Lecture 4: Geocentric Universe
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Scientific Method
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Earth was thought to be the center of the universe until only a few centuries ago!
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Man has been around for millions of years
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Our scientific revolution began only a few thousand years ago
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The origins of the Scientific Method are in the Natural Philosophy of the ancient Greeks
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The Greek philosophers were mainly interested in the logical consequences of pure thought
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They studied the properties of an ideal universe
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Observations of imperfect reality were generally thought to be useless
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This changed a bit when Aristotle (384BC – 322BC) first used the Scientific Method:
Observation, Theory, Prediction, and Testing.
Aristotle
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He noted that the Earth’s shadow (as projected onto the Moon during a lunar eclipse) is curved
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He theorized that the Earth must therefore be round
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He predicted that bright stars would have different positions in the sky depending on the latitude
of the observed
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This prediction was confirmed by observations
Earth’s Radius
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Eratosthenes (276BC – 194BC) used Aristotle’s geometrical approach to measure the radius of
the Earth
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The angle between the zenith and the Sun at noon on June 22 is 7.2 degrees as observed from
Alexandria Egypt
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On the same day, the Sun is exactly at the zenith (at noon) in Syene, which is 5000 stadia to the
north of Alexandria
Earth’s Radius
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We can combine the 5000 stadia distance from Syene to Alexandria with the fact that the angle
between the zenith and the Sun at noon on June 22 is 7.2 degrees in Alexandria to compute the
Earth’s radius using the relation
=
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This yields for the radius of the Earth R = 39,789 stadia
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Converting to kilometers gives the result R = 6,366 km since there are 0.16 km/stadium.
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This is very close to the correct value 6,378km
Earth’s Radius
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The calculation yields for the Earth’s radius 39,789 stadia, or 6,366 km
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Modern satellite observations give 6,378 km, which is extremely close to the value obtained by
Eratosthenes
Planetary Motions
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To the Greeks, the motions of the stars, Sun, and Moon could be understood in terms of the
geocentric universe. They thought there were 7 planets.
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The planets (“wanderers”) created some problems with this idea because their motions are
complex
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 Spring '12
 lkj
 Nicolaus Copernicus, Heliocentrism, History of astronomy

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