ASTRO 101

1. ASTRO 101 Principles of Astronomy

2. Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: orosz@sciences.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00

3. Homework • Homework due February 5: Question 11 from Chapter 2 (In what ways did the astronomical observations of Galileo support a heliocentric cosmology?) • Write down the answer on a sheet of paper and hand it in before the end of class on Febuary 5.

4. Dispatches from the Deparment of Anal Retention • The Earth’s axis is tilted… • What is the tilt relative to? • The Northern Hemisphere is more exposed to the Sun… • Why is it warmer in the summer than in the winter?

5. Homework • Go to a planetarium show in PA 209: • Tuesday, January 29   1:00 -- 2:00 PM • Wednesday, January 30   2:00 -- 3:00 PM • Friday, February 1   1:00 -- 2:00 PM • Monday, February 4   11:00 AM -- 12:00 PM • Tuesday, February 5   5:30 PM -- 6:30 PM • Wednesday, February 6   2:00 -- 3:00 PM • Thursday, February 7   5:30 -- 6:30 PM • Friday, February 8   2:30 -- 3:30 PM • Get 10 points extra credit for homework part of grade. • Sign up for a session outside PA 209. • Hand in a sheet of paper with your name and the date and time of the session.

7. Watch Your Head! • http://www.cnn.com/2013/02/05/world/space-asteroid/index.html?hpt=hp_c4

8. A Brief History of Astronomy

9. A Brief History of Astronomy • An early view of the skies: • The Sun: it rises and sets, rises and sets… • The Moon: it has a monthly cycle of phases. • The “fixed stars”: the patterns stay fixed, and the appearance of different constellations marks the different seasons. • Keep in mind there were no telescopes, no cameras, no computers, etc.

10. A Brief History of Astronomy • But then there were the 5 “planets”: • These are star-like objects that move through the constellations. • Mercury: the “fastest” planet, always near the Sun. • Venus: the brightest planet, always near the Sun. • Mars: the red planet, “slower” than Venus. • Jupiter: the second brightest planet, “slower” than Mars. • Saturn: the “slowest” planet.

11. A Brief History of Astronomy • By the time of the ancient Greeks (around 500 B.C.), extensive observations of the planetary positions existed. Note, however, the accuracy of these data were limited. • An important philosophical issue of the time was how to explain the motion of the Sun, Moon, and planets.

12. What is a model? • A model is an idea about how something works. • It contains assumptions about certain things, and rules on how certain things behave. • Ideally, a model will explain existing observations and be able to predict the outcome of future experiments.

13. Aristotle (385-322 B.C.) • Aristotle was perhaps the most influential Greek philosopher. He favored a geocentric model for the Universe: • The Earth is at the center of the Universe. • The heavens are ordered, harmonious, and perfect. The perfect shape is a sphere, and the natural motion was rotation.

14. Geocentric Model • The motion of the Sun around the Earth accounts for the rising and setting of the Sun. • The motion of the Moon around the Earth accounts for the rising and setting of the Moon. • You have to fiddle a bit to get the Moon phases.

15. Geocentric Model • The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars.

16. The constellations rise and set each night, and individual stars make a curved path across the sky. • The curvature of the tracks depend on where you look.

17. Geocentric Model • The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars. • However, the detailed motions of the planets were much harder to explain…

18. Planetary Motion • The motion of a planet with respect to the background stars is not a simple curve. This shows the motion of Mars. • Sometimes a planet will go “backwards”, which is called “retrograde motion.”

19. Planetary Motion • Here is a plot of the path of Mars. • Other planets show similar behavior. Image from Nick Strobel Astronomy Notes (http://www.astronomynotes.com/)

20. Aristotle’s Model • Aristotle’s model had 55 nested spheres. • Although it did not work well in detail, this model was widely adopted for nearly 1800 years.

21. Better Predictions • Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions. • Claudius Ptolomy (85-165) presented a detailed model of the Universe that explained retrograde motion by using complicated placement of circles.

22. Ptolomy’s Epicycles • By adding epicycles, very complicated motion could be explained.

23. Ptolomy’s Epicycles Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/).

24. Ptolomy’s Epicycles

25. Ptolomy’s Epicycles • Ptolomy’s model was considered a computational tool only. • Aristotle’s ideas were “true”. They eventually became a part of Church dogma in the Middle Ages.

26. The Middle Ages • Not much happened in Astronomy in the Middle Ages (100-1500 A.D.).

27. Next: The Copernican Revolution

28. The Sun-Centered Model • Nicolaus Copernicus (1473-1543) proposed a heliocentric model of the Universe. • The Sun was at the center, and the planets moved around it in perfect circles.

29. The Sun-Centered Model • The Sun was at the center. Each planet moved on a circle, and the speed of the planet’s motion decreased with increasing distance from the Sun.

30. The Sun-Centered Model • Retrograde motion of the planets could be explained as a projection effect.

31. The Sun-Centered Model • Retrograde motion of the planets could be explained as a projection effect. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/)

32. Copernican Model • The model of Copernicus did not any better than Ptolomy’s model in explaining the planetary motions in detail. • He did work out the relative distances of the planets from the Sun. • The philosophical shift was important (i.e. the Earth is not at the center of the Universe).

33. Tycho Brahe (1546-1601) • Tycho was born in a very wealthy family. • From an early age, he devoted himself to making accurate astronomical observations. • He received a great deal of support from the king of Denmark, including the use of his own island.

34. Tycho • Tycho lived before the invention of the telescope. • His observations of Mars were about 10 times more accurate than what had been done before.

35. Johannes Kepler (1571-1630) • Kepler was a mathematician by training. • He believed in the Copernican view with the Sun at the center and the motions of the planets on perfect circles. • Tycho hired Kepler to analyize his observational data.

36. Johannes Kepler (1571-1630) • Kepler was a mathematician by training. • He believed in the Copernican view with the Sun at the center and the motions of the planets on perfect circles. • Tycho hired Kepler to analyize his observational data. • After years of failure, Kepler dropped the notion of motion on perfect circles.

37. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion:

38. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion: • Planets orbit the Sun in ellipses, with the Sun at one focus.

39. Ellipses • An ellipse is a “flattened circle” described by a particular mathematical equation. • The eccentricity tells you how flat the ellipse is: e=0 for circular, and e=1 for infinitely flat.

40. Ellipses • You can draw an ellipsed with a loop of string and two tacks.

41. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion: • Planets orbit the Sun in ellipses, with the Sun at one focus.

42. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion: • Planets orbit the Sun in ellipses, with the Sun at one focus. • The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away.

43. Kepler’s Second Law • The time it takes for the planet to move through the green sector is the same as it is to move through the blue sector. • Both sectors have the same area.

44. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion: • Planets orbit the Sun in ellipses, with the Sun at one focus. • The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away.

45. Kepler’s Three Laws of Planetary Motion • Starting in 1609, Kepler published three “laws” of planetary motion: • Planets orbit the Sun in ellipses, with the Sun at one focus. • The planets sweep out equal areas in equal times. That is, a planet moves faster when it is closer to the Sun, and slower when it is further away. • (Period)2 = (semimajor axis)3

46. Kepler’s Third Law

47. The Kepler’s Law Simulator • There are some animations on the web illustrating Kepler’s Laws: • http://www.astro.utoronto.ca/~zhu/ast210/kepler.html

48. Heliocentric or Geocentric? • The year is around 1610. The “old” school is Aristotle and a geocentric view. The “new” school is the heliocentric view (Copernicus and Kepler). • Which one is correct?

49. Heliocentric or Geocentric? • The year is around 1610. The “old” school is Aristotle and a geocentric view. The “new” school is the heliocentric view (Copernicus and Kepler). • Which one is correct? • Observational support for the heliocentric model would come from Galileo.

50. Heliocentric or Geocentric? • The year is around 1610. The “old” school is Aristotle and a geocentric view. The “new” school is the heliocentric view (Copernicus and Kepler). • Which one is correct? • Observational support for the heliocentric model would come from Galileo. • Theoretical support for the heliocentric model would come from Isaac Newton.