Ancient Mystery of Planets: Apparent Retrograde Motion & Historical Astronomy

1. Class 4 (Sept 2, 2010): End of Ch 2 and Ch. 3 Last time: 2.1….. celestial coordinates 2.2 Seasons,…Precession 2.2 (cont.). 2.3. The Moon and Eclipses Lunar Phases and Eclipses Today: 2.4 Ancient Mystery of the Planets: Apparent Retrograde motion of planets Parallax Ch 3 (Histrory of Astronomy)

2. 2.4 The Ancient Mystery of the Planets • Why do planets sometimes seem to move backwards relative to the stars?

3. Apparent retrograde motion — try it yourself!

4. Retrograde Motion • Planets usually appear to move eastward relative to the stars. • But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months. • Noticeable over many nights

5. Explaining Apparent Retrograde Motion • Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us) • But very difficult to explain if you think that Earth is the center of the universe! • In fact, ancients considered but rejected the correct explanation…

6. Why did the ancient Greeks reject the notion that the Earth orbits the Sun? • It ran contrary to their senses. • Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

7. Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun The nearest stars are much farther away than the Greeks thought. So the parallax angles of the star are so small, that you need a telescope to observe them.

10. Possible reasons why ancient Greeks could not detect stellar parallax : • Stars are so far away that stellar parallax is too small for naked eye to notice • Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the ancient Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

11. Chapter 3The Science of Astronomy(I prefer to call it History of Astronomy)

12. Ch 3 (Histrory of Astronomy) Ancient Roots of Science (Archeoastronomy) Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Plato and Aristotle: Geocentric universe Ptolemy: mathematical models of geocentric views The Copernican revolution The Nature of Science

13. Ch 3 (Histrory of Astronomy) Ancient Roots of Science (Archeoastronomy) Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Plato and Aristotle: Geocentric universe Ptolemy: mathematical models of geocentric views The Copernican revolution The Nature of Science Histrory of Astronomy is more like a Soap Opera than a Sitcom

14. Ch 3 (Histrory of Astronomy) “Does the World Turn?” Ancient Roots of Science (Archeoastronomy) Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Plato and Aristotle: Geocentric universe Ptolemy: mathematical models of geocentric views The Copernican revolution The Nature of Science

15. Ch 3 (Histrory of Astronomy) “Does the World Turn?” Ancient Roots of Science (Archeoastronomy) Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Plato and Aristotle: Geocentric universe Ptolemy: mathematical models of geocentric views The Copernican revolution “The Church Strikes back” The Nature of Science

16. 3.1 The Ancient Roots of Science(Archeoastronomy) How did astronomical observations benefit ancient societies? • Keeping track of time and seasons • for practical purposes, including agriculture • for religious and ceremonial purposes • Aid to navigation

17. 3.1 The Ancient Roots of Science(Archeoastronomy) What did ancient civilizations achieve in astronomy? • daily timekeeping • tracking the seasons and calendar • predicting eclipses • and more…

18. Days of week were named for Sun, Moon, and visible planets

19. England: Stonehenge (completed around 1550 B.C.) was used as an observatory

20. Yucatan, Mexico: Mayan Observatory at Chichen Itza

21. 3.2 Ancient Greek Science Our goals for learning: • Why does modern science trace its roots to the Greeks? • How did the Greeks explain planetary motion? • How did Islamic scientists preserve and extend Greek science?

22. Why does modern science trace its roots to the Greeks? (some not in book) • Greeks: good records and more rational approach than previous civilizations • were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 BC)

23. Prominent Ancient Greeks (some not in book): • Thales and Pithagoras: roots of science • Thales: Universe is rational (our mind can understan it) • Pithagoras: nature is governed by musical (mathematical) principles • Plato and Aristotle: Geocentric universe • Earth at center of universe • Earth is imperfect Heavens are perfect • “uniform circular motion” is perfect. Hence, heavenly bodies must follow uniform circular motion • Ptolemy: mathematical models of geocentric views (sufficiently accurate to remain in use for 1,500 years) Plato Aristotle Ptolemy

24. Ptolemy: mathematical models of geocentric views, but this made it difficult to explain apparent retrograde motion of planets…

25. So how does the Ptolemaic model explain retrograde motion? Planets do go backward in this model…

26. Ptolemy’s Model

27. What have we learned? • Why does modern science trace its roots to the Greeks? • Greeks: good records and more rational approach than previous civilizations. • How did the Greeks explain planetary motion? • The Ptolemaic model had each planet move on a small circle whose center moves around Earth on a larger circle. • How did Islamic scientists preserve and extend Greek science? • While Europe was in its Dark Ages, Islamic scientists preserved and extended Greek science, later helping to ignite the European Renaissance.

28. Much of our mathematical and scientific heritage originated with the civilizations of the Middle East

29. 3.3 The Copernican Revolution Histrory of Astronomy PART II: “The Church Strikes Back” Problems with Ptolemy’s models Copernicus: publishes heliocentric model and dies Tycho observes planetary motions and dies Kepler: uses Tycho’s observations & writes 3 laws (on pages 67-69 of book): Each planet moves in ellipse with the Sun at one focus. The line between the Sun ……. (faster near Sun and vice versa) p2 = a3 Galileo: Uses telescope to discover moons of Jupiter, study sunspots and phases of Venus Supports Copernicus’ theory & gets in trouble with Church (bad boy! go to your room and stay there till you die)

30. Copernicus’ Heliocentric Model • With better observations more problems with the Ptolemaic model became apparent…..and created more complex fixes • Copernucus’ solution was to put the Sun at the center. • The math was much simpler in Copernucus’ solution

31. Copernicus’ Model

32. Ptolemy’s Model

33. Copernicus’ Heliocentric Model • Sun is at center • Earth orbits like any other planet • Inferior planet orbits are smaller • Retrograde motion occurs when we “lap” Mars & the other planets

34. Occam’s Razor • Among competing theories (otherwise equal), the best theory is the simplest theory-that is, the one with the fewest assumptions. • Heliocentric model is much simpler than Ptolemy’s model

35. Tycho Brahe • Father of observational astronomy. • Danish nobleman and Scientist. Lost his nose in a duel…. (ask me how he died) • He had a young assistant, Johannes Kepler

36. Johannes Kepler • German astronomer and mathematician • Used Tycho’s observations to write 3 laws of planetary motion • He was deeply religious and actually strongly believed in the predictive power of astrology (interesting characters in our soap opera, don’t you think so?)

37. Kepler’s Laws (on page 67-69 of book) • Each planet’s orbit around the Sun is an ellipse, with the Sun at one focus.

38. Kepler’s Laws • A planet moves along its orbit with a speed that changes in such a way that a line from the planet to the Sun sweeps out equal areas in equal intervals of time. (the closer to the Sun, the faster it moves)

39. Kepler’s Laws • The ratio of the cube of a planet’s average distance from the Sun to the square of its orbital period is the same for each planet. a3/ P2 = 1 a3 = P2 a in AU P in years

40. Kepler first tried to match Tycho’s observations with circular orbits • But an 8 arcminute discrepancy led him eventually to ellipses… • “If I had believed that we could ignore these eight minutes [of arc], I would • have patched up my hypothesis accordingly.” • But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy. Johannes Kepler(1571-1630)

41. Galileo Galilei (1564-1642) • First man to point a telescope at the sky • wanted to connect physics on Earth with the heavens • Wrote the book “Dialogue Concerning the Two Chief World Systems”. This book got him in trouble with the Church! Go to your room!

42. Galileo discovered that Jupiter had four moons of its own. Jupiter was the center of its own system. Heavenly bodies existed which did not orbit the earth. Galileo’s Observations

43. Galileo’s observation of the phases of Venus was the final evidence which buried the geocentric model. Geocentric Heliocentric No gibbous or full phases! All phases are seen! Galileo observed all phases!

44. The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in 1633 His book on the subject was removed from the Church’s index of banned books in 1824 Galileo was formally vindicated (pardoned) by the Church in 1992 (ask me if you want to know more juicy details) Galileo Galilei

45. What have we learned? • How did Copernicus, Tycho and Kepler challenge the Earth-centered idea? • Copernicus created a sun-centered model; Tycho provided the data needed to improve this model; Kepler found a model that fit Tycho’s data. • What was Galileo’s role in the Copernican revolution? • His experiments and observations overcame the remaining objections to the Sun-centered solar system

46. 3.4 The Nature of Science Ch 3 (Histrory of Astronomy) “Does the World Turn?” Archeoastronomy Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Plato and Aristotle: Geocentric universe Ptolemy: mathematical models of geocentric views The Copernican revolution “The Church Strikes back” The Nature of Science Our goals for learning: How can we distinguish science from nonscience?

47. Science and Pseudoscience (not exactly like the book) Science: Based on observations and theory Open to criticism and constant challenge in light of new evidence Not perfect (human), not always right, but is the most successful discipline at predicting the way nature works. Pseudoscience Generally holds “absolute truth” Considers only part of evidence available Tends to play on emotions and fears instead of logic

48. Hallmarks of Good Science • Science seeks explanations that rely solely on natural causes. • Science progresses by creating and testing models of nature that explain observations as simply as possible: Occam’s Razor • A scientific model must make testable predictions that could force us to revise or abandon the model. • Scientific Theory: a model which survives repeated testing

49. Astrology (not exactly like book) Ancient Astronomers were also Astrologers (e.g., Kepler) Astrology is not a science Empirical Discipline (no theoretical explanation) Easily abused by charlatans Big problem: astrologers do not agree on what any given celestial configuration means In other words, who will you believe if one astrologer tells you this is a great time to travel and another tells you to stay home?