The Solar System. PLATO 2011: Planets - Foreign Worlds 2

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2 The Far Side of the Moon Have you ever noticed that we can only ever see one side of the moon? PLATO 2011: Planets - Foreign Worlds 1 Our Neighborhood 2

3 Phase locking (=co- PLATO 2011: Planets - Foreign Worlds 1 Our Neighborhood 3

4 Lunar Size The moon appears to change size. At the horizon, it appears bigger. This is an optical illusion. 4

5 Lunar Size The moon and the sun happen to be about the same angular size on the sky: 1/2 Angular size measures (use out-stretched arm):

6 Eclipses 6

7 Lunar Eclipses Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7

8 Lunar Eclipses Copyright 2004 Pearson Education, publishing as Addison Wesley 8

9 Why are Lunar Eclipses Rare? Copyright 2004 Pearson Education, publishing as Addison Wesley 9

10 Solar Eclipses 10

11 Solar Eclipses What you see from Earth 11

12 Solar Eclipses From Space International Space Station (2006) Mir (1999) 12

13 2011 Eclipses: Partial Solar eclipse on January 4 (visible from Europe, North Africa, Central Asia) Partial Solar eclipse on June 1 (Arctic region). Partial Solar eclipse on July 1 (Antarctica) Total lunar eclipse on June 15 (Africa, Europe). Partial Solar eclipse on November 25 (Antarctica) Total lunar eclipse on December 10 (Visible from part of North America, Africa, Europe) 13

14 Future Solar Eclipses Copyright 2004 Pearson Education, publishing as Addison Wesley 14

15 Planets The phases of Venus 15

16 Stellarium 16

17 Planetos: Wanderer Planets Mercury, Venus, Mars, Jupiter, Saturn, (Uranus, Neptune) Like Sun and Moon, planets move relative to the stars Planets always move along the ecliptic. Planets usually move West to East, relative to stars. But sometimes they move backwards, East to West. This backward motion is called retrograde motion. Planets don t twinkle - they are not points! 17

18 Venus and Mercury: Inferior Planets Always close to the Sun in the sky Venus:! Within 47 from Sun Mercury:! Within 28 from Sun They spend about equal time East and West of Sun They show phases Conclusion: Their orbits are smaller than Earth s orbit They are closer to the sun 18

19 Inferior Planets Inferior Conjunction r o ri ion e p ct u S jun n o C 19

20 Superior Planets Mars, Jupiter, Saturn, Uranus, Neptune: Can be anywhere on the ecliptic (close or far from Sun) Travel mostly Eastward, slower than Sun. Show retrograde motion once a year for shorter period They do not show phases Conclusion: Their orbits are bigger than Earth s orbit They are further away from the sun 20

21 Superior Planets Opposition n o i ct n u j n o C 21

22 Retrograde Motion 22

23 Summary: The Sun is the largest, most massive body at the center of the Solar System All planets orbit the Sun The moon orbits the rotating Earth This explains: The phases of the Moon, Venus, and Mercury Eclipses Planetary motion Diurnal motion 23

24 The Historical Perspective Since prehistoric time: Sky was part of culture Important for Time keeping (time of day) Seasons (agriculture) Orientation, navigation Mystical and metaphysical connotation Fortune telling (Astrology) 24

25 The Greek Enlightenment Early Greek Astronomy (Thales and the Ionian School, 7th century BC): There exists an underlying order to the universe This removes the influence of the supernatural It possible for mortals to understand this order with rational thought 25

26 The Greek Enlightenment Pythagoras (5th century BC): Mathematics is part of this order Heavenly bodies adhere to ideal form: Sphere, circle The universe is harmonic, rational numbers express order, but

27 The Greek Enlightenment Plato (4th century BC): Geometric ideals underly reality All celestial bodies should be ideal spheres Universe should be harmonious Geometric representation of Universe Using only circular motion All insight derives from reason Senses are imperfect, observation is secondary to thought experiment and reason (different from modern science) 27

28 The Greek Enlightenment Aristotle (350 BC): Sun is further away than Moon Moon shines by reflected light - phases Moon occults the Sun during eclipse Spherical Earth Round shadow during lunar eclipses Travel in latitude alters star positions Earth is at center, celestial sphere revolves around it If Earth were moving we would see parallax of nearby stars 28

29 The Scale of the Solar System Aristarchus of Samos (~300 B.C.) Relative distance to Moon and Sun Estimated α 87 (actually: α = 89.8) A/D = Cos(α) 0.05 Sun 20x further away (actually: 390x) A D 29

30 The Scale of the Solar System Aristarchus of Samos (~300 B.C.) Relative size of Moon and Sun Sun 20x further away (actually: 390x) Same angular size Sun 20x bigger than Moon (actually: 390x) 1/2 30

31 The First Heliocentric Model Aristarchus of Samos (~300 B.C.) Relative size of Moon and Sun Sun 20x (390x) bigger than Moon Lunar eclipse: Shadow ~ 3x bigger than Moon Earth ~ 3x bigger than Moon Sun ~ 7x bigger than Earth! Sun should be at Center, not Earth 31

32 A Huge Leap Backwards Aristarchus heliocentric model was rejected because stellar motion showed no observable parallax. Underlying assumption: The stars are about as far away as other objects in the solar system. "[Cleanthes, a contemporary of Aristarchus] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth],... supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis." Plutarch 32

33 Parallax Parallax: The same principle as retrograde motion Foreground objects appear to move opposite to our motion relative to background objects Radius R Distance D Parallax angle α 33

34 The Size of the Earth Eratosthenes (~200 BC) All of Aristarchus results were relative sizes (to Earth s radius) Eratosthenes set out to measure the absolute size of Earth Sun at zenith in Syene (Egypt) on Summer Solstice In Alexandria, Sun casts a shadow at a 7 angle on Summer Solstice 34

35 The Size of the Earth From Syene to Alexandria: 5,000 stadia (800 km) 7 = 5,000 stadia 360 = stadia Earth Radius: 41,000 stadia (6,500 km) 35

36 The Geocentric Model Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 36

37 The Geocentric Model Eudoxus (~340 BC) Spherical Earth at center Celestial objects on crystalline spheres orbiting the Earth This model explains: Phases of the Moon Daily Motion Eclipses It does not explain: Retrograde motion of planets 37

38 The Geocentric Model Eudoxus (~340 BC) To explain retrograde motion: Put each planet on yet another, smaller rotating sphere Smaller sphere attached to larger sphere The double-spinning motion is called epicycle 38

39 Daily Rotation 39

40 Ptolemy (140 AD) Ptolemaic Model Collected large volume of archival data Synthesized the most accurate Geocentric model possible from data Account for all known observations With an accuracy of ~ 5 Astronomy now data-driven Models must explain existing data New data used to refine or invalidate models 40

41 Revised Geocentric Model Non-uniform motion of sun and planets required: Eccentric circles Epicycles upon epicycles Non-uniform circular motion: Constant rotation around the equant Slowest along its track when closest to equant Earth not at center equant 41

42 Copernican Revolution Nicolaus Copernicus (1543) De Revolutionibus Orbium Coelstium (On the Revolution of the Celestial Spheres) Copernicus re-introduced the heliocentric model Assumptions: Sun at center Earth rotates Earth-Sun distance << Earth-star distances (solves parallax) Uniform circular motion 42

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45 Copernican DNA Library book read by Copernicus with hair used for DNA match 45

46 Ockham s Razor If two scientific models explain the existing data equally well, the one with the fewer assumption (the simpler one) is favored. Example: Ptolemy needed a large number of epicycles and corrections, while Copernicus needed many fewer assumptions. This principal is called Ockham s Razor (in that it erases any unnecessary stubbles ). 46

47 The Scientific Method A scientific theory......must explain the existing body of evidence...must be based on logic and understandable (in principle) by humans...must make testable predictions...if a prediction is falsified, the hypothesis must be modified or discarded...should adhere to Ockham s Razor (if a simpler hypothesis is equally capable, it is favored) 47

48 Astrology Aquarius: Your usual outgoing and often radical nature may be far more conservative today. This can be a result of the day's planetary influences, and if used well, this mood can prove most beneficial. It can add seriousness to your day that suits such activities as goal setting, budget planning, or recovery. Make the most of this energy by digging into the areas of your life that could use some rethinking. (from my.horoscope.com) Horoscopes are generic and make vague statements that can most often not be tested. When a prediction of a horoscope is testable, it is often wrong, thus falsifying the method used to generate horoscopes (mainly, making stuff up). It does not explain existing evidence (since past predictions are also mostly wrong) It does not adhere to Ockham s razor (since Astrology is not actually an explanation) 48

49 Copernicus Copernican vs. Ptolemaic model: Copernicus not any more accurate Favored because of Ockham s razor In fact, non-uniform motion of sun still requires equant In the early 17th century, how could you have disproved the geocentric model? 49

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51 Galileo (1610) The First Telescope Invented the Astronomical telescope First to use it to look at the sky Breakthrough technology leads to new data revolution Observations published in Sidereus Nuncius (Star Messenger) 51

52 The Moon Galileo discovered: Craters, mountains, valleys Moon like Earth: Not a perfect reflecting sphere 52

53 The Sun Galileo discovered: Sun spots Solar rotation Again, sun not a perfect, unblemished sphere 53

54 Saturn Galileo discovered: Saturn s rings. Initially thought Saturn was three stars in a row: Later he re-observed. Again, not an ideal sphere. 54

55 Galileo discovered: The Milky Way Milky Way is made up of stars Only looks like clouds because stars are incredibly far away Solves the parallax problem of the heliocentric model: If stars are so far away, parallax must be tiny. 55

56 Galileo discovered: Jupiter s Moons Jupiter has satellites: Io, Europa, Ganymede, Callisto (from inside out) Inferred that they were moons just like Earth s moon But orbiting Jupiter 56

57 The end of Geocentrism Galileo discovered: Phases of Venus Tied to angular size and position relative to sun This disproved the Geocentric model Venus tied to solar orbit We would never see illuminated side in geocentric model 57

58 The end of Geocentrism Galileo discovered: Phases of Venus Tied to angular size and position relative to sun This disproved the Geocentric model Venus tied to solar orbit We would never see illuminated side in geocentric model 58

59 First Quarter Earth and Moon 59

60 Tycho Brahe (1575 AD) Tycho Brahe Finest observer since Hipparchus Collected accurate positions Collected many positions Allowed accurate orbits of planets to be constructed Fun fact: Tycho had lost his nose in a duel Has been rumored to have worn a golden nose 60

61 Kepler s Laws Johannes Kepler (1600 AD) Driven by Tycho s data Rudolphine Tables (with Tycho) ~1500 star positions Goal: More accurate predictions of planetary positions To make those predictions: He needed more accurate model to predict positions Better than Copernicus and Ptolemy 61

62 Kepler s First Law The Orbits of the planets are ellipses with the Sun at one focus focal distance f semi-major axis a Eccentricity: e = f / a 62

63 Kepler s Second Law The straight line between the Sun and a planet sweeps out equal areas in equal times 30 days 30 days Both areas are equal 63

64 Kepler s Third Law The square of a planet s orbital period P (measured in years) is equal to the cube of the semi-major axis A of its orbit (measured in AU): P 2 = A 3 The orbital period is the duration of one orbit (e.g., one year for Earth) 64

65 Kepler s Third Law Planet: Period P Semi-major axis A P 2 A 3 Mercury Venus Earth Mars Jupiter Saturn Uranus ,058 7,067 Neptune ,160 27,160 65

66 The Birth of Modern Physics Ptolemy s, Copernicus, and Kepler s models are phenomenological: They describe the motion of bodies They do not explain why the bodies move this way They do not allow calculation of motion of other objects Ptolemy and Copernicus: Uniform circular motion Motivated by Aristotelian desire for harmony Kepler motion: Neither circular nor uniform This called for a deeper understanding 66

67 Mechanics Galilieo: Father of experimental physics Law of inertia: An undisturbed body in motion stays in motion 67

68 Newton One of histories greatest geniuses Father of theoretical physics Invented calculus Critical for all of physics Newton s laws of motion Newton s law of gravity Discovered the spectrum of light Invented the reflecting telescope 68

69 Newton s First Law An object remains at rest or moves at uniform speed along a straight line, unless acted upon by a force. 69

70 Newton s Second Law The acceleration of an object is proportional to the force acting on it and is in the direction of the force. The acceleration is inversely proportional to the mass of the object (the heavier an object, the less acceleration a given amount of force produces) or a = F m F = ma 70

71 Newton s Third Law To every action (force) there is an equal and opposite reaction (counter-force) 71

72 Newton s Third Law To every action (force) there is an equal and opposite reaction (counter-force) Examples: Gun recoil Rockets Collisions Hamster wheel 72

73 Click Now You are an astronaut taking a spacewalk to fix your spacecraft with a hammer. Your lifeline breaks and the jets on your back are out of fuel. To return safely to your spacecraft you should A. Throw your hammer at your spacecraft to get someone s attention B. Throw your hammer in the direction away from the spacecraft C. Use a swimming motion with your arms D. Kiss your spacecraft goodbye 73