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CHAPTER 3 THE ORIGIN OF MODERN ASTRONOMY Topic Summaries 3. 1 Astronomy before Copernicus How did people in ancient civilizations describe Earth's place in the universe? • Ancient philosophers accepted as a first principle that Earth was the unmoving center of the universe. Another first principle w as that the heavens were perfect; and, because the circle was the only perfect geometrical form, objects in the heavens must move in uniform (constant speed) circular motion. • The geocentric (Earth-centered) universe became part of the teachings of the great philosopher Aristotle, who argued that the sun, moon, and stars were carried around Earth on rotating crystalline spheres. About AD 140, Ptolemy gave mathematical form to Aristotl e's model. Ptolemy kept the geocentric (Earth-centered) and uniform circular motion principles, but he added off-center circles and variable speeds to better predict the motions of the planets. 3. 2 Nicolaus Copernicus How did Copernicus revise that ancient model? Copernicus devised a heliocentric (sun-centered) model. He preserved the principle of uniform circular motion, but he argued that Eart rotates Ojl itS::Pxis and circles the sun once a.year. His t heory w controve ial be ause i( contradic e Church teaching. Copernicus publi bed his theory in his book De Revolutionibus in 1543, the year he died. • Why was the Copernican model gradually acce teCI 7 Because Copernicus kept uniform circular motion, his model did not pre ict th motions of the planets well, but it did offer a simpl e explanation of retrograde motion of planets without using large epicycles (circles on circles). He did have to include small epicycles to account for some observed planetary motions. The Copernican model was also more eloquent and straightforward. Venus and Mercury were treated the same as all the other planets, and the velocity of each planet was related to its distance from the sun. first principle Something that seems obviously true and needs no further examination. geocentric universe A model universe with Earth at the center, such as the Ptolemaic universe. heliocentric universe A model of the universe with the sun at the center, such as the Copernican universe. uniform circular motion The classical belief that the perfect heavens could move only by the combination of uniform motion along circular orbits. parallax The apparent change in position of an object due to a change in the location of the observer. Astronomical parallax is measured in secon(js of arc. retrograde motion The apparent ~backward (westward) motion of planets as seen against the background of stars. epicycle The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around Earth. paradigm A commonly accepted set of scientific ideas and 3. 3 rycho Brahe, Johannes Kepler, and Planetary Motion assumptions. ellipse A closed curve around t wo How did Tycho Bra he and Johannes Kepler contribute to the Copernican Revolution? points, called the foci, such that the • Tycho's great contribution was to compile the most preci se and detailed nakedeye observations of the planets and stars ever made, observations that were later analyzed by Kepler. curve and back to the other focus Kepler inherited Tycho's books of observations in 1601 and used them to uncover three laws of planetary motion. He found that the planets follow ellipses with th e sun at one focus, that they move faster w hen near the sun, and that a planet's orbital period squared is proportional to its orbital radius cubed. total distance from one focus to the remain s constant. semi-major axis (a) Half of the longest diameter of an ellipse. eccentricity (e) A number between 1 and 0 that describes the shape of an ellipse; the distance from one focus to the center of the ellipse divided by the semi major axis. Key Terrns empirical Description of a phenomenon without explaining why it occurs. Topic Surnrnaries 3. 4 Galileo Galilei Why was Galileo condemned by the Inquisition? Galileo used the newly invented telescope to observe the heavens, and he recognized the significance of what he saw there. His discoveries of the phases of Venus, the satellites of Jupiter, the mountains of the moon, and other phenomena helped undermine the Ptolemaic model. hypothesis A conjecture, subject to further tests, that accounts for a set of facts. theory A system of assumptions Galileo based his analysis on observational evidence. In 1633, he was condemned before the Inquisition for refusing to obey an order to halt his defense of Copernicus's model. and principles applicable to a wide range of phenomena that has been repeatedly verified. natural law A theory that has been 3.5 Isaac Newton, Gravity, and Orbital Motion so well confirmed that it is almost How did Isaac Newton change humanity's view of nature? universally accepted as correct. Newton used the work of Kepler and Galileo to discover three laws of motion and the law of gravity. These laws made it possible to understand such phenomena as orbital motion and the tides. mass A measure of the amount of matter making up an object. weight The force that gravity exerts on an object. inverse square relation A rule that the strength of an effect (such as • Newton's laws gave scientists a unified way to think about nature. Every effect has a cause, and science is the search for those causes. The 144 years from Copernicus's book De Revolutionibus to Newton's book Principia marked the beginning of modern science. From that time on, science depended on evidence to test theories and relied on the methods of gravity) decreases in proportion as the distance squared increases. spring tide Ocean tide of jarge range that occurs at full and new moon. neap tide Ocean tide of small range occurring at first- and third-quarter moon. circular velocity The velocity an object needs to stay in orbit around another object. geosynchronous satellite A satellite that orbits eastward around Earth with a period of 24 hours and remains above the same spot on Earth's surface. center of mass The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces. closed orbit An orbit that repeatedly returns to the same starting point. escape velocity The initial velocity an object needs to escape from the surface of a celestial body. open orbit An orbit that carries an object away, never to return to its starting point. heavens were made up of perfect crystalline spheres moving at constant speeds? 2. How did the Ptolemaic model explain retrograde motion of the planets? 3. In what ways were the models of Ptolemy and Copernicus similar? 4. Why did the Copernican hypothesis gradually win acceptance? 5. Why is it difficult for scientists to replace an old paradigm with a new paradigm? 6. Why did Tycho Bra he expect the new star of 1572 to show parallax? Why was the lack of parallax evidence against the Ptolemaic model? 7. Explain how Kepler's laws contradict uniform circular motion. etween a hypothesis, a theory, and a law? 9. Review Galilee's telescope discoveries and explain why they supported the Copernican model and contradicted the Ptolemaic model. 10. If you lived on Mars, which planets would describe retrograde loops? Which would be always seen near the sun? Which wou ld never be visible as crescent phases? 11. Galileo was condemned by the Inquisition, but Kepler, also a supporter of Copernicus, was not. Why not? 12. Why did Newton conclude that gravitation had to be universal? 13. Explain w hy you might describe the orbital motion of the moon with the statement, "The moon is falling."