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Scientific Models &
Kepler’s Laws
Scientific Models
We know that science is done using the
Scientific Method, which includes the
following steps :
Recognize a Problem
Form a Hypothesis
Predict Consequences of the
Hypothesis
Perform an Experiment to test
Predictions
Communicate your results
Scientific Models
When communicating your results, you
are trying to organize your hypothesis,
prediction, experimental steps and
conclusions so that they can be
communicated and understood.
A good way to do this is to develop a
Scientific Model.
A Scientific Model is just a description
of a scientific idea.
Examples of Scientific Models
a word
actual
description.
physical
A scientific model can be an
mathematical
model.
equation or graph.
For Example :
For Example :
A star is big ball of burning gasses.
A globe.E = mc2 or F = ma
Developing a Scientific Model
This is where the scientific method comes in!
REAL
WORLD
Initial Observations
or Assumptions
Model
Make Predictions
Based on Model
• It is important to understand, that any model
must be capable of producing predictions that
can be tested.
• If these predictions are verified by observation
or experiment, this gives evidence that
supports the model.
• If these preictions are
shown to be false then the
Revise
model MUST be
Model
revised!!!
• If it can’t be revised,
then it MUST be thrown
Compare
Observations to
out!!!!
Predictions
Mathematics
Physics
Simplicity
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The ancient Babylonians
were prolific astronomers
who created very detailed
records of the positions of
the “heavenly bodies”.
These charts showed a
phenomena of planetary
motion called retrograde
motion.
Retrograde motion is the
apparent backwards
motion of the planets in the
This motion is not a true motion of the planets,
night sky.
but any good model of the solar system must be
able to explain this observation.
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In ancient Greece, a
scientist by the name of
Aristotle (384-322 BC)
developed a model of
the universe (5 planets
and the background
stars).
His model was
“geocentric”.
This means the Earth
was at the center of our
Solar System and
everything, including
the Sun moved around
the Earth.
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Claudius Ptolemy, around
AD 125, revised earlier
attempts at a geocentric
model.
In his model the Earth was a
little off center.
The Sun and the Moon each
orbited the Earth.
Each planet orbited a point,
called the epicenter, that
orbited the Earth at varying
speeds.
This model allowed for
retrograde motion and made
fairly accurate predictions
for the position of the stars
and planets (5 – 10% error).
C
Claudius Ptolemy
 In 1514 Nicolaus
Copernicus developed a
heliocentric (Suncentered) model.
 In his model, the
“Heavenly Spheres”,
revolved around the Sun
in circular orbits and the
Earth spun on its axis.
 The stars were also much
farther from the Sun than
the planets.
Copernicus’ Heliocentric
Model
 Retrograde motion is
explained by the relative
motion of the planets to
each other.
 Copernicus’ model was
only as accurate in
predicting the positions of
the planets, the Sun and
the Moon as Ptolemy’s
model. (5 – 10%)
Kepler’s Heliocentric Model
 In the 1600’s Johannes
That
predictions
must match
observations.
Kepler used
astronomical data
recorded by his former
boss to develop a
heliocentric model of
our Solar System.
 To do this he applied a
condition on his model:
Kepler’s Heliocentric Model
 Between 1609 and
1618, Kepler
developed his
three Laws of
Planetary Motion.
 These are still
used today !!!!!
Law 1 : The Law of Ellipses

The orbit of each
planet is an ellipse,
with the Sun at one
focus.
 An ellipse is like an
oval where the
distance from one
focus to a point on
the ellipse and back
to the focus is the
same.
Law 1: The Law of Ellipses

We can tell how
close an ellipse is
to a circle by its
eccentricity.
 An ellipse with an
eccentricity of 0,
would be a circle.
 As the eccentricity
approaches 1, the
circle becomes
more elliptical.
Animation
A ellipsehas
has anan
A circle
eccentricity
eccentricity
between of
0&1
0
Law 2 : Law of Equal Areas

A line drawn from a planet to the Sun
sweeps out equal areas in equal time.
 What this means : The farther a planet is
from the Sun the slower it moves.
Faster
orbital
speed
Slower
orbital
speed
Animation
Law 3 : Harmonic Law
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
The square of the orbital period, P, (the time it takes a
planet revolve around the Sun one time) of a planet is
directly proportional to the cube of the planet’s average
distance from the Sun, R.
What this means for us : The planets farther from the
Sun take longer to orbit the Sun. (Much weaker than
the above statement)
𝟐
𝐏
=
𝑪𝒐𝒏𝒔𝒕𝒂𝒏𝒕
𝟑
𝐑
Where’s the Science ?
Each of these models, from Aristotle to Kepler had one problem.
There was no scientific reason to accept one over the other.
It took Isaac Newton, with a little help from
Galileo, to establish a central force that held the
universe (Solar System) together.
Gravity