The Copernican Revolution

1. The Copernican Revolution and the Development of Modern Astronomy

2. In 1543, Nicolaus Copernicus published the book De Revolutionibus Orbium Cœlestium, “On the Revolutions of the Celestial Spheres” in which he proposed a new model of the Solar System, in which the Sun is at the center (Heliocentric).

3. The new model kept some archaic ideas (perfect circles for the orbits, and even some epicycles), but it was revolutionary because it displaced the Earth from the center of the Solar System, and thus the Universe. Its great advantage lay in its ability to explain simply the two basic observational properties of the Solar System.

4. The Retrograde • Motion of the • Superior Planets

5. 2) The Behavior of the Inferior Planets

6. Recall how awkward these explanations were with the Ptolemaic System

7. What was the single greatest advantage of the Copernican System over the Ptolemaic system It did away with epicycles It reproduced the motions of the planets without having to use complicated and artificial devices It was heliocentric instead of geocentric It made Venus the closest planet to the Earth instead of Mercury It was able to predict positions of the planets more accurately than the Ptolemaic system

8. Before we continue the story of the ultimate triumph of the Copernican system over the Ptolemaic system, let us digress, and consider some terminology associated with the Copernican system.

10. Opposition • Conjunction • Quadrature

11. Synodic Period: The time between two successive identical configurations. Sidereal Period: The true orbital period of the planet Let: P = Sidereal Period of the Planet E = Sidereal Period of the Earth (about 365.25 days) S = Synodic Period of the Planet Inferior Planet: Superior Planet:

12. Example: Venus P = ? (Sidereal Period) E = 365.25 days S = 584 days (Synodic Period) P = 1/0.004450 = 225 days

13. You try: Mars E = 365.25 days S = 780 days (synodic period of Mars) P = ? (sidereal period of Mars) P = 687 days

14. The Copernican System was not immediately accepted by astronomers, and especially by the Church because: • Displacing the Earth from the center was counter • to Aristotelian philosophy and Church doctrine. • Though simpler than the Ptolemaic system the • Copernican system did not predict the positions • of the planets any more accurately. • Contemporary astronomers had their whole careers • invested in the Ptolemaic system.

15. Copernicus was aware of these difficulties, and did not publish his book until the last year of his life. Defense of the Copernican system thus fell to other astronomers. Some important figures in the Copernican Debate: Copernicus: 1473 – 1543 Galileo Galilei: 1564 – 1642 Tycho Brahe: 1546 – 1601 Johannes Kepler: 1571 – 1630 Issac Newton: 1642 - 1727

16. Galileo Galileo was the first astronomer to use the teles-cope (invented in Holland in 1609) for astro- nomical obser-vations. His discoveries ultimately disproved the Ptolemaic system, and helped establish the Copernican system.

17. The Discoveries of Galileo 1) Craters on the Moon - cast doubt on the philosophy of Aristotle

18. 2) Discovery of Sunspots

19. 3) Discovery of the Moons of Jupiter

20. Discovery that Venus goes through a complete • set of phases.

21. The discovery Venus goes through a complete set of phases is consistent with the Copernican system, but is impossible under the Ptolemaic system. Copernican System Ptolemaic System

23. Why were believers in the Ptolemaic system reluctant to believe that Jupiter had moons? Because they had not discovered them first themselves Because astronomical bodies being in motion seemed to contradict Aristotles ideas of the unchanging heavens Because this meant there was another “center” in the solar system instead of the Earth being the only center Because the Ptolemaic system did not predict the phases of these moons.

24. Galileo’s efforts to prove the Copernican system got him in trouble with the Church. His book Siderius Nuncius (1610) raised controversy, and he was warned by the Church not to teach the Copernican system as the truth. After the publication of his second major book, Dialogue Concerning the Two Chief World Systems (1632), he was placed under house arrest.

25. Tycho Brahe Tycho Brahe was a Danish nobleman who was convinced that both the Copernican and Ptolemaic models were incorrect.

26. Tycho Brahe thought that both systems were in error because: • In the year 1572, a supernova • appeared, and in 1577 a comet, • disproving the idea that the • “heavens were immutable” and • thus casting doubt on the • Ptolemaic system. • Tycho Brahe could not observe the phenomenon of • stellar parallax, and thus he did not believe that the • Earth was in motion – suggesting to him that the • Copernican system was wrong. In addition, neither theory gave very good predictions of planetary positions.

27. He came up with his own system, called the Tychonic System. He decided the only way to prove the correctness of any system was to make a large number of very accurate observations of the positions of the planets over a number of years. For that purpose, he established a major observatory called Uraniburg.

28. How did the Tychonic system differ from the Copernican and Ptolemaic system? It was heliocentric, but all the planets orbited around the earth It was geocentric, but all the planets orbited around the sun It was Venus-centric It completely did away with epicycles and introduced The idea of elliptical orbits

29. Tycho’s observatory was established before the invention of the telescope, so all his observations were naked-eye. However, he was able to compile the most accurate and extensive collection of planetary observations at that time.

30. Tycho Brahe had an assistant, Johannes Kepler. On his deathbed, Tycho willed his observations to Kepler, and instructed him to use them to prove the Tychonic system. Kepler used them to prove the Copernican system, and to discover elliptical orbits.

32. Over a period of 25 years, Johannes Kepler used Tycho’s observations to deduce his Three Laws of Planetary Motion. Kepler’s First Law: The orbits of the Planets are ellipses with the sun at one focus.

34. Over a period of 25 years, Johannes Kepler used Tycho’s observations to deduce his Three Laws of Planetary Motion. Kepler’s First Law: The orbits of the Planets are ellipses with the sun at one focus.

35. The semi-major axis (a) is half the major axis The eccentricity (e) runs from 0 (circle) to 1

36. Perihelion Aphelion Major axis = 2a

37. The semi-major axis, a, is the distance between the sun and the planet at perihelion the distance between the sun and the planet at aphelion the average distance between the sun and planet the length of the major axis of the elliptical orbit

38. Kepler’s Second Law: A line from the planet to the sun sweeps over equal areas in equal times. This implies that the planet travels faster in its orbit near Perihelion than at Aphelion.

39. Kepler’s Third Law: The Orbital Period of a planet squared is proportional to the length of its semi-major axis cubed. If we use the units of Years for the Period and Astronomical Units (A.U.) for the semi-major axis, the equation becomes:

40. Kepler’s Third Law: An Example Mars has an orbital period P = 1.881 years. Its semi-major axis a = 1.524 A.U.

41. Kepler’s Third Law: Another Example The orbit of a comet has a semi-major axis (a) of 9 A.U. What is its orbital period?

42. Try this one yourself: Venus has an orbital period of 0.615 years. What is The semi-major axis of its orbit?

43. Yet another example! A comet is observed with a perihelion distance of 1 A.U. and an aphelion distance of 49 A.U.. What is the period in years? Aphelion Perihelion 49 A.U. 1 A.U. The major axis = 2a = 50A.U. => a = 25A.U. Thus, P2=a3 = 253 = 15625