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Astronomy 1010
Planetary Astronomy
Fall_2015
Day-13
Course Announcements
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Dark Sky nights – Mon. 10/5 & Wed. 10/7 starting at
7:30pm – at the Observatory.
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Total lunar eclipse: Sun.-Mon. 9/27-28
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Weather dependent, if clear, we’ll be setup at the
observatory about sunset. If you come out after dark, turn
your headlights OFF before you come around the last bend.
Here’s the important information
Exams will be returned on Friday. If you’re one of those
who did not take it on Monday (for some valid reason),
you have until the START of class Friday to complete it.
L-T workbooks will be picked up Friday. Make SURE
your name is in the book.
 From our
perspective on
Earth, it appears that
everything in the sky
moves and orbits us.
 Early astronomers
and philosophers
therefore crafted
mostly geocentric
models of the
universe to reflect
this.
 These models became greatly fixed in the
minds of astronomers for millennia.
 Politics and science can clash when cultural
mindsets refuse to be changed.
 Another point unwilling to be conceded was
the idea of “uniform circular motion.”
• Objects moved in perfect circles at uniform
speeds.
 As astronomers viewed the motions of the
planets, the models did not match the
observations.
 Complicated models were needed to explain
phenomena such as retrograde motion.
 Ptolemy developed a system with epicycles
in 150 CE that remained accepted for about
1,500 years.
 Copernicus was the
first to create a
mathematical model
with the Sun at the
center.
 Heliocentric model
with circular orbits.
 Could estimate
relative distances of
the planets from the
Sun and each other.
 Copernicus’s model could explain the
behavior of objects in the Solar System.
 The ordering of the planets could explain
how they sometimes interrupt their prograde
motion with retrograde motion.
Observing Retrograde Motion
Law: pg 99
 Work with a partner!
 Read the instructions and questions carefully.
 Discuss the concepts and your answers with one
another.
 Come to a consensus answer you both agree on.
 If you get stuck or are not sure of your answer, ask
another group.
 If you get really stuck or don’t understand what the
Lecture Tutorial is asking, ask one of us for help.
 Tycho Brahe spent
decades collecting
astronomical data
after building his
own observatory.
 Created his own
geocentric model
with the other
planets orbiting the
Sun, but with the
Sun orbiting Earth.
 Using Tycho’s data,
Johannes Kepler
came up with
empirical rules to
describe planetary
orbits in a
heliocentric system.
 Empirical science
describes how
something works,
not why.
Johannes Kepler
1571 - 1630
Kepler’s First Law: The orbit of a planet about
the Sun is an ellipse with the Sun at one focus.
Eccentricity, e
•how squashed or out
of round the ellipse is
•a number ranging
from 0 for a circle to 1
for a straight line
e = 0.02
e = 0.7
e = 0.9
What is the shape of Earth’s orbit
around the Sun?
Earth, e = 0.017
Nearly circular
SECOND LAW
 A line drawn from the planet to the Sun sweeps out
equal areas in equal times
 orbital speed is not constant for an ellipse only
for a circle
 planets move faster when near the Sun
(perihelion)
 planets move slower when they are far from the
Sun (aphelion)
Kepler’s Second Law: A line joining a planet and
the Sun sweeps out equal areas in equal intervals
of time.
SECOND LAW
 The speed a planet travels during its orbit is related
to the distance from the star
 When the planet is near the sun the planet goes
faster than when the planet is farther from the sun
Planet travels slow here
Planet travels fast here
Lecture – Tutorial
nd
Kepler’s 2 Law: pg 21
 Work with a partner!
 Read the instructions and questions carefully.
 Discuss the concepts and your answers with one
another.
 Come to a consensus answer you both agree on.
 If you get stuck or are not sure of your answer, ask
another group.
 If you get really stuck or don’t understand what the
Lecture Tutorial is asking, ask one of us for help.