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Survey of
Astronomy
The Universe, and all that
Public Service
Announcement #1: Your
Rocket ID
The reason you should use your
rocket id instead of your SSN:
IDENTIFY THEFT.
It can happen. Don’t let it
happen to you.
You don’t want to deal with a
collections law firm (or four).
Public Service
Announcement #2
Register to
vote by Mon.
Oct 6th (1
1/2 weeks!)
Here on campus near
student union,
rockthevote.com, many
other places online. You
need: a stamp!
Out of class event
Reminder
Fill out your impressions of the
planetarium shows and observing on M.A.
Planetarium: dome in which you sit and
watch a show projected on the inside.
Observatory: Contains telescope which you
look through to see planets/moons/galaxies/
etc.
Planetarium shows every Fri. evening/Sat
afternoon. Observing again Oct 27th.
Exam #1
You (raw) grades are available now
on your Mastering Astronomy
Account.
Strongly encourage you to take
advantage of “buy-back” extra credit.
Grades will be adjusted in computing
course grade.
The Exam Distribution
40
30
20
10
0
0-9
10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-100
Adjusting Exam #1
Still to be determined, but roughly:
75 or above: very good performance.
50-75: average to good performance.
Below 50: needs improvement.
Suggestions: use the M.A. study guides, see
me at office hours or any other time.
Buy-Back Extra Credit
Earn up to 10% back on your Exam #1 score.
Using your (hopefully marked) test, identify
up to 5 problems you answered incorrectly.
Write a short paragraph for each explaining
the problem, what yo uchose, why you chose
the answer you did, and what the correct
answer is. See last page of your test!
Turn in by next Monday before class.
Other Extra-Credit
Tune in to PBS NOVA and watch “Monsters
of the Milky Way”.
Turn in a report (email is fine) describing
what you learned, and what surprised you.
Worth 2% extra credit on your final grade!
Last Time
History of astronomy:
Ancient peoples: structures to mark
progression of sun/moon/planets.
Greeks: Earth is round, at center of “real”
celestial spheres.
1500 years of Ptolemaic model before earth
was displaced as center of universe.
Copernicus: first to gain traction with suncentered universe.
Brahe/Kepler: measured and refined the
model of motions: not circles but ellipses!
Galileo: Cemented heliocentric model using a
telescope. Moons of jupiter, sunspots, etc.
Last Time
Retrograde motion the key: very complicated
“epicycle” models required to account.
Simple to explain when all planets orbit
the sun!
APOD: Haumea
Kepler’s Breakthrough
Kepler (1571-1630), worked for
Tycho in Prague to understand his
decades of measurements of planet
positions.
8 minutes of arc (a matchstick held
at arm’s length) error in the
models compared to observations.
“If I had believed that we could
ignore these eight minutes [of
arc], I would have patched up my
hypothesis accordingly. But, since
it was not permissible to ignore,
those eight minutes pointed the
road to a complete reformation in
astronomy.”
Kepler’s Laws of
Planetary Motion
Kepler’s First Law: The orbit of each
planet around the Sun is an ellipse, with
the Sun at one focus.
Kepler’s Laws of
Planetary Motion
Kepler’s First Law: The orbit of each
planet around the Sun is an ellipse, with
the Sun at one focus.
Kepler’s Laws of
Planetary Motion
Kepler’s Second Law: As a planet moves
around its orbit, a line from the planet to
the Sun sweeps out equal area in equal time.
Kepler’s
nd
2
Law
Equal area law implies that planets move:
– Faster when closer to sun
• Perihelion = planet closest to sun
– Slower when farther from sun
• Aphelion = planet farthest from sun
Workbook Time
Stand up, move at least 3 rows up
or down the lecture hall, and
form groups of 3 or more.
Go to “Kepler’s Second Law” on
Page 21.
Kepler’s Laws of
Planetary Motion
Kepler’s Third Law: The ratio of the cube
of the average distance of the planet
from the Sun (a=semimajor axis) to the
square of the orbital period (p) is the
same for each planet.
2
3
P =A
p in units of years
a in units of Astronomical Units
What is an
Astronomical Unit?
It is Earth’s distance from the Sun
(or more technically, it is the
Earth’s semi-major axis)
1 A.U. = 1.5 x 1011 m
499 light seconds!
Does the third law
work for the Earth?
Earth: P = 1 year, a = 1 A.U.
P2 = a3 so 12 = 13 or 1 = 1
It works!
Graphical Version of
Kepler’s Third Law
The farther you are
from the Sun, the
longer your orbital
period
p2 = a3
The farther you are
from the Sun, the
slower you orbit the
Sun
One year of time
(Earth is green)
Thought Question
An asteroid orbits the sun at an average
distance of 4 A.U. What is it’s orbital
period?
A = 4 A.U.
P2 = A3 = 43 = 64
P = 8 years!
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.
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.
A3 = P2
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
c) They would take the same amount
of time.
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
✪ c) They would take the same amount
of time.
Which of the following best
describes what would
happen if Mercury and
Jupiter were to switch
places in their orbits about
the Sun?
A) Jupiter, the larger planet, would have
a shorter orbital period than before.
B) Mercury, the smaller planet, would
have a shorter orbital period than
before.
C) Neither of the two planets would have
any change in their orbital periods.
Which of the following best
describes what would
happen if Mercury and
Jupiter were to switch
places in their orbits about
the Sun?
✪ A) Jupiter, the larger planet, would have
a shorter orbital period than before.
B) Mercury, the smaller planet, would
have a shorter orbital period than
before.
C) Neither of the two planets would have
any change in their orbital periods.
Where do Kepler’s
laws work?
Everywhere!
Why do Kepler’s Laws
work everywhere?
Gravity
... which brings us to Isaac Newton.
Thought Question
If you drop a hammer and a feather with
no air resistance:
A) the hammer will hit the ground first
B) the feather will hit the ground first
C) they will hit the ground at the same
time
Thought Question
If you drop a hammer and a feather with
no air resistance:
A) the hammer will hit the ground first
B) the feather will hit the ground first
✪ C) they will hit the ground at the same
time
Apollo 15 Experiment
For Next Time
Start Reading Chapter 4.
HW #1 will be handed out
(online) on Fri, due the
following Sunday.
Don’t forget your exam buy-back
extra credit!