The Solar System

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The Solar System and Planetary Motion
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Just what orbits around what and why, did not
come easy
It seems easy for us to imagine the solar System
today, but
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Beyond the planets Aristotle placed a
crystalline sphere of stars
Aristotle was the first person in history to
devise a model of the universe
Furthermore, Aristotle Thought the universe To be
perfect, homogeneous and unchanging
His model had the earth in the center and the sun
and planets orbiting around it
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His model was known as a
The Geocentric Model
Meaning Earth Centered
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Aristotle believed all objects orbited the Earth
and beyond the planets was a permanent smooth
sphere of crystalline stars
What would Aristotle have made of Jupiters moons?
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Ptolemys Model and explanation of planetary
motion set back astronomy for 1500 years
By the 1500s, it was becoming evident that
Ptolemys model had some major flaws, as it was
no longer accurately predicting the positions of
the planets
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A much simpler model was finally put forth by the
son of a Polish clergyman in 1543. His name was
Nicholos Copernicus
Copernicus was very reluctant to publish such a
radical new idea and only did so when he knew he
was dying.
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The Copernican model was much simpler than
Ptolemys and it placed the sun rather than the
earth in the center of the solar system
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So how did the Copernican heliocentric model
explain the retrograde (looping ) motion of the
planets?
The Copernican explanation for this is rather
simple
Have you ever been on a train and watch out the
window as you pass another train?
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To an observer on the train going faster it
appears as if the other train is going backwards
70mph
100mph
See Ya
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Copernican model
This is where I belong
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But the Copernican model had problems of its
own as was proven by the great Danish
astronomer Tycho Brahe
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Tycho was the greatest observational astronomer
of the day, and data from his observatory in
Gratz showed Copernicus model to be inaccurate
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In fact, the Copernican model did no better in
predicting planetary positions than did the
Ptolemy model
How could this be?
Copernicus placed the planets in circular orbits
He placed the sun at the center of the orbit
He placed all the planets in the same plane of
orbit
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Johannes Kepler made the Copernican idea work
It would be less than 100 years later before
Johannes Kepler determined that planets actually
orbit the sun in ellipses and that the sun is not
exactly in the center of the ellipse
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After careful analysis of Tycho Brahes data
Kepler formulated what has become 3 of the most
prominent laws in astronomy
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Keplers 1st Law of Planet Motion
1. The planets go around the sun in an ellipse
with the sun at one focus of the ellipse
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Calculating Eccentricity
Eccentricity ranges between 0 and 1
.
focus 2
focus 1
asteroid
focal distance
major axis
Eccentricity focal distance major axis
Notice the units cancel out
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E 0 (circle)
E 0.1
E 0.4
E 0.9
E 0.7
The higher the eccentricity the more elongated
the orbit
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Earth Orbit 0.017 (very slightly elliptical)
Like the earths shape, the human eye probably
can not distinguish earths orbit from a circle