Some things from last timeSlide 2Slide 3Some things from last timeA total lunar eclipse occurs…A solar eclipse happens…Chapter 2Ancient Greek AstronomersEarly Ideas: Shape of the EarthEarly Ideas: Size of the EarthEarly Ideas: Relative Size of the Earth and MoonEarly Ideas: Size of the Sun and Moon The issue of angular sizeEarly Ideas: Relative Distances of the Moon and SunEarly Ideas: Earth-centered universeThe PlanetsThe Planets (“Wandering Stars”)The Planets: Retrograde MotionSlide 18Slide 19Slide 20Retrograde Motion: What’s Actually Happening?Astronomy in the RenaissanceSlide 23Tycho BraheJohannes KeplerKepler’s 1st LawKepler’s 2nd LawKepler’s 3rd LawPlanetary orbits are…Galileo GalileiGalileo’s DiscoveriesGalileo’s DiscoveriesGalileo and the Catholic ChurchWhich of the following are named after Galileo?Isaac NewtonThe Scientific MethodNew DiscoveriesSome things from last time•Is the sun’s axis of rotation tilted? YES, by about 7º.•What’s that figure 8 on a globe? An “analemma”•Photos taken same time each day at same location.•Part from axial tilt (up and down)•Part from Earth’s elliptical orbit (left and right)Considering the moon’s phases, on the same night, someone in the US and someone in China will see…A. Different sides of the moonB. Approximately the same phaseC. Obviously different phasesD. Trick question: there is no moon in ChinaSome things from last timeEclipses can only happenin these arrangementsThe moon’s phase dependsONLY on where the moon is relative to where the sun is.A total lunar eclipse occurs…A. During the new moon phaseB. When the sun blocks the moonC. During the full moon phaseD. Always around the summer solsticeA solar eclipse happens…A. Every month at the new moonB. Every week at the quarter phasesC. Every month at the full moonD. About every six months at the new moonE. Every year at the new moonChapter 2The Rise of AstronomyAncient Greek Astronomers•Through observations, they were the first to use a careful and systematic manner to explain the workings of the heavens•Limited to naked-eye observations, their idea of using logic and mathematics as tools for investigating nature is still with us todayEarly Ideas: Shape of the Earth•The shadow of the Earth on the Moon during a lunar eclipse.•Could not happen with a flat Earth.•Certain stars can be seen from one location, but not another.•Could not happen with a flat Earth.Early Ideas: Size of the Earth•Eratosthenes (276-195 B.C.) made the first measurement of the Earth’s size.•He obtained a value of 25,000 miles for the circumference, a value very close to today’s value.Early Ideas: Relative Size of the Earth and Moon•Aristarchus found the moon is 0.35 times the size of Earth. (pretty close. actual number is 0.27)•Assumption: Earth’s shadow is the same size as the Earth. (mostly true)Early Ideas: Size of the Sun and MoonThe issue of angular size•Angular size is only helpful if you know how far away the objects are!Early Ideas: Relative Distances of the Moon and Sun•Aristarchus (again) uses geometry to determine how far away the moon and sun are (relatively).•He found the sun is 20x further away, not terribly accurate (real value is closer to x400)Early Ideas: Earth-centered universe•Parallax: the shift of a foreground object with respect to the background•The parallax did exist (and still does), but it was waaaaayyyy too small to be measured using the tools at the time.•Conclusion: The Earth is at the center of the universe (which is wrong)The PlanetsThe Planets (“Wandering Stars”)•What was it that made planets behave differently than stars?•The stars stayed in the same place in the sky (relative to the other stars)•Planets “wandered” through the stars•Planets (usually) move west-to-east with respect to the stars, but they still rise in the east and set in the west.The Planets: Retrograde Motion•This type of motion was another clue to the ancients that planets were something special.•These “wandering stars” were named after gods and goddesses.Trying to Explain Retrograde Motion with a Geocentric Model (Earth at the center)•Simple Earth-centered models can explain most of the motions of the planets, but not retrograde motion.•Ptolemy lived in Egypt near the Great Library of Alexandria. •He proposed “epicycles” to explain retrograde motion.•It fit better than simple models, but was still not quite right.•Further complexity increased precision, but…the truth is usually simple (Ockham’s razor)Trying to Explain Retrograde Motion with a Geocentric Model (Earth at the center)Astronomy in the Renaissance:Nicolaus Copernicus & the Heliocentric Model•(1473 - 1543)Dude, what if the Earth isn’t at the center of the universe?Dude, what if the Earth isn’t at the center of the universe?Retrograde Motion: What’s Actually Happening?•The Earth is “passing” a planet further out.•This makes Mars’ position on the celestial sphere change direction.•Something similar can happen on the highway, passing while the road is curving.Astronomy in the Renaissance•wtf is an AU?•He was pretty close.•His ideas were met with hostility and skepticism due to their departure from contemporary Catholicism.•His theory did have issues.•Wasn’t any better at predicting the positions of the planets than Ptolemy.•This issue came from assuming the orbits of planets were perfectly circular.•Astronomers still couldn’t measure parallax precisely.What is it called when a planet temporarily reverses direction relative to the stars?A. Retired motionB. Rudimentary motionC. Retroactive motionD. Retrofitted motionE. Retrograde motionTycho Brahe•(1546-1601)•Created instruments with far greater accuracy and precision than those of the time.•Last great astronomer to believe the Earth was at the center of the Universe.Johannes Kepler•(1571 - 1630)•Planetary orbits are ellipses, not circlesKepler’s 1st Law•Planets move in elliptical orbits with the Sun at one focus of the ellipseKepler’s 2nd Law•The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep out equal areas in equal time intervals•The closer a planet is to the Sun, the faster it movesKepler’s 3rd Law•The amount of time a planet takes to orbit the Sun is related to its orbit’s size•The square of the period, P, is proportional to the cube of the