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Announcements PHYS 1411 Introduction to Astronomy • • • • • Homework 1 Extended till February 5th. Homework 2 is assigned on MindTap: Due Feb. 10th Homework 3 is assigned on MindTap: Due Feb. 14th Help Session: Every Wednesday from 5-6pm Exam 1: Wednesday February 15th during class – Multiple Choice Questions (40 or 50) on whatever we cover by the exam date – Bring a Green Scantron Sheet, No. 2 pencil – Can use a calculator if needed. – Will provide conversion and constants if needed. – No cell phones or calculator on cell phone, laptops, index card, cheat sheet etc. allowed – Be prepared to present School ID if asked. – There is no review session: use the help sessions and questions we do in class and those assigned on homework. Cycles of Moon History of Astronomy Chapter 3 This chapter is not part of this class syllabus Chapter 4 Topics in Chapter 4 Chapter 4 talks about the history of Astronomy and the development of the model of the solar system. For this class only the following topics are needed. • Overview of the model of solar system • Parallax • Kepler’s Laws of Planetary Motion • Please study the rest of the topics on your own Model of the Solar System The early Greek philosophers believed that the Earth was in the center of the solar system and all the planets and sun revolved around it. http://ecsst.blogspot.com/ 1 First Observational Challenge to Geocentric Model Attempts to Explain Retrograde Motion • ~140 BC: Ptolemy perfects the geocentric model of the solar system – Uses epicycles to explain retrograde motion. – Note the displacement of Earth from the center o f the circle. – Each epicycle has a different rate Ptolemy’s Solar System Model The Copernican Revolution The Copernican Universe The Eye is Inefficient • The early Greek philosophers believed that the earth did not moved because their eyes could not see the motion of stars • The telescope was not invented yet. • So they could not decide which model (heliocentric or geocentric) was correct. 2 What is Parallax? Apparent Motion of Stars • After the invention of the telescope it became known that a foreground star appeared to move with respect to background stars during the course of a year. • This let the acceptance of heliocentric model • Parallax can be used to find distances to nearby stars. We will look at it in detail in chapter “Properties of Stars” Attendance Johannes Kepler (1571 – 1630) • Used the precise observational tables of Tycho Brahe to study planetary motion mathematically • Found a consistent description by abandoning both: – Circular motion – Uniform motion • Planets move around the sun on elliptical paths, with non-uniform velocities Campus.kellerisd.net Kepler’s Laws of Planetary Motion Kepler’s Laws of Planetary Motion 1. The orbits of the planets are ellipses with the sun at one focus. c Eccentricity e = c/a Thinglink.com 3 Ellipse How to find Eccentricity Eccentricities of Ellipses 1) 2) e = 0.02 3) e = 0.1 Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles: e = 0.2 Earth: e = 0.0167 Most extreme example: Pluto: e = 0.248 5) 4) e = 0.4 e = 0.6 Kepler’s First Law of Planetary Motion Kepler’s Second Law of Planetary Motion • A line from a planet to the Sun sweeps over equal areas in equal intervals of time 4 Keplers Third Law Keplers Third Law • A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: (Py = period in years; aAU = distance in AU) Py2 = aAU3 Schools.wiki.com Kepler’s Third Law of Planetary Motion • A planet’s orbital period squared is proportional to its average distance from the Sun cubed ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) Planetary Orbits Worksheet http://astro.unl.edu/naap/pos/animations/kepler.html ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 5 ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) Acknowledgment • The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only • Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 6