Overview
In the previous module, we explored how the scientific method developed as astronomers grappled with the problem of describing the motion of the planets—the “wandering stars”—through the celestial sphere. We learned that science begins and ends with empirical measurements; that scientists make assumptions about the nature of physical reality by inferring from the evidence, and those assumptions subsequently form the bases of mathematical descriptions that they take into the world and compare against more evidence. We learned that the assumptions scientists make, and the elaborate descriptions resulting from them, are supposed to explain the phenomena. And we learned that the first successful explanation of all the phenomena associated with the problem of planetary motion—including retrograde motion and the apparent daily/annual cycles—was finally given by Ptolemy in the second century.
In this module we will see how science eventually corrected its own mistaken result. We will see how a few people with an idea that they thought would better explain planetary motion sought a description based on that idea and worked to ensure that it agreed precisely with the available evidence. We will see how Ptolemy’s geocentric theory was replaced by a heliocentric theory which was more consistent with the discoveries Galileo made with his telescope, which fit precision measurements of planetary motion more accurately, and which revolutionised our understanding of Earth’s place among the stars. We will then see how Newton brought together the theories and insights of his predecessors and developed a theory of universal gravity that explained not only the apparent motions of the planets, but the tides as well. Finally, we will see how further scientific progress eventually led Einstein to yet another revolutionary proposal, which led to as great a revision of our scientific worldview as the heliocentric proposal achieved during the Scientific Revolution.
Learning Objectives
When you have finished this module, you should be able to do the following:
- Explore the strengths and weaknesses of the Ptolemaic model, the Copernican model, and the Tychonic model prior to Kepler and Galileo.
- Investigate Kepler’s theoretical accomplishments and Galileo’s empirical accomplishments, and interpret within the context of scientific explanation.
- Examine Galileo’s investigation of motion and inertia, and its influence on Newton.
- Explore Newton’s explanation of both orbital motion and tides as gravitational phenomena.
- Compare Newton’s universal law of gravitation and Einstein’s general theory of relativity, as two explanations of the same phenomenon, and describe the tests that show general relativity describes gravitation more accurately.
- Assess the historical revolutions that have occurred in astronomy, and identify common themes in the works of revolutionary scientists.
Key Terms and Concepts
- Inertia
- Kepler’s laws of planetary motion
- Newton’s laws of motion
- Newton’s law of universal gravitation
- Equivalence principle
cosmiCave.org 