Models of the Solar System Ch 27.2 Page 691
Early models Around 2,000 years ago, Aristotle suggested the earth- centered or geocentric model of the solar system In this system everything revolved around the Earth
Ptolemy made modifications Around 130 CE, thought that planets moved in small circles called epicycles as they revolved in larger circles around the earth They helped explain retrograde motion
RETROGRADE MOTION The apparent backward movement of planets
Heliocentric model In 1543 CE, Nicolaus Copernicus proposed a sun-centered, or heliocentric model of the solar system Planets revolved around the sun in the same direction but at different speeds and distances from the sun Fast moving planets passed slow moving planets This explained retrograde motion
Galileo Galileo’s observation of four moons that traveled around Jupiter gave support to Copernicus's model
Kepler’s Laws of Planetary Motion
Law 1 Law 1 : Each planet moves in an elliptical orbit having the sun at one focus.
Law 2: An imaginary line from the sun to any planet sweeps out equal areas in equal time intervals.
Law 3: The square of any planet’s period is proportional to the cube of the planet’s average distance from the Sun. p 2 = d 3
Newton’s Explanation of Kepler’s Laws Newton said a moving body will remain in motion and resist a change in speed or direction until an outside force acts on it This concept is called inertia It is Newton’s First Law
orbit Since a planet does not follow a straight path, some outside force must be acting on it Newton called this force gravity He said this force exists between any two objects
Two forces combine to create orbit Inertia makes things move in a straight line Gravity pulls it downward The result is an ellipse