Our Solar System:The Geocentric vs. Heliocentric

DebateThe Answer to the Debate

Learning GoalsStudents will:1) Develop scientific argumentation skills

(students will use Earth-based observations to prove the commonly understood notion that the sun is in the center of our universe).

2) Understand that ancient humans could only use Earth-based observations to understand that our solar system is heliocentric.

Success CriteriaStudents will display learning goals by:1) Using research skills to obtain information.2) Collecting data to disprove the points

suggested by geocentric theory.

AstronomyI don’t care what your grade 6 teacher told you – but the Earth is in the center of the Universe and all other celestial bodies (including the sun, the planets and the stars) revolve around the Earth!

AstronomyIf you don’t believe me, I’ve got proof!

It’s all just common sense – all you have to do is watch the sky during the day and night and even collect data for several weeks.

Let me list the observations you will make:

Astronomy1) Over the course of the day we

watch the sun rise in the east, move across the sky and set in the west – the sun is obviously revolving around the Earth!

2) Similarly – during the day and night we can watch the moon revolve around the Earth.

Astronomy3) If we watch the stars

carefully at night, we notice that they circle around us. A camera taking a long exposure photograph of the sky will produce a picture with star trails. This proves that the positions of the stars are moving around us from East to West.

Star trails photographed in a long term exposure near an astronomical observatory

Astronomy4) The Parallax

EffectDepending upon the position of the Earth in its orbit, nearer stars should be in different positions relative to more distant stars. Have you ever observed this? I haven’t!

Astronomy

A simple test of the parallax effect is to place your thumb in front of your face and observe an object with your left eye only. Now switch to using your right eye only. The object will be in a different position relative to your thumb. We can see that the building appears to be in a different place relative to the thumb in the two pictures. The same thing should happen as the Earth travels in orbit. More distant stars should appear to be in different positions relative to closer stars or planets.

Astronomy5) If we plot the position of planets carefully

at night, we notice that they also move but at a different (slower) speed than the stars.

6) If the Earth were orbiting the Sun and rotating on its axis, wouldn’t we be swept away by the wind and then fly off into space?

7) God created the Earth on the first day but didn’t create the Sun and Stars until the fourth day. God created man in his own image and put humans as the masters of the universe and therefore must have placed humans in the central location in the universe

Prove or Disprove my Statement!

1. Using evidence that can be obtained by looking at the sky, argue against my claim that the Earth is in the Center of the Universe (Geocentric Theory).

2. OR IF YOU REALLY WANT TO - AGREE WITH MY STATEMENT – but you must add to my argument! (Aristotle is on my side as well.) I would suggest that you disprove that the Heliocentric Theory.

We will now head to the library for a research sessionCollect evidence from the library and record your facts on

paper. NO plagiarizing! You must put the evidence into your own words and be prepared to discuss the data in front of the class.

YOU MUST UNDERSTAND THE DATA IN ORDER TO BE ABLE TO ARGUE YOUR POINT!!

1) Sun and Moon revolve around Earth2) Stars revolve around Earth3) The Parallax Effect is not observed.4) Planets move across sky – only slower5) We’d fall off the Earth if it was moving in

orbit6) God created us and put us in the center of

the Universe

This data is included on the Argument Summary

Here is the summary of my evidence:

What I need to get from you:

1) A completed argument summary – a page of written evidence is sufficient

2) HOWEVER MUCH GREATER VALUE WILL BE GIVEN TO THOSE WHO:

a) provide diagrams or astronomical charts to help prove your point (make them big enough to show the class)

b) add clear written support of the arguments you find – you can add extra pages if you wish!

3) I will not make life easy for you! Photographs taken from spacecraft (a picture of the planets revolving around the Sun, photos from Satellites, pictures from the Mars Rover etc.) make too simple an argument. Remember – 100 years ago scientists such as Claudius Ptolemy, Galileo and Copernicus did not have the luxury of spacecraft and were able to make good arguments on both sides of this debate. What evidence could an Earth-bound observer use to convince me that I am wrong!

4) Reference your work – write down the names of books or web sites that you used to gain information

5) YOU MAY WORK WITH A PARTNER – BUT IF YOU DO I WILL EXPECT MORE WORK!

6) Prepare to defend yourself IN A SCIENTIFIC ARGUMENT!

7) Remember that simple textbook or internet pages showing photographs are not sufficient. It is likely that none of you can produce the background facts that support the Sun-centered theory shown in these pictures. I need the written explanations to go with your ideas

Astronomy

Need a few research hints to get you started?

Here are some keywords:

Heliocentric GeocentricCopernicus Aristotle PtolemyGalileo ParallaxRetrograde Motion

Answering the Debate!Please submit your assignment before we continue!

Aristotle vs. CopernicusGeocentric vs. Heliocentric Views

Geocentric = Earth Centered

Heliocentric = Sun Centered

Let’s Follow Up on Our Debate

The Earth-Centered Universe proposed by Aristotle and PtolemyThe Greek philosopher

Aristotle proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities (but the angular velocity was constant for a given sphere), with the Earth at the center.

The Earth-Centered Universe proposed by Aristotle and PtolemyThe following figure

illustrates the ordering of the spheres to which the Sun, Moon, and visible planets were attached.

An outermost sphere was the domain of the "Prime Mover". The Prime Mover caused the outermost sphere to rotate at constant angular velocity, and this motion was imparted from sphere to sphere, thus causing the whole thing to rotate.

Aristotle, we have a problem!Ancient Astronomers

noticed that while stars had fixed positions in the sky, this is not true of the planets.

The planets not only wandered on the celestial sphere, but occasionally appeared to stop and retrace their steps for a while, sometimes moving in a great loop, before advancing once again. This motion is called retrograde motion.

Watch the video showing the motion of Mars in the sky over several weeks. The position of Mars is plotted at the same time each night.

Note that the position of the stars is static but that Mars wanders across the sky in a retrograde path.

The Retrograde Motion of Mars

This pictures shows the location of Mars over a 14-week period at the same time of night against an identical backdrop of stars.

This type of motion can be seen with the other planets. Such as this picture of Jupiter.

Note that astronomical data would had to have been collected (at the same time of night) for years to prove that retrograde motion exists

Aristotle, we have a problem!Watch the video showing the retrograde motion of Venus in the sky over several weeks. The second diagram shows that retrograde motion is due to the fact that each planet is travelling in its own orbit AND at different velocities (planets closer to the sun travel faster due to Gravitational effects. Note (in the left frame) that the apparent size of Venus also changes.

Ptolemy's ModelThe planetary phenomena of

retrograde motion fascinated and frustrated the ancient astronomers.

A second century mathematician -astronomer-geographer Ptolemy (Claudius Ptolemy) improved on the theory proposed by Aristotle by adding the idea of epicycles and off-center orbits.

Ptolemy wrote the Almagest. This great work dominated astronomical thought right up to the early years of the seventeenth century.

Understand that the Ptolemaic model of the Universe is still Earth-centred, or Geocentric

Ptolemy (c. 87 – 170 A.D.)

The "solution" to the problems with Aristotle’s model were in the form of a clever proposal:

1) planets do not travel along the concentric spheres themselves, but to circles attached to the concentric spheres, as illustrated in the previous diagram.

2) These circles were called "Epicycles", and the concentric spheres to which they were attached were termed the "Deferents".

This image shows the complex but ingenious system of off-center orbits and epicycles (smaller circular orbits in which planets moved as they orbited the Earth). The is the model developed by astronomer Claudius Ptolemy of Alexandria

Epicycles and Planetary MotionThe Ptolemaic model was remarkably successful

in predicting where the planets, moon, and sun would appear in the sky at future dates.

It was only in the Middle Ages, when Arabian astronomers had accumulated more accurate observations of the planets, that the validity of the Ptolemaic model began to be questioned.

Only by further complicating the model by adding epicycles to epicycles and by tilting the orbits was it possible to explain the latest data. Ptolemy's model was accepted throughout much of the world for nearly 1,500 years.

The Epicycle ModelThe net effect of the Ptolemaic Epicycle Model

is illustrated in the animation on the next slide.As the center of the epicycle (white dot) moves

around the deferent at constant angular velocity, the red planet moves around the epicycle, also at constant angular velocity.

The apparent position of the planet on the celestial sphere at each time is indicated by the line drawn from the earth through the planet and projected onto the celestial sphere.

The resulting apparent path against the background stars is indicated by the blue line.

http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html

The Epicycle Model

Aristotle, Aquinas and the ChurchBy the Middle Ages, the ideas of

Ptolemy took on a new power as the philosophy of Aristotle (newly rediscovered in Europe) was added to Christian Medieval theology

This great synthesis of Christianity and (Aristotle’s) Reason undertaken by philosopher-theologians such as Thomas Aquinas.

The Prime Mover of Aristotle's universe became the God of Christian theology, the outermost sphere of the Prime Mover became identified with the Christian Heaven

The position of the Earth at the center of it all was understood in terms of the concern that the Christian God had for the affairs of mankind.

St. Thomas Aquinas (1225-1274) – Italian born Dominican Friar and one of the greatest Theologians of the Catholic Church. His word carried a lot of weight in the Church.

Aristotle, Aquinas and the ChurchThus, the ideas largely originating

with pagan Greek philosophers were baptized into the Catholic church and eventually assumed the power of religious dogma

To challenge this view of the Universe was not merely a scientific issue; it became a theological one as well

In the age of the Renaissance, challenging the views of the church was considered blasphemy and subject to great punishment.

The church was not a benevolent force. Questioning Aristotle’s model was the equivalent of questioning Aquinas, which was the equivalent of questioning the church (= blasphemy)

The Copernican Model:A Sun-Centered Solar System

The Earth-centered Universe of Aristotle and Ptolemy held sway on Western thinking for almost 2000 years. Then, in the 16th century a new idea was proposed by the Polish astronomer Nicolai Copernicus (1473-1543).

In a book called On the Revolutions of the Heavenly Bodies (that was published as Copernicus lay on his deathbed), Copernicus proposed that the Sun, not the Earth, was the center of the Solar System.

Such a model is called a heliocentric system. The ordering of the planets known to Copernicus in this new system is illustrated in the following figure, which we recognize as the modern ordering of those planets.

The Copernican UniverseIn this new ordering the

Earth is just another planet (the third outward from the Sun), and the Moon is in orbit around the Earth, not the Sun.

The stars are distant objects that do not revolve around the Sun.

Instead, the Earth is assumed to rotate once in 24 hours, causing the stars to appear to revolve around the Earth in the opposite direction.

Retrograde Motion of MarsPtolemy Copernicus

Retrograde Motion and Varying Brightness of the PlanetsBy banishing the idea that the Earth was the center of the Solar System, Copernicus’ idea immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion:

Another problem was also answered – that of the varying brightness of the planets!

The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth.

Copernicus was able to refute the long-held notion that the Earth was the center of the Solar system, but he did not question the assumption of uniform circular motion. Thus, in the Copernican model the Sun was at the center, but the planets still executed uniform circular motion about it.

As we shall see later, the orbits of the planets are not circles, they are actually ellipses. The work of Johannes Kepler (based on the data of Tycho Brahe) proved the elliptical orbits of planets

The Copernican Revolution

As a consequence, the Copernican model (and its assumption of uniform circular motion) still could not explain all the details of planetary motion.

Astronomers following Ptolemy tried to solve the elliptical motion problem by using epicycles with epicycles.

In Science we call models that are simpler, a more ELEGANT MODEL. Usually simpler models that explain all of the facts tend to be more accepted than very complex models such as the Ptolemy’s multi-epicycle model.

The Copernican Revolution

The Copernican Revolution

Copernicus was an unlikely revolutionary. It is believed by many that his book was only published at the end of his life because he feared ridicule and disfavor by his peers and by the Church, which had elevated the ideas of Aristotle to the level of religious dogma.

However, this reluctant revolutionary set in motion a chain of events that would eventually (long after his lifetime) produce the greatest revolution in thinking that Western civilization has seen.

His ideas remained rather obscure for about 100 years after his death. Much of Copernicus’ theory was based on data from earlier Islamic astronomers.

But, in the 17th century the work of Kepler, Galileo, and Newton would build on the Heliocentric Universe of Copernicus and produce the revolution that would sweep away completely the astronomical ideas of Aristotle and replace them with the modern view of astronomy and natural science. This sequence is commonly called the Copernican Revolution.

The idea of Copernicus was not really new! A sun-centered Solar System had been proposed as early as about 200 B.C. by Aristarchus of Samos (an island off the coast of Turkey). However, it did not survive long under the weight of Aristotle's influence and "common sense":

Aristotle’s Common Sense:

1)If the Earth actually spun on an axis (as required in a heliocentric system to explain the diurnal motion of the sky), why didn't objects fly off the spinning Earth?

2)If the Earth was in motion around the sun, why didn't it leave behind the birds flying in the air?

Been There, Done That Aristarchus of Samos

Finally – the Answer to Mr. Snyder's challengeHow can the Geocentric view of the universe be disproved?

1. Sun and Moon revolve around Earth2. Stars revolve around Earth

1. Earth (and other planets) rotate on their own axes – this refutes for the first two arguments very easily. The sun rises in the east and sets in the west since the planet rotates counterclockwise.

2. The stars rotate around the north pole slowly during the night as the earth spins. Since the north pole points directly at Polaris (The North Star), it does not move.

3. The Parallax Effect is not observedo Review - stars should appear

to change their position with the respect to the other background stars as the Earth moved about its orbit, because of viewing them from a different perspective – Astronomers said that parallax could not be seen!

3) NOT TRUE! Since planets and stars are so distant, the parallax is very small – BUT IT IS THERE! It can be measured with more sensitive equipment.

o The parallax method allowed Astronomers to measure the distance to close stars for the first time.

Parallax is larger for closer objects

4) Planets move across sky, but only slower!5) Planets travel in retrograde motions

because of epicycles

4) Planets follow orbits around the sun that do not match the speed of the Earth’s rotation – as a result planets appear to travel across the sky more slowly than the stars.

5) The heliocentric vs. Geocentric View of the Solar System is a far more elegant solution to explain retrograde motion!

Geocentric

Heliocentric

5) We’d fall off the Earth if it was moving in orbit This a response to

“Aristotle’s Common Sense Argument”

This objection is not valid because it represents an inadequate understanding of the physics of gravity that would only be corrected in the 17th century.

6) God created us and put us in the center of the Universe

This is a classic example of trying to use religion to solve a scientific argument. Obviously, the Catholic Church of the Renaissance Era would have been seriously humbled by this mistake and it would have exposed great weakness in their “heavy handed” approach.

A much more benevolent Catholic Church today would be far less likely to make a similar mistake

HOWEVER, many religious groups continue to put “faith” or a literal interpretation of the Bible in front of scientific knowledge – for example the debate over Evolution (by the way the Catholic Church agrees with this theory and we teach it in Grade 12 Univ. Biology.

There ARE still supporters of the FLAT EARTH theory still living today.

Modern Astronomy vs. The Church

Galileo recants and his life is spared but he is placed under house arrest for the rest of his life.

More Evidence to refute the Geocentric Theory not included in this presentation1) Phases of Venus (and

other planets)o Like the moon, planets

display different phases as they travel in their orbits around the sun. Without the invention of the telescope by Galileo, the phases of planets could not be determined.

o Seeing phases of planets would have helped astronomers refute the Geocentric Theory more quickly.

Phases of the MoonWe are very familiar

with the concept of the phases of the moon. This diagram shows why we see phases of the moon as it rotates around us.

The phases of planets occur for exactly the same reason

2. The Ancients knew that Jupiter was a planet. Planets with their own moons suggest that the Earth might be like Jupiter – just another planet with moons.

Galileo first saw the 4 largest moons of Jupiter in 1610. He plotted their positions as they rotated around Jupiter. He determined that Jupiter had its own “system”.

By the way you can see the 4 “Galilean” moons of Jupiter with just a pair of binoculars.

3) In the early 17th century, Isaac Newton stated the Laws of Gravity

These laws were used to dictate the Laws of Motion which in turn helped to explain how objects in space could travel in orbital paths.

Kepler refined the orbital motion of planets from perfect circles to ellipses.

More Evidence to refute the Geocentric Theory not included in this presentation

Activities1) Retrograde Motion Activity2) Kepler’s Laws activity (tomorrow)