Lecture 3 Geocentrism vs. Heliocentrism Dennis Papadopoulos ASTR 340 Fall 2006

Aristotle’s Universe Planets on physical spheres – each moving the other – earth and sphere of prime mover stationary

Aristotle-Ptolemy and the Church

Aristotle’s influence Science roots of cosmology Pythagoras and Plato described the world through mathematical relationships. Many common ideas – Aristotle started from these with focus on Universe is dynamic, living, organic structure perfectly ordered on musically mathematical proportions Earth is center of the universe and planets revolve in perfect circles Aristotle used Mathematics and Observations for his cosmos Cosmology based on the fact that there are natural laws that describe how objects behave in the Universe Using these laws and sufficient observations we can understand the universe

Aristotle’s laws of motion Motion – Kinematics-On Earth Why do objects move the way the do on earth? Why do objects fall when dropped? Why stones fall while bubbles rise on water Answer: They move till they found their natural state- rest Different elements move differently Earth towards the center of the earth Fire away from center of the earth Water and air occupy the in between space Motion linear Concept of force Force=(Resistance)x(speed) Continuous motion required application of force - inertia

Laws of motion The heavens - Ether Motion – kinematics – heavens Fifth element ether Objects in heavens (above moon) move in circles Why? Universe finite – linear motion would force them to fall off the end of universe Why earthly objects cannot reach the end of universe? – they move in circles as soon they approach heavens Aristotle’s cosmology inextricably linked to laws of motion (physics) as today’s cosmology

Aristotle vs. Aristarchos

Geocentric vs. heliocentric Pros and cons

Medieval Universe

Enter Copernicus Copernican Principle: THE EARTH IS NOT LOCATED IN A PRIVILEDGED POSITION

Tycho Brahe Johannes Keppler Tycho Brahe aristocrat/nobleman observer – naked eye Observations from island observatory Amassed data with extreme precision In 1572 observed first supernova in the constellation of Cassiopeia. Could not measure parallax but he realized that it was not an atmospheric brightening but it belonged to the realm of fixed stars -> Heavens not immutable Used parallax to show that comet orbits lay outside the orbit of the moon. Another strike against Aristotle

Brahe on observing Supernova Amazed, and as if astonished and stupefied, I stood still, gazing for a certain length of time with my eyes fixed intently upon it and noticing that same star placed close to the stars which antiquity attributed to Cassiopeia. When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes.

Brahe’s Universe Earth at the center orbited by moon and Sun Planets orbit Sun Failed to confirm Copernicus by testing parallax change of Mars, Jupiter due to inaccurate knowledge of earth’s orbit (by a factor of 20)

Keppler Elliptical paths Conic section Foci Major axis Minor axis

Circular vs. elliptical motion

KEPLER’S LAWS The orbits of the planets are ellipses, with the Sun at one focus of the ellipse. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. R is semi-major axis (P/yr )2=( R/1 AU) 3 1 AU=150 million km

From ellipse to parabola

Keppler vs. Copernicus Wrong assumptions of Copernicus Planets move in perfect circles Planets move in constant speed Sun at the center of these orbits Keppler showed Planets move in ellipses Planetary speed varies constantly Sun is not quite at the center of the planetary orbits

Galileo Galilei

Galileo Looking at the sky PHASES OF VENUS MOON

Geocentric vs. heliocentric Post Galileo

Galileo’s drawing Huyghen’s drawing Hubble image

Zeno Achilles and the turtle Atomists – Zeno-Democritus-Lefkippos Universe made up of little particles – atoms- indivisible and eternals –quanta They move in vacuum – Aristotle -> no vacuum

Stoic Universe