September 21, 2009

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Read textbook Unit 12
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Take Moon observations
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6 Observations due September 28
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Test #1 – September 18 to September 21
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Sept 16 class – last day of test material
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Reserve your test date/time ASAP
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InQsit instructions on Blackboard

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How were Tycho Brahe’s observations critical to the development of astronomy?
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He brought up the idea of the planets positions again, and tried to improve them
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Made a new model

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The orbit of each planet about the Sun is an ellipse with the Sun at one focus

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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 than it does at perihelion

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More distant planets orbit the Sun at slower average speeds, obeying the mathematical relationship of p 2 =a 3
(p 2 =ka 3 ) where p is the planet’s orbital period in years and a is the average distance from the Sun in
AU

 What about comets? Assume a comet has a high eccentricity of 0.95
(e) with a semi-major axis of
40 (a) AU. How long will it take this comet to orbit the Sun? Where at in its orbit will it spend most of its time? Near the Sun at perihelion? Far from the
Sun at aphelion? Or maybe somewhere in between? p 2 = ka 3
P=253 years, at the aphelion when it is furthest away
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If p is measured in years and a is measured in AU, then k = 1.

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If a small weather satellite and the large
International Space Station are orbiting Earth at the same altitude above Earth’s surface, which object takes longer to orbit once around Earth?
 a) the large space station
 b) the small weather satellite
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c) They would take the same amount of time.

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Consider a planet orbiting the Sun. If the mass of the planet doubled but the planet stayed at the same orbital distance, then the planet would take a) more than twice as long to orbit the Sun.
b) exactly twice as long to orbit the Sun.
c) the same amount of time to orbit the Sun.
d) exactly half as long to orbit the Sun.
e) less than half as long to orbit the Sun.