Astro 201: Aug. 26, 2010

1. Astro 201: Aug. 26, 2010 • Turn in HW #1 in front of room • Pick up Telescope Lab Handout • Reading: • Hester et al: Chapter 2 • Gleiser: Chapters 1-6 • Wait List students have been added; class limit will be increased, so everyone can be enrolled • Today: • Discuss Telescope Lab, On-Line quiz • History of cosmology and the scientific method

2. On-line Quiz, go to d2l: Open book, 10 questions, available after class today, do it before class next Thursday Which of the following is the largest? a. the diameter of the Moon b. the diameter of Earth c. the diameter of the Milky Way d. the diameter of the Sun e. 1 astronomical unit

3. Cosmology: Some History

4. Lascaux Cave Paintings: Cro-Magnons recorded the lunar cycle 15,000 years ago http://news.bbc.co.uk/2/hi/science/nature/975360.stm

5. PATTERNS IN THE SKY • Ancient peoples observed the Sun and stars and created calendars that marked the yearly passage of time • Earth rotates on its axis once every 24 hours • Earth revolves around the Sun once every 365 days • The stars appear to rise in the east and set in the west every night, on a big dome called the celestial sphere.

7. The Earth's rotation is counter clockwise looking down on the N pole, and results in the apparent motion of the celestial sphere -- everything rises in the East and sets in the west. 1 solar day = time from one noon to the next = 24 hours

8. Circumpolar Stars Always above Horizon In the Northern Hemisphere, they Appear to rotate around The North Star, Polaris In the Southern Hemisphere, No bright star at rotation center

9. Celestial Sphere and Constellations The sky is divided into 88 "official" constellations, although in reality the stars in any particular constellation are at different distances from Earth.

10. Figure 2.3: The celestial sphere as a useful fiction

11. Figure 2.4: As viewed from the Earth’s North Pole

12. Figure 2.5: Our perspective on the sky depends on our location

13. Figure 2.9, Right: Celestial sphere at different latitudes

14. Figure 2.10: Earth’s orbit and constellations along the ecliptic The Zodiac

15. Rise and set of the Sun and Starshas 2 possible explanations • GEOCENTRIC Earth at the center Sun and Stars go around the Earth once per day • HELIOCENTRIC • Earth rotates on its axis and so the Sun and stars • appear to go around the Earth

16. Ancient Egyptian celestial sphere:the goddess Nut (“noot”) c. 3000 BC Nut gave birth to the Sun god “RA” every morning

17. Hindu Rig Veda (1500 BC) • Written cosmology (Sanskrit) • Cyclical or recursive universe • Universe originates from the Hiranyagarbha, or golden egg • Timescales eerily coincident with modern results, e.g. age of the Earth

18. Hebrew Cosmology And God said, “Let there be a vault in the midst of the waters, and let it divide water from water.” And God made the vault and it divided the water beneath the vault from the water above the vault, and so it was. - Genesis 1:6 Michaelangelo’s depiction of the creation Of the planets and stars

19. God had divided the waters "above" from the waters "below" by constructing an immense dome that held open the space for dry land. In the Hebrew Bible the dome is called "raqi'a," meaning a firm substance, and rendered in the King James translation as "the firmament” The firmament in Biblical times was understood to be firm only by the will of God.

20. If God were angered, "the windows of heaven" and "the fountains of the deep” could burst open once again and those lovely blue waters would destroy the Earth. A painting by the American Edward Hicks (1780–1849), showing the animals boarding Noah's Ark two by two.

21. Aristotle • 4th century BC, Greece • First to give reasons why the Earth is spherical

22. Aristotle’s 1st reason: Gravity pulls matter to center of Earth, compressing the Earth into as compact a shape as possible. …a sphere Thanks to Barbara Ryden for this discussion

23. 2nd reason: Big Dipper You see different stars from the south than from the north. Southern Cross

24. If the Earth were flat: Southern Cross Big Dipper

25. 3rd reason: The shape of the Earth’s shadow. During a lunar eclipse, Earth’s shadow is always circular. Only object whose shadow is always circular is a sphere.

26. Another Greek: Aristarchus of Samos 3rd century BC Proposed that the Earth rotates on its axis & goes around the Sun. Rejected by contemporaries He used geometry to estimate the size of the Sun and Moon, and their relative distances

27. What was his reasoning? • The Sun is farther away than the Moon because the Moon comes between the Sun and the Earth during a solar eclipse. ANIMATION

28. The phases of the Moon result from the Moon orbiting • the Earth ANOTHER ANIMATION

29. Aristarchus measured the angle between the Moon and the Sun when the Moon was half lit, and deduced the ratio of the Sun-Earth distance to the Moon-Earth distance.

30. 90° 3° 87° Here’s a picture more to scale M S E Earth – Sun distance (E-S) is 19 times Earth – Moon distance (E-M). (actual value 400x) Thus, Aristarchus correctly discovered that the Sun is much farther away from the Earth than the Moon. He then concluded that the Sun was MUCH bigger than the Earth or Moon. He reasoned that it was more plausible that the smaller body would orbit the bigger body: thus the Earth orbits the Sun

31. Despite Aristarchus, for 2000 years, a GEOCENTRIC model of the Universe was favored EARTH at the center STARS affixed to the celestial sphere Moon, Planets and the Sun are between the EARTH and STARS

32. PTOLEMY 2nd century AD Ptolemy’s ALMAGEST was translated into Latin in 1496 GEOCENTRIC: Earth at center

33. Copernicus (1473-1543)Heliocentric Model

34. Ptolemy’s Epicycles • In order to understand the motion of the planets in the sky Ptolemy’s model had to be modified • In terms of the heliocentric model, the planets orbit the Sun in ellipses, not circles • Retrograde motion of the planets with respect to the stars required EPICYCLES

35. Eclipticthe fact that the planets orbit the Sun in a plane means that they always appear to lie on a great circle on the sky, called the ecliptic

36. Retrograde motion The planets sometimes appear to be traveling west to east, unlike the stars and Sun which always go east to west Next slide: animation Astronomy Picture of the Day for Dec. 20, 2001. Jupiter and Saturn showing retrograde motion. Combining 23 pictures taken at 2 week intervals from June 2000 - May 2001. Planet = “wanderer”

38. Ptolemy’s explanation for retrograde motion of the planets

40. Earth Off Center Earth Equant Epicycle Deferent The planet moves along its epicycle as the epicycle moves along the deferent around the Earth. To make the observations as accurate as possible, it was necessary to place the Earth slightly off center of the orbits, but to preserve symmetry that meant that there was an equal place (“Equant”) opposite the Earth from the center. The combined motion of the planet and the resulting retrograde motion are shown.

41. Copernicus’ explanation for retrograde motion

42. Copernicus: “On the Revolution of the Heavenly Spheres” Published at the time of his death, in 1543 • Major Conclusions: • The planets orbit the Sun • The apparent daily motion of the Sun and stars is the result of the Earth’s rotation • The stars are much farther away than the Sun

43. Heliocentric model: distance from Sun to stars must be much greater than distance from Sun to Earth. Since Earth orbits Sun, stars should show parallax (a shift in apparent position) over the course of half a year.

44. OBSERVATION: Parallax of stars is TOO SMALL to be seen by the naked eye. Not observed until 1800s. Implication: distance to stars is several thousandtimes Earth – Sun distance.

45. Parallax Unit of distance: the PARSEC A star which is one parsec from Earth shows a parallax of one arcsecond Remember: 360 degrees in a circle, 60 arcminutes per degree, 60 arcseconds per arcminute

46. Reactions to Copernicus: On March 5, 1616, Copernicus' work was banned from being taught and discussed by the Congregation of the Index "until corrected." It stayed on this list of prohibited books and teachings until 1822. Martin Luther (1483-1546): [Copernicus] “is a fool who wishes to reverse the entire scheme of astronomy; but sacred scripture tells us that Joshua commanded the Earth to stand still, not the Sun.” Giordano Bruno (1548-1600): burned at the stake for advocating that stars are suns in their own right, and that there is a plurality of worlds like the Earth.

47. Cosmological Models: v. 1.0 v. 3.0 v. 2.0 Version 1.0: “Superdome” model Version 2.0: Geocentric model Version 3.0: Heliocentric model

48. Which is right? Occam's Razor: Pluralitas non est ponenda sine neccesitate "plurality should not be posited without necessity." Given a set of otherwise equivalent models of a phenomenon, the simplest one is the best. Keep it simple, stupid.

49. William of Occam 1285-1349 English philosopher from Ockham Franciscan Monk Got into trouble with the Pope for advocating “apostolic poverty” Died of the Black Death while in exile Advocated Epistomological Parsimony c.f. Ontological parsimony Epistomology = theory of knowledge Ontology = what exists?

50. Galileo Galilei (1564 – 1642) Italian First to use a telescope to look at the sky • 1610 “Siderius Nuncius” (The Starry Messenger) • “Spots” on the Sun; the Sun rotates • The Moon has mountains, craters, rocky surface with imperfections • The “planet” Jupiter is not a pinpoint star – but a disc in the sky WITH MOONS that orbit it • Venus has “PHASES” like the MOON