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Asia-Pacific Forum on Science Learning and Teaching, Volume
7, Issue 2, Article 1 (Dec., 2006) Shu-Chiu LIU Historical models and science instruction: A cross-cultural analysis based on students’ views
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From the Ptolemaic to Copernican universe
The Ptolemaic universe had been the basis of astronomical thought in Europe for many centuries, in line with the Greek sphere-making tradition in constructing cosmological models. In this system, every planet is treated as an exclusive fact, being independent of all others, and having its own motion and its own parameters. The theory was made flexible: it could be “stretched, compressed, cut, almost everything” (Bechler, 1991, p. 83).
Copernicus (1473-1543 A.D.) is probably the first in the Western history of science to give statements that distinguish the form of a theory (logical structure) from the content (Bechler, 1991). Indeed there had been criticisms on Ptolemaic astronomy before Copernicus, but they concentrated on its opposition to Aristotle’s principle of uniform planetary motion. Their concern had been only upon the contents. It is Copernicus who started to look at the structure and to argue for a systematic, harmonic and logically coherent astronomy. He made his point that traditional astronomy is “fundamentally hypothetical”- nothing is certain, all in pieces, and thus one can freely add, delete and correct any parts to match new empirical data. He thus proposed a revolutionary model of the universe in which the central position of the Earth was replaced by the Sun and contended thoroughly how this model achieved a systematic, harmonic and logically coherent structure in explaining the heavenly phenomena.
As Kuhn (1996) argued, the Copernican model is in fact not more informative than Ptolemaic astronomy, yet it is for the first time in history not only informative but also harmonic. It is the essence of the Copernican Revolution that there is a drive for harmony in addition to mere information. In order to compare these two structures, Nowak and Thagard (1992) took out their propositions and further identified the “explanatory relations” between these propositions. They constructed an elaborate model of the explanatory structures of both Ptolemy and Copernicus, and showed that from Copernicus’s perspective, his astronomical system gave a more coherent account of the observable features of the heavens. Furthermore, they arrived at following conclusions:
1. The Copernican system had greater simplicity as it needed fewer hypotheses than the Ptolemaic. In the representation of Copernicus, far fewer propositions were used to describe the motions of the planets;
2. Most of the evidence is common for both systems, and therefore the main issue is simply which provides a better explanation;
3. It took a hundred years for Copernicus’s theory to be generally accepted, even though it explains more with fewer hypotheses, because the Ptolemaic system of astronomy was consistent with the authoritative principles of Aristotelian physics.
Apart from revising the system in terms of harmony, the shift from a geocentric model to a heliocentric model can be viewed as a change of “reference system” for the understanding of the universe. We naturally use a geocentric reference when we watch the sky from the Earth; we see apparently the heavenly objects move around the Earth. Only when we are not constrained by the Earth’s surface can we come to realize that the daily observation of the Sun going up one side of the horizon and going down the other side is not the evidence of the Sun moving around Earth. That is, the Sun moving up on the horizon would have the same effect as the horizon moving down to the Sun as observed on the Earth. This is a crucial point in scientific progress: The awareness of reference systems affects physical time and space magnitudes. The Copernican universe put forward a new reference system in which relative motions between the Sun and Earth were first brought into light. The model is revolutionary because it is not based upon direct sensorial observation, but rather a complex relation between what we see and cannot.
In the following, we turn to students’ views before the instructional meanings of these historical accounts can be further analyzed. The discussion of students’ ideas and models is taken along with a consideration of their linkage to the historical aspect of science.
Copyright (C) 2006 HKIEd APFSLT. Volume 7, Issue 2, Article 1 (Dec., 2006). All Rights Reserved.