1-theories of the universe

8/10/2019 1-Theories of the Universe

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The Geocentric Theory vs.

The Heliocentric Theory


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Aristotle (384-322 B.C.)

Developed an early modelbased on the concept ofuniform circular motion:

Earth was at the center

of the Universe. Planets Sun and stars

revolved around it.


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Aristotle reasoned that if the Earth

rotated about its axis, we should flyoff into space. Since we don't, theEarth must be stationary.

At that time they didnt know about

gravity and inertia (Galileo, 1900 yearsafter)


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Ptolemy (85-165 A.D.)

Ptolemy proposed the Geocentric theory :The Earth is the center

of the Universe .

Later he addedmathematics to supportthe theory, and itprevailed for 1400 years .


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The Geocentric theory:Ptolemaic Universe

Universe consists ofconcentric spheres withEarth at the center.

Moon, Sun, Mercury, Venus, Mars, Jupiter, andSaturn each have theirown sphere.

The stars are all onesphere.

All heavenly(astronomical) motionconsists of perfectuniform circular motion.


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Geocentric modelWhy ?

The Geocentric theory was firmly believed by theCatholic church which taught that God had put the

Earth as the center of the Universe .

They believed that the natural motion of earthymaterials was always to seek the center of the

Universe.


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But geocentric theory had a

gap Many supporters of the

geocentric theory had onepiece of evidence theycouldnt explain themovement of MARS .

Lets investigate thismovement


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Mars Motion

http:// alpha .lasalle.edu/~smithsc/Astronomy/retrograd.html
http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.htmlhttp://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html


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How did Ptolemy Explain this

Problem? Ptolemy used geometric models to predict

the positions of the Sun, Moon, andplanets using epicycles .

An epicycle is an orbit (small loop) withinan orbit (planet around the Sun).


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Epicyclesof

PtolemysModel


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The Heliocentric Theory

The Sun is the center of the Universe


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Copernicus (1473-1543)

Polish astronomer.He proposed the Heliocentrictheory: the Earth and otherplanets revolve around theSun. This was highlycontroversial at the time.


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The Theories: HeliocentricCopernican UniversePlanets orbit around theSun, which is the center ofthe Universe.

The Earth rotates.

The moon orbits aroundthe Earth.

The stars have a fixedlocation.

Galileos legendarycomment:

But it does move.


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How did the Heliocentric model explainthe movement of Mars?

http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html
http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.htmlhttp://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html


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This theory also solves:

The daily movement of the Sun, Moon,

planets and stars from east to west.

The annual movement of the Sun towardsEast (changing daily hours of Sun light).

-The retrograde motion of planets.But still does not solve the stellar parallax


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Galileo (1564-1642) Galileo supported the Copernican

model:

He was condemned by the Catholic

Church for his view of the cosmosbased on the theory of Copernicus,

He was accused of heresy by

Inquisition and sentenced to housearrest for life .


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In 1992 , 350 years afterGalileos death, the RomanCatholic Church gave apardon to Galileo andadmitted that theheliocentric theory wascorrect.


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Keplers Laws (Phisician 1609)

He analyzed data collected fromobservations and wrote three lawsto explain the movements of thestars and planets.

Orbits are always ellipses.

They sweep out equal areas in equal times. The period of the orbit is related to the

average separation .


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Keplers First Law The orbits of the

planets are elliptical(not circular) withthe Sun at one focusof the ellipse.

'a' = semi-major axis: Avg. distance betweensun and planet


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Kepler's Second Law

A line connecting the Sun and a planetsweeps out equal areas in equal times.

Planets move faster when closer to the

Sun.

slower faster


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Kepler's Third Law

The square of a planet's orbital period isproportional to the cube of its semi-majoraxis.

P2 is proportional to a 3

or

P2

(in Earth years)

= a3

(in A.U.)

The further the planet is from theSun, the longer the period.


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Stephen Hawking theories of the Universe

http://www.youtube.com/watch?v=xjBIsp8mS-c
http://www.youtube.com/watch?v=xjBIsp8mS-chttp://www.youtube.com/watch?v=xjBIsp8mS-chttp://www.youtube.com/watch?v=xjBIsp8mS-chttp://www.youtube.com/watch?v=xjBIsp8mS-c

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