Lecture 3Geocentrism vs. Heliocentrism

Dennis PapadopoulosASTR 340Fall 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 - 1543

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

PHASEPHASES OF S OF VENUSVENUS

MOON

Geocentric vs. heliocentric Post Galileo

Galileo’sdrawing

Huyghen’sdrawing

Hubbleimage

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