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Chapter 2

Chapter Two Review: The Copernican Revolution

Course Code
Brian Wilson

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September 20th, 2010
Intro to Astronomy and Astro Part I
Chapter Two: The Copernican Revolution
Ancient Astronomy (pg.32)
Probably wasnt developed for religious or scientific reasoning
Instead, it was extremely practical and down to earth, used for:
Time Keeping
Tracking the Seasons
In many ways human survival was dependent on the knowledge of the
Some ancient sites (such as Stonehenge) served as a three dimensional
astronomical calendar
Also used to indicate cosmic events through alignment with stones (ie:
Summer solstice)
The Chinese associated astrology with omens
Many mathematical techniques involved in trigonometry were developed by
Islamic astronomers
Astronomys roots come into existence because of the need to track time
(mainly the Sun, due to food production) but more importantly, from human
nature and the need to know where we came from and how we fit into the
The Geocentric Universe (pg34)
The Greeks of antiquity, built models of the universe
Cosmology: The study of the workings of the universe on the largest scale

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To the Greeks the universe was basically seen as only our solar system; the
stars beyond that were clearly seen as part of the universe by the Greeks, but
they were seen as fixed and unchanging beacons of the celestial sphere
The objects of our solar system had patterns of behaviour which set them
apart from being believed to be within the celestial sphere
Observations of the Planets
The Greeks noticed movement in the heavens which occurred over time
Planets vary in behaviour from the regular and predictable movements of the
Sun, Moon and the stars
They vary in brightness, and
The dont maintain a fixed position in the sky
Planet means wanderer in Greek
Observations in the past showed that the planets:
Sped up or slowed down at times
At times, even appearing to loop back and forth relative to the stars
Periods where a planets eastward motion (normal path) stops,
relative to the stars; and the planet appears to move westward,
these backward (westward) loops are known as retrograde
The Greeks knew that periods of retrograde motion were closely correlated
with other planetary properties, such as:
Apparent brightness
Position in the sky
Inferior Planets: Those whose orbits lie between the Earth and the Sun
Superior Planets: Those Whose orbits lie outside Earths
Key observations of planetary orbits:
Inferior planet:

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Never stray too far from the Sun, as seen from Earth
Its path on the celestial sphere is close to the ecliptic, and
therefore an inferior planet makes two conjunctions (or close
approaches) with the Sun during each orbit (it doesnt actually come
close to the Sun, conjunction is simple the occasion when the planet
and the Sun are in the same direction in the sky)
At inferior conjunction, the planet is closest to Earth and moves past
the Sun from east to west the retrograde sense
At superior conjunction, the planet is farthest from Earth and passes
the Sun in the opposite (prograde) direction
Superior Planets:
Are not tied to the Sun, unlike inferior planets
Make one prograde (west to east) conjunction with the Sun during
each trip around the celestial sphere
Exhibit retrograde motion when they are at opposition, absolutely
opposite the Sun on the celestial sphere
Brightest at opposition, during retrograde motion
The closer a planet is to the Earth, the brighter it appears, as it doesnt create
its own light but instead reflects it from the Sun
Astronomers then and now were faced with creating a model of the solar
system that worked with these observations
A Theoretical Model (pg.36)
Geocentric: Earth-centered universe
This simplest possible description uniform motion around a circle with Earth
at its center was a good approximation to the orbits of the Sun and Moon,
but failed to account for the observed variations in the movements of the
planets and their brightness
Geocentric Model: Each planet moved uniformly around a small circle,
called an epicycle, whose center moved in a bigger circle around the earth,
known as a deferent
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