Transcript Models of the Soar System

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:
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.
p2 = d3
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