ASTRONOMY 114 Survey of Astronomy

1. http://www.plavchan.com/msu/ast114/ ASTRONOMY 114Survey of Astronomy Monday,Tuesday,Wednesday,Thursday 2:30-3:20pm Temple Hall 0001 Dr. Peter Plavchan 626-234-1628 PeterPlavchan@missouristate.edu Office Hours: Mon-Thurs 1-2pm “Textbook:” Sapling Learning

2. Introductions

3. Introductions Introductions

4. Introductions

5. You Please take out a piece of paper, or compose an email to me on your phone/computer, and answer: • Name & Student ID# (also for attendance) • How many semesters have you completed at MSU? • What do you know or have you heard about astronomy? • What is your dream career? • Why are you taking this class? • What do you dread doing? What do you love doing? • Can you give me tips on how you learn best? • What would you like to know about me? • What would you like me to know about you? • What can I teach you? • What can you teach me?

6. Syllabus & Exams

7. Extra Credit Options Extra credit is available and can contribute up to 5-10%. The purpose of extra credit is to find a balance between your interests and the subject material of astronomy. Possible extra credit options must be approved by the instructor, but can include for example: • Visit Baker Observatory.Bring back proof - photographic - of your visit. To qualify for the extra credit, please write a one page summary of your visit. • Astronomy as art. Many of you are non-science majors, and excel in other areas of specialization - art, writing, music, etc. There is a vast history of art inspired by astronomy. See http://www.spitzer.caltech.edu/Media/happenings/20081007/ for an example. For another example, consider constructing a scale model of our galaxy or local group of galaxies. In order to qualify for the extra credit, please create an original work of art, music or writing inspired by astronomy and the material you have learned in class. You may team with up to two other people. ALL EXTRA CREDIT REPORTS ARE DUE BY LAST CLASS, DIRECTLY TO ME.

8. Group Projects • Find two friends for groups of 3. Sign up for a Thursday to present to the class. • Two groups will present each week, and there will be a competition for the better presentation. Your classmates will vote! • Presentations can be voice only, Powerpoint, Keynote, PDF. Please blog about your presentation here: • Presentations are 10-15 minutes each • From each lecture’s topic, pick a subject to go into detail on. Consult with the professor on the topics covered in class well before you prepare your presentation. • Will count towards 10% of final grade.

9. The Copernican Revolution Scotland: 4,000-year-old stone circle; the Moon rises as shown here every 18.6 years.

10. What did ancient civilizations achieve in astronomy? • Daily timekeeping • Tracking the seasons and calendar • Monitoring lunar cycles • Monitoring planets and stars • Predicting eclipses • And more… Mexico: model of the Templo Mayor England: 1550 BC Our mathematical and scientific heritage originated with the civilizations of the Middle East Mexico: model of the Templo Mayor

11. Why does modern science trace its roots to the Greeks? • Greeks were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. • This is a very important step in understanding the natural world. Greek geocentric model (c. 400 B.C.)

12. Eratosthenes measures the Earth (c. 240 BC) Measurements: Syene to Alexandria distance ≈ 5000 stadia angle = 7° Calculate circumference of Earth: 7/360  (circum. Earth) = 5000 stadia  circum. Earth = 5000  360/7 stadia ≈ 250,000 stadia Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km

13. APPARENT RETROGRADE MOTION OF THE PLANETS We see apparent retrograde motion when we pass by a planet in its orbit.Easy for us to explain today: occurs when we “lap” another planet (or when Mercury or Venus laps 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 …

14. Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor. p If the angle p = 1 second of arc then d =206265 AU. We define this distance to be 1 parsec (pc). Can you show that 1 pc = 3.26 light-years? tan p = 1AU/d (AU) For small angles: p = 1/d d Here p is in radians, but there are 206265 arcseconds in 1 radian. 1 AU Not to scale

15. The Greeks knew that the lack of observable parallax could mean one of two things: • Stars are so far away that stellar parallax is too small to notice with the naked eye • The Moon is 30 minutes of arc and the eye can separate objects about 1 minute of arc apart • Earth does not orbit the Sun; the Earth is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

16. Explaining retrograde motion with the Geocentric model Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) Ptolemy Most sophisticated geocentric model was that of Ptolemy (A.D. 100-170). Sufficiently accurate to remain in use for 1,500 years.

17. The Copernican Revolution Our goals for learning: • How did Copernicus, Tycho, and Kepler challenge the Earth-centered (geocentric) idea? • What are Kepler’s three laws of planetary motion? • How did Galileo solidify the Copernican revolution?

18. How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea? Copernicus (1473-1543): • Proposed Sun-centered model (published 1543) • Used model to determine layout of solar system (planetary distances in AU) But . . . • He had the right idea, yet Copernicus still assumed that the orbits of the planets were perfect circles and so his heliocentric model was no more accurate than Ptolemy’s geocentric model in predicting planetary positions!

19. Danish astronomer Tycho Brahe (1546-1601) • Compiled the most accurate (one arcminute) naked eyemeasurements ever made of planetary positions. • He still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets must go around the Sun) • Hired Johannes Kepler, who later used these detailed observations to discover the truth about planetary motion. Brahe’s observatory was visual – before the telescope – and really a giant protractor for measuring angles.

20. Johannes Kepler (1571-1630) • 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.”

21. What is an ellipse? The semi-major axis is also the “average” distance from the focus. An ellipse looks like an elongated circle

22. Eccentricity and Planetary Orbits • Define c to be the distance from the center to either focus • Define a to be the semi-major axis length • The eccentricity e of an ellipse is then e = c/a [ For a circle e = 0 because c = 0] a c • Perihelion distance = a(1 – e) • Aphelion distance = a(1 + e) Earth: e = 0.017; a = 149.6 million km; closest = 147.1 million km and farthest = 152.1 million km. Note that a is the average.

23. Eccentricity of an Ellipse

24. Kepler’s Three Laws of Planetary Motion? Kepler’s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus. NOTE: None of the planets have orbits as elliptical as this, only some comets.

25. Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times. • this law means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.

27. Kepler’s Third Law More distant planets orbit the Sun at slower average speeds, obeying the relationship p2 = a3 p = orbital period in years a = avg. distance from Sun in AU If a = 1 AU then 13=1 and p2=1 therefore p=1 (year).

28. Graphical version of Kepler’s Third Law p2 plotted against a3 v plotted against a Planet’s average speed is v = 2πa/p (circumference of orbit divided by period), so v2 = (2π)2a2/p2 but p2 = a3 so, v2 = (2π)2a2/a3 = (2π)2/a, thus v = 2π/√a. Using v in km/s then v = 30/√a.

29. Thought Question:An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun? • 4 years • 8 years • 16 years • 64 years Hint: Remember that p2 = a3

30. An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun? • 4 years • 8 years • 16 years • 64 years We need to find p so that p2 = a3 Since a = 4, a3 = 43 = 64 Therefore p = 8, p2 = 82 = 64

31. THURS How did Galileo Galilei solidify the Copernican revolution? Galileo (1564-1642) overcame major objections to Copernican view. Three key objections rooted in Aristotelian view were: Earth could not be moving because objects in air would be left behind. Non-circular orbits are not “perfect”as heavens should be. If Earth were really orbiting Sun,we’d detect stellar parallax.

32. Overcoming the first objection (nature of motion): Galileo’s experiments showed that objects in air would stay with a moving Earth. • Aristotle thought that all objects naturally come to rest. • Galileo showed by experiment that an object will stay in motion unless a force acts to slow it down. • Galileo proved this with countless experiments involving falling and rolling objects.

33. Overcoming the second objection (heavenly perfection): • Tycho’s observations of a comet and a supernova (new star) already challenged this idea. • Using his telescope (1609), Galileo saw: • Sunspots or “imperfections” on the Sun • Mountains and valleys on the Moon (proving it is not a perfect sphere)

34. Overcoming the third objection (parallax): • Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth. • Galileo showed stars must be much farther than Tycho thought — in part by using his telescope to resolve the Milky Way into countless individual stars. • If stars were much farther away, then lack of detectable parallax was no longer so troubling. • The eventual discovery of the tiny angular shifts in a star’s position due to parallax as the Earth orbits the Sun required more than another century of technology development to make much bigger telescopes.

35. Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth. Galileo saw four moons orbiting Jupiter, proving that not all objects orbit the Earth

36. The Nature of Science Our goals for learning: • How can we distinguish science from nonscience? • What is a scientific theory? The previous example of the evolution of ideas from the false geocentric model to the true heliocentric model leads us nicely to a discussion of the Scientific Method.

37. The idealized Scientific Method • Based on proposing and testing hypotheses through careful and repeated experiments • hypothesis = educated guess • Around the time of Galileo there was a fundamental shift away from mere speculation and towards tangible experimental evidence.

38. Hallmarks of Science: #1 Modern science seeks explanations for observed phenomena that rely solely on natural causes. (A scientific model cannot include divine intervention) Example: Kepler sought a natural explanation for observations made by Tycho.

39. Hallmarks of Science: #2 Science progresses through the creation and testing of models of nature that explain the observations as simply as possible. (Simplicity = “Occam’s razor”) Occam’s principle states that “all things being equal, the simplest explanation tends to be the right one.” The heliocentric model is simpler than the geocentric one.

40. Hallmarks of Science: #3 A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations. Example: each of the competing models offered predictions that were tested. Kepler’s model worked best. Kepler’s model can still be tested. In fact, slight discrepancies found at much later dates led to new discoveries, such as Einstein’s theories…

41. What is a scientific theory? • The word theory has a different meaning in science than in everyday life. • In science, a theory is NOT the same as a hypothesis, it is NOT a guess, rather: • A scientific theory must: • Explain a wide variety of observations with a few simple principles, AND • Must be supported by a large, compelling body of evidence. • Must NOT have failed any crucial test of its validity.

42. Scientific Theories • The theory of gravity • The theory of electromagnetism and light • The theory of atoms • The theory of evolution by natural selection • The theory of relativity • The theory of plate tectonics • The quantum theory

43. Other modes of reasoning • Appeal to authority – weak because does not rely on experimental evidence and advance predictions, even if the authority figure is a famous scientist. • Opposing advocates (the legal system) – good in principle if both sides present all the evidence and argue rationally for the truth. Fails in practice because each advocate presents only those facts that support their position. • Myths, superstitions and divine intervention – weakest of all because there is no rational way to seek truth when explanations are attributed to unseen and unknowable forces. Today, after millennia of study, no belief systems based on superstitions stand the test of experiment. • The Scientific Method, especially in modern times where many scientists of equal skill compete to verify facts by experiment, is the most powerful method of obtaining new knowledge.

44. What have we learned? • How can we distinguish science from non-science? • Science: seeks explanations that rely solely on natural causes; progresses through the creation and testing of models of nature; models must make testable predictions • What is a scientific theory? • A model that explains a wide variety of observations in terms of a few general principles and that has survived repeated and varied testing