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History of Astronomy:
1. Ancient view: Earth is the center of the solar system.
 This is called the geocentric model.
The Sun and other planets revolve around Earth
in circles.
Sun
E
Jupiter
Mars
It gets much worse…..
and worser…..
Very weird!
2. Newer view: The Sun is the center of the solar system.
 This is called the heliocentric model.
The Earth and other planets revolve around the Sun
in circles.
E
Mars
Sun
Jupiter
 Kepler had to change the heliocentric model because
it did not predict the planets’ positions accurately.
3. Kepler’s Laws of Planetary Motion
Kepler’s 1st Law: The orbit of a planet or asteroid orbiting
a star is an ellipse. The star is at one focus.
This is also true for the Moon orbiting Earth.
In this case, the Earth is at one focus.
planet
Sun
x
Earth
x
x
Moon
x
Kepler’s 2nd Law： As a planet approaches the Sun,
it moves faster. A planet moves fastest when it reaches its
closest point: perihelion. As a planet moves away from
the Sun, it slows down. It moves most slowly when it
reaches its furthest point: aphelion.
speeding up
perihelion
fastest
(Jan 1st)
Sun
x
x
slowing down
aphelion
slowest
(July 1st)
Kepler’s 3rd Law：
1. The further a planet is from the Sun,
the longer it takes to revolve.
2. The further a planet is from the Sun,
the slower its orbital speed.
orbital
speed
Period of
Revolution
(time for
one year)
MVEMJSUN
(distance from Sun)
MVEMJSUN
(distance from Sun)
Kepler did not know why his 3 laws were correct!
Ex: 3 of the 4 Galilean moons of Jupiter:
How do we know Saturn's rings are not one solid disk?
4. Newton explained why Kepler’s Laws were correct
a. Gravity is the force that holds the planet’s in
their orbits.
 The bigger the planet, the more the gravity.
 The farther a planet, the less the gravity
b. Object tend to move in straight lines (inertia)
 The orbit of a planet
is the result of a
balance between
gravity and inertia.
Sun
gravity
planet
inertia
orbit