The History of Astronomy II
September 18, 2009
Taking Care of Business (TCB)
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Read textbook Unit 12
Take Moon observations
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6 Observations due September 28
Test #1 – September 18 to September 21
Sept 16 class – last day of test material
 Reserve your test date/time ASAP
 InQsit instructions on Blackboard
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Parallax Review
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A star located 25 parsecs from the Sun has a
parallax angle of
A. 25 arcseconds
B. 50 arcseconds
C. 0.04 arcseconds
D. 0.02 arcseconds
E. None of the above
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1/25= 1/d
Review Question
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What is the distance to a star (in parsecs) that
has a parallax angle of 0.1 arcseconds?
a) 0.01
b) 0.1
c) 1
d) 10
e) 100
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1/= 1/0.1
Ptolemy
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Earth-centered system
Earth located slightly off
the center of orbiting
circles
Ptolemaic model was
inconsistent over many
years
Epicycles adds to prove
his method, instead it’s
too complicated
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Geocentric Method
Ptolemy – 100 – 170 A.D.
Ptolemy’s Geocentric Model
Ptolemy’s Epicycles
Epicycle
Deferent
Earth
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Ptolemy’s Epicycles
Where is retrograde motion
occurring?
Planet’s Path
Ptolemy’s Epicycles
Where is retrograde motion?
Planet’s Path
Astronomy Revolution
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Idea of an Earthcentered system held to
be true for close to 2000
years
Believed that there must
be a simpler explanation
of the Solar System
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Usually true when it is the
simplest
Placed the Sun at the
center
 Heliocentric system
Nicholas Copernicus – 1473 1543
Discussion
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Why was the Copernican model not widely
accepted at first?
Tycho Brahe
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Great interest in observing
the motions of the sky
Noticed that positions of
planets were not accurate
compared to available tables
Did his own detailed
observations to mark
positions
Discovered a “new star”
 Supernova
Brahe – 1546 - 1601
Kepler
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Wanted to accurately match
the observations of Mars –
off by as much as 8
arcminutes
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Developed a new model that
did not rely on circles, but
instead used ellipses
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Kepler’s 3 Laws of Motion
Johannes Kepler – 1571 - 1630
Kepler’s First Law
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The orbit of each planet about the Sun is an
ellipse with the Sun at one focus
Kepler’s Second Law
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As a planet moves
around its orbit, it
sweeps out equal areas in
equal times
To do this, a planet must
move slower at aphelion
(point furthest from sun)
than it does at perihelion
(point closest to the sun:
fastest)
Post Tutorial Question
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During which part
of the planet’s orbit
(A, B, C, or D)
would the planet
move with the
greatest speed?
Post Tutorial Question
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During how many portions of
the planet’s orbit (A, B, C and
D) would the planet be
speeding up the entire time?
a. Only during one of the
portions shown.
b. During two of the portions
shown.
c. During three of the portions
shown.
d. During four of the portions
shown.
e. None of the above.
Post Tutorial Question
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According to Kepler’s
Second Law, during
which one of the portion
of the planets orbit “B”,
“C”, or “D”, would the
planet take the same
amount of time as it
took for the portion of
the orbit identified with
letter “A”? If you think
all the portions of the
orbit take the same
amount of time, answer
“E”.