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The Copernican The Copernican RevolutionRevolution

The Birth of Modern ScienceThe Birth of Modern Science

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What do we see in the sky?What do we see in the sky?• The stars move in the sky but not

with respect to each other• The planets (or “wanderers”) move

differently from stars– They move with respect to the

stars– They exhibit strange retrograde

motion• What does all this mean?• How can we explain these

movements?• What does the universe

look like?

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Geocentric vs. HeliocentricGeocentric vs. Heliocentric

• How do we decide between two theories?

• Use the Scientific method:– These are both explanations

based on the observation of retrograde motion

– What predictions do the models make?

– How can these predictions be tested?

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Geocentric vs. Heliocentric Models of the Solar System

Aristotle vs. Aristarchus (3rd century B.C.)

Aristotle: Sun, Moon, Planets and Stars revolved around fixed Earth.

Aristarchus: 1st Heliocentric model

Aristotle: But there's no wind - Doesn’t “feel” like we are moving. (Actually orbiting sun at 70,000 mph!)

Difficulties with "Geocentric" model - Retrograde (Backwards) motion of planets

- Phases of Venus

Ancient Greeks knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.

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What are some reasons that the geocentric model of the universe seems to make intuitive sense?

It doesn't feel like we are It doesn't feel like we are moving – wouldn't there be a moving – wouldn't there be a

wind or something?wind or something?Why would things fall down and Why would things fall down and

not towards the center of the not towards the center of the universe?universe?

Why don't we see stellar Why don't we see stellar parallax?parallax?

Geocentric Model of Solar System (Earth Centered)

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GeoCentricGeoCentric

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Geocentric TheoryGeocentric Theory

Ancient Greece

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Geocentric ModelGeocentric Model(Earth Centered)(Earth Centered)

• Fairly good agreement with retrograde motion of planets

• Some predictive power• More precise

measurements showed errors

• To account for unusual planetary motion epicycles were introduced

• Fit the Greek model of heavenly perfection – spheres are the perfect shape, circular the perfect motion

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Ptolemy’s Prediction: Future planetary positions

Observation: retrograde motion of planets

Refine: epicycles

Success! For 1500 years

cfa-www.harvard.edu/seuforum/mtu/MTUcosmology.ppt

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Motion of the other planetsMotion of the other planets

Mars Retrograde Motion

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Ptolemy's geocentric model (A.D. 140) (VIDEO CLIP)

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Problems with Geocentric Problems with Geocentric TheoryTheory

1. This model does not explain all apparent motions of celestial (space) objects

2. Cannot explain Foucault’s Pendulum or the Coriolis Effect

Ptolemy’s system provided the first framework for all discussion of the universe for nearly 1500 years!!

But… there was a problem

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Geocentric model fails to Geocentric model fails to account for phases of Venus account for phases of Venus

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Aside: AristarchusAside: Aristarchus Written in the second century BC he calculated the ratio of the distance between the Earth and Sun to that between the Earth and the Moon. (His estimate was more than an order of magnitude too small, but the fault was in his lack of accurate instruments rather than in his method of reasoning.) -This image compares the line subtending the arc dividing the light and dark portions of the Moon in a lunar eclipse with the relative diameters of the Moon and Sun. -Aristarchus also found an improved value for the length of the solar year.

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Heliocentric (Copernican) SystemHeliocentric (Copernican) System• Sun at center (heliocentric)• Uniform, circular motion

– No epicycles (almost)• Moon orbited the earth, the earth

orbited the sun as another planet• Planets and stars still on fixed

spheres, stars don’t move• The daily motion of the stars

results from the Earth’s spin• The annual motion of the stars

results from the Earth’s orbit

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Heliocentric model easily Heliocentric model easily accounts for phases of Venusaccounts for phases of Venus

TimelineTimeline

1473-1543Copernicus

Tycho

1546-1601

Galileo

1564-1642 1642-1727 Newton

1571-1630 Kepler

Heliocentric Model

“Rediscovered” by Copernicus in 16th century.

Much simpler was the main attraction for Copernicus. Simply explains retrograde motion. Put Sun at center of everything, but still insisted on circles, thereby retainig unnecessary complexity.

Opposed by Catholic Church

Copernican revolution – critical realization that Earth is not at the center of the universe, only accepted after his death.

Copernicus 1473-1543

Heliocentric TheoryHeliocentric Theory

• In the heliocentric model, apparent retrograde motion of the planets is a direct consequence of the Earth’s motion

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Planets sometimes appear to loop back - retrograde motion

Loops are called "epicycles"

1

2

3

4

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7

12

3

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67Earth

Mars

Apparent motion of Mars against "fixed" stars

*

*

***

*

January

July

Retrograde Motion of PlanetsEarth overtakes slow outer planet so the outer planet appears to slow down, move in reverse, and then move forward again with respect to the fixed stars

Tycho Brahe (1546-1601): the greatest of the pre-telescope observers in Europe.20+ years measuring the positions of the Sun, Moon and planets with great accuracy.

Tycho BraheTycho Brahe• Had two sets of astronomical

tables: one based on Ptolemy’s theory and one based on Copernicus’.

• He found that both tables’ predictions were off by days to a month.

• He believed that much better tables could be constructed just by more accurate observations.

• Tycho’s homemade instruments improved measurement precision from ten minutes of arc (which had held since Ptolemy) to less than one

The skies changeThe skies change• Tycho observed 2 phenomena that

showed the heavens DO change:– In November 1572, Tycho noticed

a new star in the constellation Cassiopeia

– Comet of 1577• Prior to this sighting,

comets were thought to be atmospheric phenomena because of the immutability of the heavens

• But neither the star nor the comet changed position as the observer moved, as expected for atmospheric phenomena

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• Johannes Kepler (1609)• Johannes Kepler was an assistant to

Tycho Brahe. He used Brahe’s observations to study the orbit of Mars

• Discovered three laws of planetary motion: predict with accuracy the motions of the planets

Galileo (1564-1642), Experimentalist

Built his own telescope.

Discovered four moons orbiting Jupiter. What does this suggest?

Discovered sunspots. What might we infer about the Sun from these observations?

Observed phases of Venus.

Was imprisoned for the last 9 years of his life for his scientific discoveries.

Galileo GalileiGalileo Galilei

• Turned a telescope toward the heavens• Made observations that:

– contradicted the perfection of the heavens• Mountains, valleys, and craters on the Moon• Imperfections on the Sun (sunspots)

– Supported the heliocentric universe• Moons of Jupiter• Phases of Venus – shows a full phase

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• Galileo (1564-1642) first scientist to use a telescope to observe the sky. He observed the phases of Venus and the moons of Jupiter. Both observations supported the heliocentric model.

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Modern UniverseModern Universe• Is the Sun the center of the

Universe?