Models of Our Solar System A Study in Developing a Scientific Model

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 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 = mc 2

Developing a Scientific Model  You start with the real world.  You then make your initial observations or assumptions.  This then leads to a model.  But this isn’t where this process ends. Initial Observations or Assumptions Model

Developing a Scientific Model  Once you have your model you then can make some predictions based on your model.  Then it is time to test your predictions by comparing these predictions to observations. Initial Observations or Assumptions Model Make Predictions Based on Model Compare Observations to Predictions

Developing a Scientific Model  Were your predictions correct or not ? If not revise (fix) your model. You don’t have to throw it out.  You revise your model based on Mathematics, Physics and Simplicity (simpler is usually better). Initial Observations or Assumptions Model Make Predictions Based on Model Compare Observations to Predictions Revise Model Mathematics Physics Simplicity

Developing a Scientific Model  Now you get to start the process over again.  And Again!  And Again!!  Don’t worry, be happy. This is how science is done.  Remember, science is self-correcting. Initial Observations or Assumptions Model Make Predictions Based on Model Compare Observations to Predictions Revise Model Mathematics Physics Simplicity

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 ( 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 ( 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.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

CE

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. CE

This was about to change !!!!!! Enter Nicolaus Copernicus.

Copernicus’ Heliocentric Model  Nicolaus Copernicus (his friends called him Copper) ( ) developed a heliocentric model and distributed a copy among his friends in  It wasn’t until late in his life that Copernicus finally agreed to have his work published.

Copernicus’ Heliocentric Model  All the heavenly spheres revolve around the Sun.  The distance from the Earth to the sphere of Stars is much greater than the distance from the Earth to the Sun.  The Earth spins on its axis, which explains the motion of the planets.

Copernicus’ Heliocentric Model  The motion of the Sun relative to the stars results from the annual revolution of the Earth around the Sun.  The planet’s retrograde motions occur from the motion of the Earth relative to the other planets. Animation II

Copernicus’ Heliocentric Model  The Copernican model wasn’t any better than Ptolemy’s at matching observations. Copernican modelCopernican model  But some scientists began to accept it.  However many, including the church, felt it shouldn’t be confused with reality.

Kepler’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 predictions must match observations.

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.

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