Ssolsys.htm, on the web http://www.phy6.org/stargaze/Solsys.htm
Both are sections of "From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web
Goals: The student will|
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.
The teacher may start with a discussion:
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.
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.
Guiding questions and additional tidbits (With suggested answers).
--Who was Ptolemy?
--How did the Greeks view the motion of the inner planets, Venus and Mercury?
-- What made the motion of the outer planets: Mars, Jupiter and Saturn, hard to understand?
-- How did Ptolemy explain the motion of the outer planets? (Actually, that theory was by Hipparchus)
-- Did this predict correct planetary motions?
-- Who was Nicolaus Copernicus, and what is his great contribution?
-- How did Copernicus explain the motion of Venus and Mercury?
-- How did Copernicus explain the "retrograde" (backward) motion of Mars, Jupiter and Saturn?
-- Did Copernicus convert others to his thinking?
-- Who was Galileo Galilei (1564-1642), and what was his main contribution?
-- What did Galileo discover with his telescope?
-- What got Galileo into trouble with the Catholic Church?
-- What was his punishment?
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Material last updated: 14 September 2004
File last updated: 21 January 2008