Early Models of the Early Models of the UniverseUniverse

Pythagoreans (500 B.C.)Pythagoreans (500 B.C.)

Believed the stars, planets, sun, Believed the stars, planets, sun, and moon were attached to and moon were attached to crystalline spheres which rotated crystalline spheres which rotated around the Earth.around the Earth.

GeocentricGeocentric- everything moved - everything moved around the Earth.around the Earth.

Aristotle (350 B.C.)Aristotle (350 B.C.)

Believed the Earth is motionless Believed the Earth is motionless at the center of the universe and at the center of the universe and all the stars and planets rotated all the stars and planets rotated around it.around it.

Stars and planets moved in Stars and planets moved in circular paths.circular paths.

GeocentricGeocentric

Ptolemy (140 A.D.)Ptolemy (140 A.D.)

Expanded AristotleExpanded Aristotle’’s theories to try to s theories to try to account for account for ““retrograde motionretrograde motion”” of the of the planets planets

Planets traveled in smaller circular paths Planets traveled in smaller circular paths as they traveled around the Earth as they traveled around the Earth (epicycles and deferents)(epicycles and deferents)

Popular model of the universe for 1,500 Popular model of the universe for 1,500 years.years.

Copernicus (1543)Copernicus (1543)

Sun at the center of universe and the Sun at the center of universe and the planets orbit the sun. planets orbit the sun. Heliocentric.Heliocentric.

Solved the problem of Solved the problem of ““retrograde motionretrograde motion””

Theory did not immediately replace Ptolemy’s theory.

Tycho Brahe (1600)Tycho Brahe (1600)

Favored an Earth-centered universe Favored an Earth-centered universe different from Ptolemydifferent from Ptolemy’’s theory.s theory.

Thought that other planets revolved Thought that other planets revolved around the Sun, and that the sun and around the Sun, and that the sun and moon revolved around Earth.moon revolved around Earth.

His theory was incorrect, but made many His theory was incorrect, but made many precise observations of planets and stars.precise observations of planets and stars.

Johannes Kepler (1609)Johannes Kepler (1609)

MathematicianMathematicianTychoTycho’’s assistants assistantUsed TychoUsed Tycho’’s observational data to s observational data to

develop laws of planetary motion.develop laws of planetary motion.KeplerKepler’’s 3 Laws of Planetary Motions 3 Laws of Planetary Motion

Law of Ellipses Law of Ellipses Law of Equal AreasLaw of Equal AreasLaw of PeriodsLaw of Periods

Galileo Galilei (1609)Galileo Galilei (1609)

First person to use telescope for astronomical observations

Discoveries:Galilean Satellites (Jupiter’s moons)Craters on the moonSunspots on the Sun Phases of Venus

Favored Copernicus’s theory over Ptolemy’s

Isaac NewtonIsaac Newton

16871687Explained why planets orbit the Sun, and Explained why planets orbit the Sun, and

why moons orbit planets.why moons orbit planets.NewtonNewton’’s Law of Motions Law of MotionNewtonNewton’’s Law of Gravitations Law of GravitationCompleted the work of Copernicus, Tycho, Completed the work of Copernicus, Tycho,

Kepler, and GalileoKepler, and Galileo