Models of Our Solar Models of Our Solar SystemSystem

A Study in Developing a Scientific A Study in Developing a Scientific ModelModel

Scientific ModelsWe 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

Formulate the Simplest General Rule

Scientific Models

When you formulate the simplest general rule, 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.

Scientific Models

A scientific model can be a word description.

For Example :

A star is big ball of burning gasses.

Scientific Models

A scientific model can be an actual physical model.

For Example :

A globe.

Scientific Models

A scientific model can be a metaphor.

For Example :

Mars is like a large ball of rusting metal.

Yes, I know this is really a simile, but give me a break.

Scientific Models

A scientific model can be a mathematical equation.

For Example :

E = mc2

Developing a Scientific ModelDeveloping a Scientific Model

You start with the real You start with the real world.world.

You then make your You then make your initial observations or initial observations or assumptions.assumptions.

This then leads to a This then leads to a model.model.

But this isn’t where But this isn’t where this process ends.this process ends.

REALWORLD

Initial Observations or Assumptions

Model

Developing a Scientific ModelDeveloping a Scientific Model

Once you have your Once you have your model you then can model you then can make some make some predictions based on predictions based on your model.your model.

Then it is time to test Then it is time to test your predictions by your predictions by comparing these comparing these predictions to predictions to observations.observations.

REALWORLD

Initial Observations or Assumptions

Model

Make Predictions Based on Model

Compare

Observations to Predictions

Developing a Scientific ModelDeveloping a Scientific Model

Were your predictions Were your predictions correct or not ? If not correct or not ? If not revise (fix) your model. revise (fix) your model. You don’t have to throw You don’t have to throw it out.it out.

You revise your model You revise your model based on Mathematics, based on Mathematics, Physics and Simplicity Physics and Simplicity (simpler is usually (simpler is usually better).better).

REALWORLD

Initial Observations or Assumptions

Model

Make Predictions Based on Model

Compare

Observations to Predictions

Revise Model

MathematicsPhysicsSimplicity

Developing a Scientific ModelDeveloping a Scientific Model

Now you get to start Now you get to start the process over the process over again.again.

And Again!And Again! And Again!!And Again!! Don’t worry, be Don’t worry, be

happy. This is how happy. This is how science is done.science is done.

Remember, science Remember, science is self-correcting.is self-correcting.

REALWORLD

Initial Observations or Assumptions

Model

Make Predictions Based on Model

Compare

Observations to Predictions

Revise Model

MathematicsPhysicsSimplicity

Over time our understanding of the Solar System has changed.

It has evolved.

As our knowledge increased and our technology improved we were able to make better scientific models of our Solar System.

We will start around 1600 BC with the Babylonians.

The Babylonians made extremely detailed catalogs of star positions and began to keep long-term records of planetary positions.

By 800 BC they had fixed the positions of the planets compared to the stars. These were recorded on clay tablets.

By 240 BC the Babylonians were able to use their charts to predict solar and lunar eclipses.

These charts also showed a phenomena of planetary motion called retrograde motion. The apparent backwards motion of the planets in the night sky. This motion is not a true motion of the planet,

but these observations will require many revisions of the model of our Solar System

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).

Aristotle’s model built on the models of Pythagoras and Plato, and consisted of nested spheres (a ball within a ball).

Aristotle observed that the Earth was a sphere and consisted of objects made of the element Earth.

Other spheres were centered on the Earth and included water, air and fire (Sun). Outside of this were the observed planets (Mercury, Venus, Mars and Jupiter. Outside of this would be the sphere of the stars (the Zodiac).

Aristotle’s Geocentric Universe

Aristotle even had a Physics to explain the motion of objects.

Basically motion consisted of natural and forced motions.

The spheres rotated naturally, but to move something, like lift a rock required a force.

One problem of this model was its inability to explain retrograde motion. Aristotle’s Geocentric Universe

The Geocentric model persisted and other scientists tried to refine it so that they could explain retrograde motion.

One such person was Hipparchus (160-127 BC). A Greek astronomer

He introduced eccentrics, epicycles and deferents to his models of the Solar System.

Hipparchus

The eccentric is a point where the Earth is which is not exactly at the center of the other spheres.

This helped explain the apparent changes in speed for the planets.

Eccentric

C

The path around the Earth is called the deferent.

A planet rides along a small circle centered at a point on the deferent. This path is called the epicycle.

The planet moves along the epicycle counter-clockwise while the center of the epicycle moves counter-clockwise along the deferent. Watch.

C

Deferent

Epicycle

This model didn’t match observations as well as we would like.

But it did allow retrograde motion.

The major problem was that it was too complicated. Remember, simpler is better.

This didn’t stop people from accepting it, actually other scientists actually added to it.

C

Deferent

Epicycle

Claudius Ptolemy, around AD 125, revised the geocentric model more.

Ptolemy first allowed for the planets to move in a non-uniform way. This means they could speed up or slow down.

But he added in a point opposite the center from the eccentric which one could observe the planets moving in a uniform way. This point was called the Equant.

C E

Claudius Ptolemy

C E

Even though this model was extremely complicated it did match observations fairly well.

This model was accepted for nearly 1400 years, and there are actually people who believe it is true today.

C E

This was about to change !!!!!!This was about to change !!!!!!

Enter Nicolaus Copernicus.Enter Nicolaus Copernicus.

Copernicus’ Heliocentric Copernicus’ Heliocentric ModelModel

Nicolaus Copernicus Nicolaus Copernicus (his friends called him (his friends called him Copper) (1473-1543) Copper) (1473-1543) developed a heliocentric developed a heliocentric model and distributed a model and distributed a copy among his friends copy among his friends in 1514.in 1514.

It wasn’t until late in his It wasn’t until late in his life that Copernicus life that Copernicus finally agreed to have finally agreed to have his work published.his work published.

Copernicus’ Heliocentric Copernicus’ Heliocentric ModelModel

All the heavenly spheres All the heavenly spheres revolve around the Sun.revolve around the Sun.

The distance from the The distance from the Earth to the sphere of Earth to the sphere of Stars is much greater Stars is much greater than the distance from than the distance from the Earth to the Sun.the Earth to the Sun.

The Earth spins on its The Earth spins on its axis, which explains the axis, which explains the motion of the planets.motion of the planets.

Copernicus’ Heliocentric Copernicus’ Heliocentric ModelModel

The motion of the Sun The motion of the Sun relative to the stars relative to the stars results from the annual results from the annual revolution of the Earth revolution of the Earth around the Sun.around the Sun.

The planet’s retrograde The planet’s retrograde motions occur from the motions occur from the motion of the Earth motion of the Earth relative to the other relative to the other planets.planets.

Animation II

Copernicus’ Heliocentric Copernicus’ Heliocentric ModelModel

The The Copernican modelCopernican model wasn’t any better than wasn’t any better than Ptolemy’s at matching Ptolemy’s at matching observations.observations.

But some scientists But some scientists began to accept it.began to accept it.

However many, However many, including the church, including the church, felt it shouldn’t be felt it shouldn’t be confused with reality.confused with reality.

Kepler’s Heliocentric ModelKepler’s Heliocentric Model

In the 1600’s Johannes 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:

That That predictions predictions must match must match

observations.observations.

Kepler’s Heliocentric ModelKepler’s Heliocentric Model

Between 1609 and Between 1609 and 1618, Kepler 1618, Kepler developed his developed his three Laws of three Laws of Planetary Motion.Planetary Motion.

These are still These are still used today !!!!!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.

A circle has an eccentricity of

0

A ellipse has an eccentricity

between 0 & 1

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.

Slower orbital speed

Faster orbital speed

Law 3 : Harmonic Law The square of the orbital period (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.

What this means for us : The planets farther from the Sun take longer to orbit the Sun. (Much weaker than the above statement)

 

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