PHYS 1411 Introduction to Astronomy History of Astronomy Chapter 4 Renaissance Period Copernicus new (and correct) explanation for retrograde motion of the planets Copernicus new (and correct) explanation for retrograde motion of the planets Retrograde (westward) motion of a planet occurs when the Earth passes the planet. Retrograde (westward) motion of a planet occurs when the Earth passes the planet. This made Ptolemy s epicycles unnecessary. This made Ptolemy s epicycles unnecessary. http://astro.unl.edu/classaction/animations/renaissance/retrograde.html Tycho Brahe (1546-1601) He proposed a model of the solar system. The Sun and the Moon orbit the earth while the other planets orbit the Sun. Invented instruments for planetary motion studies. Discovered exploding stars and comets. Debated the validity of the heliocentric model. Realized that the Universe is changing and is complex. Tycho Brahe and Exploding Stars Tycho reasons that because there is no Parallax observed for exploding stars, it should be located at the celestial sphere 1
Tycho Brahe Model of Solar System Tycho s model is geocentric with the Sun and Moon revolving around Earth, but the planets revolved around the Sun. All motion was along circular paths. 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 Kepler s Laws of Planetary Kepler s Laws of Planetary 1. The orbits of the planets are ellipses with the sun at one focus. c Eccentricity e = c/a Thinglink.com Ellipse How to find Eccentricity 2
Eccentricities of Ellipses 1) 2) 3) e = 0.02 e = 0.1 e = 0.2 Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles: Earth: e = 0.0167 Most extreme example: Pluto: e = 0.248 4) 5) e = 0.4 e = 0.6 Kepler s First Law of Planetary Kepler s Second Law of Planetary A line from a planet to the Sun sweeps over equal areas in equal intervals of time Keplers Third Law Keplers Third Law A planet s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: (P y = period in years; a AU = distance in AU) P y2 = a AU 3 Schools.wiki.com 3
Kepler s Third Law of Planetary A planet s orbital period squared is proportional to its average distance from the Sun cubed Planetary Orbits Worksheet http://astro.unl.edu/naap/pos/animations/kepler.html Bode's Law Titus-Bode Law is a rough rule that predicts the spacing of planets in the Solar System. Relationship was first proposed by Johann Titus in 1766 and later formulated as mathematical expression by J.E. Bode in 1778. Consider the numbers 0 3 6 12 24 48 96 192 384, except the first two all others are twice the previous one. Now add 4 to each number and divide by 10, 4 7 10 16 28 52 100 196 388 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 The sequence is very close to the distribution of mean distance of a planet from the Sun. Bode's Law Predicts the existence of another planet between Mars and Jupiter. The asteroid belt. Source: astro.cornell.edu Galileo Galilei Galileo Finds Conclusive Evidence Galileo Galilei (1594 1642) Invented the modern view of science Transition from faith-based to observation-based science First to meticulously write down telescopic observations and apply them to the problem of the day: The place of Earth Famouspeople.com (a) If Venus moved in an epicycle centered on the Earth Sun line, it would always appear as a crescent. (b) Galileo observed through his telescope that Venus goes through a full set of phases, proving that it must orbit the Sun. 4
Surface structures on the moon; first estimates of the height of mountains on the moon The Moon is not perfect Stargazerspaul.com Four new planets circling Jupiter, known today as the Galilean moons of Jupiter Strong evidence for Copernican model The Milky Way contains many stars too faint to see with the unaided eye Observations of the Sun Discovered sunspots The Sun is imperfect Movement of sunspots Sun is a rotating sphere Acknowledgment The slides in this lecture are 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. 5