Last time we finished Ch. 2

Transcription

1 Last time we finished Ch. 2 Everyday astronomy 1. Patterns in the Sky 2. The Cause of Seasons 3. The Phases of the Moon 4. The Ancient Mystery of the Planets

2 Last time we finished Ch. 2 You re getting good! I got to stay sharp ;-)

3 Last time we finished Ch. 2 Precession changes the place in orbit where max tilt occur (winter/ summer solstice). However, our current calendar adjusts for this. You re getting good! got to stay sharp ;-)

4 Last time we finished Ch. 2 Cause of the seasons

5 Last time we finished Ch. 2 Rising and Setting of Stars

6 Last time we finished Ch. 2 Phases of the moon

7 Last time we finished Ch. 2 Retrograde motion of planets on sky

8 Last time stellar collisions Order of Magnitude Rate = Vel Area N Vol t av = 1 Rate t av is average time to coll. ~10^5 ly ~10^3 ly N ~ 10^11 stars Vel 10 2 km/sec Area 3 (10 6 km) 2 = km 2 N Vol (50000) ly ly km km 3 Rate 10 2 km sec km = sec year km 3! t av years!!! Age of Universe? ( years)

9 Last time stellar collisions However, if we lived here, things would be different. Globular Cluster of stars in Milky Way à Ch 12

10 Admin -- Any questions about first HW? Next HW 2 (mostly on Ch. 2) will be posted on MA tonight deadline ** Monday 3.00 PM ** Updates as usual on course homepage:

11 Lecture Outline Chapter 3: The Science of Astronomy

12 3.1 The Ancient Roots of Science Some goals for learning: In what ways do all humans use scientific thinking? How is modern science rooted in ancient astronomy?

13 In what ways do all humans employ scientific thinking? Scientific thinking is based on everyday ideas of observation and trial-and-error experiments. Essentially goes for all science natural as well as social This can be contrasted to belief by pure faith.

14 How is modern science rooted in ancient astronomy? Astronomy is the oldest of the sciences. It was often practiced for practical reasons. In keeping track of time and seasons for practical purposes, including agriculture for religious and ceremonial purposes In aiding navigation (recall our discussion about Polaris)

15 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 Read about it yourself -- here follows just a few illustrating slides

16 Nazca, Peru: Lines and patterns, some aligned with stars when I was a kid, I thought these were made by ancient astronauts

17 Machu Picchu, Peru: we believe Inca people aligned their cities in the Andes according to their stars in the sky.

18 "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." "On the Xinwei day the new star dwindled." Bone or tortoiseshell inscription from the 14th century B.C. China: Earliest known records of supernova explosions (1400 B.C.) à We ll discuss Supernovae later during course

19 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?

20 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. Greek geocentric model (c. 400 B.C.)

21 Eratosthenes measures the Earth (c. 240 B.C.) -- we repeat his calculation now!

22 But this made it difficult to explain the apparent retrograde motion of planets Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

23 The most sophisticated geocentric model was that of Ptolemy (A.D ) the Ptolemaic model: Remained in use for 1500 years Ptolemy See interactive Fig. in MA

24 Thought Question Which of the following is NOT a fundamental difference between the geocentric and Sun-centered models of the solar system? A. Earth is stationary in the geocentric model but moves around the Sun in Sun-centered model. B. Retrograde motion is real (planets really go backward) in the geocentric model but only apparent (planets don't really turn around) in the Sun-centered model. C. Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model. D. The geocentric model is useless for predicting planetary positions in the sky, whereas even the earliest Sun-centered models worked almost perfectly.

25 Thought Question Which of the following is NOT a fundamental difference between the geocentric and Sun-centered models of the solar system? A. Earth is stationary in the geocentric model but moves around the Sun in Sun-centered model. B. Retrograde motion is real (planets really go backward) in the geocentric model but only apparent (planets don't really turn around) in the Sun-centered model. C. Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model. D. The geocentric model is useless for predicting planetary positions in the sky, whereas even the earliest Sun-centered models worked almost perfectly.

26 Preserving the ideas of the Greeks The Muslim world preserved and enhanced the knowledge they received from the Greeks while Europe was in its Dark Ages. Al-Mamun's House of Wisdom in Baghdad was a great center of learning around A.D With the fall of Constantinople (Istanbul) in 1453, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance.

27 3.3 The Copernican Revolution Some goals for learning: How did Copernicus, Tycho, and Kepler challenge the Earth-centered model? What are Kepler's three laws of planetary motion? How did Galileo solidify the Copernican revolution?

28 How did Copernicus, Tycho, and Kepler challenge the Earth-centered model? Copernicus ( ) Copernicus proposed the Sun-centered model (published 1543). He used the model to determine the layout of the solar system (planetary distances in AU). But... The model was no more accurate than the Ptolemaic model in predicting planetary positions, because it still used perfect circles.

29 Tycho Brahe ( ) Brahe compiled the most accurate (1 arcminute) naked eye measurements ever made of planetary positions. He still could not detect stellar parallax, and thus still thought Earth must be at the center of the solar system (but recognized that other planets go around the Sun). He hired Kepler, who used Tycho's observations to discover the truth about planetary motion.

30 Kepler first tried to match Tycho's observations with circular orbits. But an 8-arcminute discrepancy led him eventually to ellipses. Johannes Kepler ( ) "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."

31 What is an ellipse? An ellipse looks like an elongated circle.

32 What are 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.

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

34 Kepler's Third Law More distant planets orbit the Sun at slower average speeds, obeying the relationship p 2 = a 3 p = orbital period in years a = average distance from Sun in AU

35 Prelude Kepler never knew the underlying physical laws governing these planetary motions it took Newton to figure that out. We will study Newton s laws of motion and the fundamental force of gravity in Ch. 4. Here s a prelude, essentially leading up to Kepler s third law. (By equating the gravitational to the centrifugal force for a planet in circular orbit of the sun.)

36 Prelude à Don t panic! We will do this again in Ch. 4.