Geocentric model, Lecture notes of Astronomy

Download Geocentric model and more Lecture notes Astronomy in PDF only on Docsity! 1 Lecture 4 The Geocentric Model of the Universe January 3a, 2014 2 Size of the Earth – Eratosthenes (200 BC) • In Syene, Egypt, the Sun would shine straight down wells at noon on the summer solstice (June 21) – The Sun is at zenith at noon on the solstice along the Tropic of Cancer (23.5° north latitude) • In Alexandria, the Sun was 7° 12’ south of zenith at that time. 5 Basic Geocentric View E Sun Moon Mercury Saturn Celestial Sphere Jupiter Mars Venus 6 Basic Geocentric View • Order of planets somewhat arbitrary. • Mercury and Venus always orbit at a velocity that keeps them near the Sun. E Sun Moon Mercury Saturn Celestial Sphere Jupiter Mars Venus 7 Retrograde Motion • Planets usually move from west-to-east in the sky with respect to the stars, just like the Sun. • Sometimes planets go east-to-west in sky, an event called retrograde motion. • Retrograde motion was very hard to explain, representing the main challenge to ancient models of the solar system. 10 • Predicted positions of planets with respect to stars with great accuracy. • This theory was accepted for ~15 centuries! Ptolemy (140 AD) • Size and rates of motion on epicycles was calculated using tabulated data – Hipparchus – Ptolemy Java animation of Ptolemy's model ll Epicycle and deferent Seeds/Horizons, 3rd ed., Fig. 4-6; Foundations of Astronomy, 1990 ed., Fig. 4-12 Planet Epicycle + oO e Earth Equant Deferent © 1991 Wadsworth, Inc. The Ptolemaic system Seeds/Horizons, 3rd ed., Fig: 4-7; Foundations of Astronomy, 1990 ed., Fig. 4-13 os (punter e Sphere of send fixed stars Venus Moon (3 Saturn Mars