Download Early Observers (The Beginnings of Astronomy)

Ancient cultures studied the sky and
developed calendars based upon the
patterns they saw
Motion of the sun
 Motion of the moon
 Motion of the stars
Ancient culture based their calendars on
repeating cycles they observed in the
sky.
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What is a day?
What they saw:
 The time it takes the sun to cross the
sky and return to the same point
What we know:
 Time required for the Earth to rotate
once on its axis
What is a month?
What they saw:
 The amount of time required for the
moon to move through a complete set
of phases
What we know:
 The amount of time it takes the moon
to revolve around the Earth
What is a year?
What they saw
 The time it takes for a group of stars
(constellation) to return to the same
part of the sky at a certain time of day
What we know:
 Time required for the Earth to revolve
around the sun.
Early Models of the Universe
Pythagoreans (500 B.C.)
Believed the stars, planets, sun,
and moon were attached to
crystalline spheres which
rotated around the earth.
 (geocentric)
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Aristotle (350 B.C.)
Believed the Earth is motionless at
the center of the universe and all
the stars and planets rotated
around it.
 Stars and planets moved in circular
paths.
(geocentric)
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Ptolemy (140 A.D.)
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Expanded Aristotle’s theories to try to
account for “retrograde motion” of the
planets (Ptolemaic Theory)
Planets traveled in smaller circular
paths as they traveled around the Earth
(epicycles and deferents)
Popular model of universe for 1,500 years.
Copernicus (1543)
Sun at center of universe and the
planets orbit the sun
(Heliocentric)
 Solved the problem of “retrograde
motion”
 Theory did not immediately replace
Ptolemy’s theory.
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Tycho Brahe (1600)
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Favored an Earth-centered universe
different from Ptolemy’s theory
Thought that other planets
revolved around the sun, and that
the sun and moon revolved around
the Earth.
Theory incorrect, but made many
precise observations of planets and
stars.
Johannes Kepler (1609)
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Mathematician
Tycho’s assistant
Used Tycho’s observational data to develop
laws of planetary motion
Kepler’s 3 Laws of Planetary Motion
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Law of Ellipses
Law of Equal Areas
Law of Periods
Galileo Galilei (1609)
First person to use telescope for
astronomical observations
 Discoveries
* Galilean Satellites (Jupiter’s moons)
* Craters on the moon
* Sunspots on the Sun
* Phases of Venus
Favored Copernicus’s theory over Ptolemy’s
Isaac Newton
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1687
Explained why planets orbit the Sun,
and why moons orbit planets
Newton’s Law of Motion
Newton’s Law of Gravitation
Completed the work of Copernicus,
Tycho, Kepler, and Galileo