Transcript Day-13
Astronomy 1010 Planetary Astronomy Fall_2015 Day-13 Course Announcements • Dark Sky nights – Mon. 10/5 & Wed. 10/7 starting at 7:30pm – at the Observatory. • Total lunar eclipse: Sun.-Mon. 9/27-28 • • • • Weather dependent, if clear, we’ll be setup at the observatory about sunset. If you come out after dark, turn your headlights OFF before you come around the last bend. Here’s the important information Exams will be returned on Friday. If you’re one of those who did not take it on Monday (for some valid reason), you have until the START of class Friday to complete it. L-T workbooks will be picked up Friday. Make SURE your name is in the book. From our perspective on Earth, it appears that everything in the sky moves and orbits us. Early astronomers and philosophers therefore crafted mostly geocentric models of the universe to reflect this. These models became greatly fixed in the minds of astronomers for millennia. Politics and science can clash when cultural mindsets refuse to be changed. Another point unwilling to be conceded was the idea of “uniform circular motion.” • Objects moved in perfect circles at uniform speeds. As astronomers viewed the motions of the planets, the models did not match the observations. Complicated models were needed to explain phenomena such as retrograde motion. Ptolemy developed a system with epicycles in 150 CE that remained accepted for about 1,500 years. Copernicus was the first to create a mathematical model with the Sun at the center. Heliocentric model with circular orbits. Could estimate relative distances of the planets from the Sun and each other. Copernicus’s model could explain the behavior of objects in the Solar System. The ordering of the planets could explain how they sometimes interrupt their prograde motion with retrograde motion. Observing Retrograde Motion Law: pg 99 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask one of us for help. Tycho Brahe spent decades collecting astronomical data after building his own observatory. Created his own geocentric model with the other planets orbiting the Sun, but with the Sun orbiting Earth. Using Tycho’s data, Johannes Kepler came up with empirical rules to describe planetary orbits in a heliocentric system. Empirical science describes how something works, not why. Johannes Kepler 1571 - 1630 Kepler’s First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus. Eccentricity, e •how squashed or out of round the ellipse is •a number ranging from 0 for a circle to 1 for a straight line e = 0.02 e = 0.7 e = 0.9 What is the shape of Earth’s orbit around the Sun? Earth, e = 0.017 Nearly circular SECOND LAW A line drawn from the planet to the Sun sweeps out equal areas in equal times orbital speed is not constant for an ellipse only for a circle planets move faster when near the Sun (perihelion) planets move slower when they are far from the Sun (aphelion) Kepler’s Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time. SECOND LAW The speed a planet travels during its orbit is related to the distance from the star When the planet is near the sun the planet goes faster than when the planet is farther from the sun Planet travels slow here Planet travels fast here Lecture – Tutorial nd Kepler’s 2 Law: pg 21 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask one of us for help.