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Astronomy 110: SURVEY OF ASTRONOMY 2. Revolution of the Spheres 1. Earth, Sun and Moon 2. The Puzzle of the Planets This “revolution” is a revolution of ideas. Copernicus started it by describing the Earth as one of many planets. Newton completed the revolution by showing that the same laws of motion work everywhere — on Earth and in space. In between them, Kepler found the first precise laws of planetary motion, and Galileo studied motion on Earth as well as celestial objects. 1. EARTH, SUN AND MOON a. Reasons for seasons b. Phases of the Moon c. Eclipses of Sun and Moon Reasons for Seasons: Summer and Winter March equinox June solstice December solstice September equinox North: SUMMER South: WINTER North: WINTER South: SUMMER Reasons for Seasons: One Year From one Dec. solstice to next, Sun moves north and returns south. Reasons for Seasons: Hot and Cold Not true in tropics! Not true in tropics! Summer: Sun higher, days longer Winter: Sun lower, days shorter HOT COLD Reasons for Seasons: In the Tropics Why are seasons different in the Tropics? 1. Length of daylight does not change much. (At equator, does not change at all.) 2. Sun is not highest at solstice. (e.g., Lahaina noons in May & July in Hawaii.) 1. What kind of seasons would we have if the Earth’s orbit was a perfect circle centered on the Sun? A. None at all — constant temperature everywhere. B. Normal seasons much like we have now. C. North would always be cold, south always hot. D. Days and nights would be 12 hours long. E. North and south hemispheres would have winter and summer at the same time of year. 1. What kind of seasons would we have if the Earth’s orbit was a perfect circle centered on the Sun? A. None at all — constant temperature everywhere. B. Normal seasons much like we have now. C. North would always be cold, south always hot. D. Days and nights would be 12 hours long. E. North and south hemispheres would have winter and summer at the same time of year. Phases of the Moon The Moon is a ball of rock lit by the Sun. Starts With A Bang: A Moon-stravaganza We see phases because the angle between the Moon and the Sun varies as the Moon orbits every 28 days. Phases of the Moon How to Simulate Phases of the Moon Phases of the Moon Astr 110L: Shape of the Moon’s Orbit Note that (1) size varies and (2) Moon “wobbles”. The Moon’s Elliptical Orbit 2. What is wrong with this picture? A. The cell phone is bugged. B. It’s night, but the Moon’s phase shows that the Sun is up. C. The water is only 3 feet deep. New Yorker: 07/24/06 2. What is wrong with this picture? A. The cell phone is bugged. B. It’s night, but the Moon’s phase shows that the Sun is up. C. The water is only 3 feet deep. New Yorker: 07/24/06 Eclipses: Geometry and Shadows Solar Eclipse Totality only visible here! Penumbra Umbra Penumbra Eclipses: Geometry and Shadows Lunar Eclipse Penumbra Umbra Penumbra Visible everywhere on night side Total Solar Eclipse Solar Eclipse Simulation Note: Penumbra should be about twice Moon’s diameter; Earth should be shown rotating; Sun should not move with respect to background stars. Astronomy Picture of the Day: A Darkened Sky Solar Eclipses: Three Types Total Eclipse Partial Eclipse Annular Eclipse 3. During a solar eclipse, which way does the Moon’s shadow typically move? A. North to south B. East to west C. South to north D. West to east E. No motion at all 3. During a solar eclipse, which way does the Moon’s shadow typically move? A. North to south B. East to west C. South to north D. West to east E. No motion at all Total Lunar Eclipse Astronomy Picture of the Day: Kalamalka Lake Eclipse Astronomy Picture of the Day: Shadow Play Astronomy Picture of the Day: Eclipsed Moon and Stars Lunar Eclipses: Three Types EARTH, SUN AND MOON : SUMMARY a. Reasons for seasons Earth spins at an angle to its orbit, exposing us to more or less sunlight as we go around the Sun. b. Phases of the Moon The Moon is a ball of rock lit by the Sun; we see only part of the illuminated side. c. Eclipses of Sun and Moon The Earth and Moon cast shadows, and each can travel through the other’s shadow. 2. THE PUZZLE OF THE PLANETS a. Planetary motion b. Models of the Solar System c. Kepler’s laws of planetary motion Planetary Motion The Sun & Moon always move westto-east relative to the stars. The planets usually move west-to-east as well, but sometimes they move the other way! Planetary Motion: Mars Goes Retrograde Mars at Opposition All planets undergo retrograde motion. Models of the Solar System Pythagorean model: rotating Earth at center. Could not explain retrograde motion. Physics for the Inquiring Mind Models of the Solar System Aristarchus’ model: Sun at center. Explained retrograde motion, predicted stellar parallax. Physics for the Inquiring Mind Parallax If the stars are scattered through space, nearby ones should appear to shift as the Earth orbits the Sun. (They do, but it’s hard to detect.) If all stars are at the same distance, constellations should grow and shrink as the Earth moves around. Neither effect was seen by ancient astronomers, so they rejected Aristarchus’ model. Ptolemaic model Earth at center, circled by everything else. Each planet combines two uniform circular motions. This worked well enough for “practical” purposes. Copernican model 1. Heavenly motions are uniform, eternal, and circular or compounded of several circles. 2. The center of the universe is near the Sun. 3. Around the Sun, in order, are Mercury,Venus, Earth and Moon, Mars, Jupiter, Saturn, and the fixed stars. 4. The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis. 5. Retrograde motion of the planets is explained by the Earth's motion. 6. The distance from the Earth to the Sun is small compared to the distance to the stars. Tycho’s Data The Copernican model was no more accurate than the Ptolemaic model. Tycho’s very precise observations showed that both models were flawed. Galileo’s Discoveries 1. The Moons of Jupiter Physics for the Inquiring Mind 2. The Phases of Venus Conclusive proof that Venus orbits the Sun! Museum of Science, Florence Phases of Venus Ptolemaic model: Venus never more than a crescent. Copernican model: Venus shows full cycle of phases. Kepler’s Laws of Planetary Motion Law 1: Planets move in elliptical orbits with the Sun at one focus. Wikipedia: Kepler’s Laws Drawing an Ellipse Kepler’s Laws of Planetary Motion Law 1: Planets move in elliptical orbits with the Sun at one focus. Closest to Sun at perihelion, furthest from Sun at aphelion. Wikipedia: Kepler’s Laws Width of ellipse is 2a, where a is the semi-major axis. Law II: A line from the Sun to a planet sweeps out equal areas in equal times. In other words, (area)/(time) is a constant, now known as the angular momentum. Wikipedia: Kepler’s Laws Law III: The quantity P2 / a3 , where P is orbital period and a is semi-major axis, is the same for all planets. If P is measured in years and a is measured in AU, then P2 = a3. Scientific Method Observe Nature Suggest Explanation (a.k.a. Hypothesis) False? Make Predictions True? Trust Hypothesis (a little, anyway) Scientific Method: Examples Retrograde Motion: Falsified the Pythagorean model Motivated all Sun-centered models Motivated the Ptolemaic (2 circle) model Scientific Method: Examples Stellar Parallax: Falsified all Sun-centered model Verified all Sun-centered models Falsified all Earth-centered models Scientific Method: Examples Tycho’s data: Falsified Kepler’s initial model Verified all of Kepler’s laws Scientific Method: Examples Phases of Venus: Falsified all Earth-centered models Verified all Sun-centered models Position 3 None - it is always above the horizon. E N S W Imaginary plane No; the Earth blocks the view. Star A at position 4 is hidden by the Earth. Agree with student 2; star B never crosses horizon plane, so it can’t rise or set. south, high west, low not visible northeast, high north, low Rising Setting No - it is always above the horizon from this location. An observer further south would see it set.