Introduction

This is the second publication in our series called Turning Points in Science, which highlights the history and nature of science. The first publication covers the topic of germ theory and the third, atomic theory.

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Claudius Ptolemy's geocentric theory of the universe, proposed in 140 A.D., formalized what even preliterate humans had known intuitively: the sun went around the earth, the earth was stationary. The early astronomer’s earth-centered theory endured until the mid-16th century, that’s up to 1,550 years! This means the heliocentric, or sun-centered, theory of the universe is a rather young idea. How could an obviously wrong theory prevail for over one and a half millennia? How does any scientific theory become established and persist?

A theory that provides both adequate explanation, or cause, for known observations and predictive power for future events is easily assimilated. Ptolemy's theory fit the bill; it worked on many levels. It accommodated most observations of the known celestial bodies. It also accommodated the notion that human beings were supreme beings, "created in the image of God," and thus rightly positioned in the center of the universe. Why then did the theory become obsolete? Why does any scientific theory become obsolete? When new observations suggest incongruence with existing theories, the theories must be re-examined and modified to accommodate the new observations; such is the nature of science. If the necessary modifications negate the original theory, it must be discarded and a new theory put in its place, which was the fate of Ptolemy's geocentric theory.

In his theory, Ptolemy had to account for observations that suggested some celestial bodies move with varying speed and apparently sometimes reverse their paths! Each complication represented an opportunity for falsification; one reason why the best scientific theories are simple and generalized. The geocentric theory also rested upon false assumptions, such as the "fact" that the paths of heavenly bodies are perfectly circular and the earth is at rest, notions promulgated by Aristotle and thus "true" by authority. These facts were not disconfirmed until technological innovation allowed.

This story of how one radical theory supplanted another illustrates and exemplifies the history and nature of science, an important domain in the National Science Education Standards. In recommending the history of science as a component of science literacy, the American Association for the Advancement of Science said in Science for All Americans,

There are two principal reasons for including some knowledge of history among the recommendations. One reason is that generalizations about how the scientific enterprise operates would be empty without concrete examples. Consider, for example, the proposition that new ideas are limited by the context in which they are conceived; are often rejected by the scientific establishment; sometimes spring from unexpected findings; and usually grow slowly, through contributions from many different investigators. Without historical examples, these generalizations would be no more than slogans, however well they might be remembered. . . .

A second reason is that some episodes in the history of the scientific endeavor are of surpassing significance to our cultural heritage. Such episodes certainly include Galileo's role in changing our perception of our place in the universe; Newton's demonstration that the same laws apply to motion in the heavens and on earth; Darwin's long observations of the variety and relatedness of life forms that led to his postulating a mechanism for how they came about; Lyell's careful documentation of the unbelievable age of the earth; and Pasteur's identification of infectious disease with tiny organisms that could be seen only with a microscope. These stories stand among the milestones of the development of all thought in Western civilization. (Retrieved October 2, 2007, from http://www.project2061.org/publications/sfaa/online/chap10.htm)

Turning Points in Science: Copernican Revolution is the second in a series regarding historic, pivotal scientific advances such as the elucidation of atomic theory and the germ theory of disease. To avoid the pitfall of isolating the history and nature of science from science content, these publications focus on specific scientific advances relevant to the NSES content standards in life, physical, and earth sciences. In this way, the science becomes personal and understandable as a human endeavor, and thus its impact on society is more easily conceived. A potentially useful tool will be James Burke's Knowledge Web, which provides the connections between historic figures in science that enabled the advancement of science. Although an explanatory video is in beta version only at the time of this writing, users can view it and bookmark the site for future use.

As students become aware of the various methods of science — some unique to different disciplines — they will notice the methods’ unity in the use of logical argumentation based on empirical evidence. For example, in Turning Points in Science: Germ Theory, the methods included controlled laboratory exercises as well as field tests, yielding many observations which by induction led to generalizations, or theory. This theory enables prediction and deduction.

Methods in astronomy rely more heavily on field observations, induction, and mathematical modeling accommodating the observations. These models are theories that enable predictions, such as when certain heavenly bodies will be in what positions relative to each other, and deductions, such as there will be a solar eclipse if the moon passes between the sun and earth.

Pedagogically speaking, it is important to avoid the temptation of verification exercises. Rather, allow students to make their own "discoveries," to interpret their observations, to make logical inferences, and to derive supported conclusions. If properly modified and presented competently, laboratory exercises are discovery opportunities for students, allowing students direct experience with the nature of science and inquiry in science. The National Science Teachers Association's position statement on Scientific Inquiry provides guidance regarding how teachers can conduct inquiry teaching and what teachers can expect from students.

This publication focuses on the evolution of the heliocentric theory of the universe. Resources provided here will facilitate understanding of the early concepts of the universe; the thinking that led to hypotheses in astronomy; the observations and experiments that yielded information allowing for theorizing; reaction to and acceptance of the investigators' findings; and the impacts of the theory on humanity.

In the background information for teachers section of Turning Points in Science: Germ Theory are a number of resources that will be helpful here but are not repeated here. They include research articles on naive conceptions, how to conduct inquiry teaching, and a video description of the nature of scientific theory. The Background Information for Teachers of this publication provides resources that show how science does not progress in a vacuum, but within the cultural parameters of a society. Historical Figures in Astronomy offers an overview of individuals whose work impacted the evolution of the heliocentric theory.

The Lessons on Theories of the Universe and Lessons in Modern Cosmology are meant to help students understand the context, the emergence, and the impact of the heliocentric theory of the universe, touching on the NSES content standards of History and Nature of Science, Science in Personal and Social Perspectives, Earth and Space Science, and Science as Inquiry.

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Copyright November 2007 — The Ohio State University. This material is based upon work supported by the National Science Foundation under Grant No. 0424671. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons License.