Astronomical History Ptolemy expresses geocentric model 127AD Copernicus offers heliocentric model 1530 AD Tycho Brahe 1546-1601 takes precision measurements Johannes Kepler analyzes Brahe’s data Galileo Galili proves heliocentric model Isaac Newton explains heliocentric model

Claudius Ptolemaeus An Alexandrian (from Alexandria in Egypt) astronomer, about 150 AD Like most of the Aristotlean world, they did not believe the natural world was orderly and rational and did not get involved in the practical world

Ptolomaen Astronomy This view is called “geocentric” because the earth is placed at the center of the universe To make the model work and explain retrograde motion, planets revolved in both cycles and epicycles around the earth

Retrograde Motion of Mars

Ptolemy’s Explanation of Retrograde Motion

Nicolaus Copernicus Copernicus was a Polish canon of the Roman Catholic church. He tried to find another astronomical system that was more philosophically and aesthetically pleasing than Ptolemy’s.

Copernican Model The Copernican model held that planets revolved around the sun instead of the earth. Copernicus did not provide much evidence in support of his model, he argued from philosophy.

Tycho Brahe Tycho Brahe was court astronomer and astrologer to King Rudolph II of Bohemia. He used his position to build precision equipment to take precise astronomical measurements, for the first time in history.

Galileo Galilei Galileo 1564-1642 In this picture, he is developing the pendulum laws Galileo observed mountains on the moon Galileo observed moons orbiting Jupiter 1610 Galileo observed phases in Venus

Johannes Kepler Johannes Kepler 1571-1630 Becomes Tycho Brahe’s assistant 1600 Analyzes data and determines the elliptical orbit of planets 1609 These laws removed all doubt that the Earth and planets go around the sun. Later Sir Isaac Newton used Kepler's laws to establish his law of universal gravitation.

KEPLER’S LAWS (1) The path of every planet in its motion about the sun forms an ellipse, with the sun at one focus. (2) The speed of a planet in its orbit varies so that a line joining it with the sun sweeps over equal areas in equal times. (3) The squares of the planets' periods of revolution are proportional to the cubes of the planets' mean distances from the sun.

An Ellipse

Kepler’s Second Law

Isaac Newton Isaac Newton 1642-1727 In 1664 explains Keplers’ Laws using the Universal Law of gravitation plus the mathematics of calculus