Class Etiquette

1. Class Etiquette Don’t distract your classmates

2. Outline - Jan. 26, 2010 • Last word on Eclipses (pgs. 45-47) • Planetary motions (pgs. 49-51) • Stellar parallax (pg. 52) • Ancient Observatories (pgs. 60-65) • Ancient Greek Astronomy (pgs. 65-68) • Copernicus, Tycho, Kepler, Galileo (pgs. 70-78)

3. Cause of Eclipses Total lunar eclipse Total solar eclipse Annular (“ring”) solar eclipse Solar eclipse: new moon passes in front of the sun, shadow of the new moon falls on the earth Lunar eclipse: earth passes between the sun and the full moon, shadow of the earth falls on the full moon

4. Why don’t we get a solar eclipse with every new moon and a lunar eclipse with every full moon? Moon’s orbit is inclined at 5o to Earth’s orbit. It is only when the moon is precisely in the same plane as the earth’s orbit that eclipses can occur.

5. How to tell a planet from a star without a telescope Five planets are visible without a telescope. They look like points of light (like stars), with one important difference. “Planets” comes from the Greek word “planetes” meaning “wanderers”.

6. Planets move with respect to the (fixed) stars Locations of Venus and Jupiter over the course of 6 nights. All photos taken when the star “Spica” was in the same location in the sky. Notice how Venus has moved much farther than Jupiter over this period of time! The primary motion of the planets is from WEST to EAST Sometimes the planets reverse their motion and travel from EAST to WEST, a phenomenon known as “retrograde motion”

7. Example: Retrograde Motion of Mars East West From Jan. 4, 1995 to March 25, 1995, Mars is moved “backward” in the sky (east to west) On astronomical maps, east is on the LEFT and west is on the RIGHT. Why????

8. From what perspective are you viewing this picture? West East Astronomers look UP at the sky!

9. Actual image of Mars undergoing “retrograde motion” Do you notice anything different about Mars when it is undergoing retrograde motion? (Assume all the individual exposures were the same length of time.) Retrograde motion is on the TOP part of the “loop” in the sky.

10. It’s all about perspective! • We see the planets move with respect to the fixed stars • Space is really 3-d (planets are much closer than the stars), but space looks 2-d (when you look at a picture of the sky you don’t have a sense of depth that tells you the planets are closer than the stars) • The planetary motion you see is due to combination of: (1) earth’s motion about the sun and (2) the planet’s motion about the sun • Retrograde motion happens when an inner planet (e.g., the earth) catches up to and “laps” an outer planet (e.g., Mars) • All planets orbit in the same direction about the sun continuously, they only appear to go “backward”

11. Retrograde Motion of Mars • Happens about once every 2 years • Only occurs when Mars and Earth are near their distance of closest approach (called “opposition” because Mars and the Sun are on opposite sides of the sky, as seen from earth) • Is currently going on! Mars will be at opposition on Friday (Jan. 29) • For every 1 orbit of the Earth around the sun, Mars makes about 2 orbits around the sun http://www.youtube.com/watch?v=72FrZz_zJFU

12. Does the earth really orbit the sun?What proof do we have? Once again, it’s all a matter of perspective! As the earth orbits the sun, nearby stars appear to shift their location in the sky compared to extremely distant (“background”) stars. The farther is a star, the smaller is its parallax. If p is measured in arcseconds, the distance to the star is d = 1/p where d is in units of “parsecs” 1 parsec = 1 pc = 3.26 ly The closest star to us (other than the sun) has a parallax of p = 0.7687 arcsec = 2.1x10-4 degree. This is much too small to see without a telescope!!

13. Ancient ObservatoriesStonehenge (3000 to 5000 years old)Salisbury Plain, west of London, England Truly Monolithic! Largest stones weigh 50 tons Rising and setting points of sun and moon from various alignments Aubry holes used to predict eclipses Heel Stone Sunrise on summer solstice

14. Heaven on Earth?Pyramids of and near Giza Plateau Looking through shaft from King’s chamber around 2600 BC would have seen Orion’s belt when it first appeared in the winter sky “Great Pyramid” of Giza (Cheyops Pyramid) built for Khufu between 2589 BC ad 2566 BC; first pyramid on the Giza plateau Osiris, god of death and resurrection, resided in Constellation of Orion

15. A scale model of Orion in Egypt?These were Master Builders!Why are the other 2 pyramids at Giza smaller than the first and “not quite” in a straight line? No pyramids at locations of “Betelguese” or “Rigel” Milky Way = Nile

16. Ancient ObservatoriesNorth America Caracol Temple, Yucatan Peninsula, built by Mayan Indians around 1000 AD Windows aligned with celestial events, particularly rising and setting of Venus (associated with deity Kulkulkan), as well as rising and setting of sun on solstices and equinoxes Big Horn Medicine Wheel (Wyoming), built by Plains Indians 27 “spokes” and other features aligned with astronomical events (rising and setting of sun and stars) Largest and most complete example

17. Heliocentric vs. Geocentric(sun-centered vs. earth-centered) We know that we live in a heliocentric (sun-centered) system. The ancient Greeks (for the most part) were completely convinced that we lived in a geocentric system. The ancient Greeks weren’t stupid people! They had reasons for their beliefs. They also didn’t have telescopes.

18. Ancient Greek Rationale for Stationary Earth If Earth does revolve/orbit about the sun, what would you expect the consequence to be for the locations of stars observed 6 months apart? If Earth does rotate on its axis once per day, what would you naively expect to observe/experience on this “moving” body? If Earth were not at the center of the universe, then why do all objects fall toward earth? Note: Ancient Greeks knew the earth was spherical for a variety of reasons (heights of specific constellations seen from different latitudes, apparent sizes of ships sailing away from harbor don’t shrink proportionately, shadow of earth on the moon during a lunar eclipse is always curved).

19. Ancient Greek “Philosophy” (not based on observation) Earth was the realm of the unpure (non-eternal, constantly changing, decay) Heavens were the realm of the pure (eternal, absolutely unchanging) Heavenly objects must be made of pure/perfect geometrical forms (flawless spheres), and must travel on the purest of orbits (perfect circles) Built up a geocentric model of the solar system based upon this philosophy (explained transient phenomena such as comets and meteors as forms of “weather”; i.e., they were earthly phenomena) Note: most of the above assertions are untestable without telescopic observations!

20. Retrograde Motion in a Geocentric System Impossibleto explain the observed planetary motions (especially retrograde motion) just by putting each planet on a circular orbit, centered on the earth Result: circular “epicyles” that made the planets literally move backward in their orbits! Resulting orbit: “loop-de-loops” in the sky; planet swings backward on the inner part of the little loops

21. Claudius Ptolemy(about 100 to 170 AD) Extraordinarily complicated geocentric model (epicycles on top of epicycles, centers of primary orbits offset from the location of the earth) Needed 80+ circles just to explain the motion of the sun, moon and 5 planets Incredibly accurate - was still working well for predicting locations of planets and occurrences of eclipses in the late-16th century!

22. There must be a simpler way…Nicolaus Copernicus (1473-1543) Sun-centered, planets placed at correct relative distances from the sun, inner planets moving more quickly than outer planets Motivation was simplicity (not greater accuracy compared to Ptolemy) Retrograde motion caused by inner planets catching up to and “lapping” outer planets Eventually had to add epicycles to make sun-centered model as accurate as Ptolemaic geocentric system

23. The heavens can change!Tycho Brahe (1546-1601) Decided to become astronomer at age 13 after witnessing partial eclipse of the sun (Aug. 21, 1560) predicted by Ptolemaic tables from 150 AD Observed supernova in 1572 (did not move in the sky, so it had to part of the heavens) Measured parallax of a comet in 1577 and proved that distance to comet was greater than distance to the moon (so comets weren’t “weather”)

24. Tycho on the 1572 Supernova “Amazed, and as if astonished and stupefied, I stood still, gazing for a certain length of time with my eyes fixed intently upon it and noticing that same star placed close to the stars which antiquity attributed to Cassiopeia. When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes.” Such an event was simply inconceivable in the ancient Greek view of the heavens.

25. Correct Model of the Solar SystemJohannes Kepler (1571-1630)using Tycho Brahe’s lifetime’s worth of planetary motion data First Law: The orbit of each planet around the sun is an ellipse with the sun at one focus. Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal time. Third Law: The relationship between a planet’s period of orbit about the sun (P) and the semi-major axis of its orbit (a) is P2 = a3 where P is measured in (earth!) years, and a is measured in AU Purely empirical laws; they only tell you “what”, not “why”

26. Kepler’s First Lawplanets move on elliptical orbits An ellipse is a geometric figure with two symmetry axes: major axis (= long axis) and minor axis (= short axis). Half the length of the major axis is called the “semi-major axis”. The farther apart are the two foci, the “flatter” the ellipse is. The closer together are the two foci, the “rounder” the ellipse is. A circle is a special case of an ellipse in which the two foci are on top of each other (in the center of the circle).

27. Kepler’s Second Lawequal areas in equal time The areas (“square footage”) of each of the blue triangles are identical. The time (t) is also identical for each triangle. Consequence of Kepler’s second law: planets move faster when they are closeto the sun than when they are far from the sun.

28. Kepler’s Third LawP2 = a3 P = the time it takes a planet to orbit once around the sun, measured in EARTH years a = half the length of the long axis of the planet’s orbit, measured in AU

29. Galileo Galilei(1564-1642) First person known to use a telescope to make detailed study of the sky Observations provided strong proof that the Aristotelian (= ancient Greek) philosophy of the heavens was wrong Observations provided direct proof that not all objects orbited the earth and at least one (Venus) orbited the sun

30. Sun, Moon and Jupiter are not “Flawless Orbs” Galileo observed that sun has black spotches (sunspots), moon has craters and mountains (very “earth-like” terrain), and Jupiter has big red spot

31. Galilean Moons of Jupiter(not all objects orbit the earth) Galileo discovered the 4 largest moons of Jupiter, and showed that they orbit Jupiter, not the earth

32. Venus Orbits the SUN!Venus could not show “full” phase in Ptolemy’s model According to Ptolemy, Venus would never show the “full” phase because it could never be on the opposite side of the sky as the sun (as seen from earth). Phases of planets can only be seen using a telescope. Venus shows the full range of phases (full, gibbous, crescent, new) just like the moon. Venus is smallest (and dimmest) in its full phase.

33. Phases of Venus in the “Keplerian” Model (Venus orbits the sun, and is always closer to the sun than Earth) Venus is in the “full” phase when it is at its farthest distance from the earth, therefore it appears very small in the sky. Venus is in the “new” phase with it is at its closest distance to the earth, so the “narrow crescent” Venus looks very large (and very bright)!

34. Minute Paper A few sentences on one of the following: * something you found particularly interesting today * something you found particularly confusing today * questions on things from today that you would like to know more about Be sure to PRINT your name legibly