From Aristotle to Newton The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics. Where do we put the other planets in our picture of the solar system? What are some reasons that the geocentric model of the universe seems to make intuitive sense?

It doesn't feel like we are moving – wouldn't there be a wind or something? Why would things fall down and not towards the center of the universe? Why don't we see stellar parallax?

Geocentric vs. Heliocentric Models of the Solar System Aristotle vs. Aristarchus (3 rd century B.C.)‏ Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth. Aristarchus: 1 st Heliocentric model Aristotle: But there's no wind or stellar parallax. Doesn’t “feel” like we are moving. Actually orbiting sun at 70,000 mph! Difficulties with "Geocentric" model - Retrograde motion of planets - Phases of Venus Ancient Greeks knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.

Aristarchus - Written in the second century BCE he calculated the ratio of the distance between the Earth and Sun to that between the Earth and the Moon. (His estimate was more than an order of magnitude too small, but the fault was in his lack of accurate instruments rather than in his method of reasoning.) -This image compares the line subtending the arc dividing the light and dark portions of the Moon in a lunar eclipse with the relative diameters of the Moon and Sun. -Aristarchus also found an improved value for the length of the solar year.

Planets sometimes appear to loop back - retrograde motion Loops are called "epicycles" Earth Mars Apparent motion of Mars against "fixed" stars * * * * * * January July

Geocentric Model (Earth Centered)‏ Fairly good agreement with retrograde motion of planets Some predictive power More precise measurements showed errors

Ptolemy's geocentric model (A.D. 140)‏

Heliocentric Model Rediscovered by Copernicus in 16 th century. Much simpler was the main attraction for Copernicus. Explains retrograde motion. Put Sun at center of everything, but still insisted on circles, thereby retainig unnecessary complexity. Opposed by Catholic Church Copernican revolution – critical realization that Earth is not at the center of the universe, only accepted after his death. Copernicus

Galileo ( )‏, Experimentalist Built his own telescope. Discovered four moons orbiting Jupiter. What does this suggest? Discovered sunspots. What might we infer about the Sun from these observations? Observed phases of Venus. Was imprisoned for the last 9 years of his life for his scientific discoveries.

Heliocentric model easily accounts for phases of Venus

Geocentric model fails to account for phases of Venus

The Scientific Method Geocentric model abandoned because of its failure, and to a lesser extent because of its complexity.