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ASTR1010 – Lecture 6 – 31 Jan ‘13
Today
•  How the Sky Works: Phases, Eclipses, Precession, Planets
Announcements:
•  Exam 1 Review sheets available up front
•  “Old-Fashioned Homework 1” emailed, due ON PAPER Tues
•  SkyGazer not needed today; laptops only for notes (in front)
•  “Moon Balls” session now & after class outside – do it!
What does retrograde motion refer to?
A.  The acceleration and deceleration of a planet on an elliptical
orbit
B.  The apparent backwards motion of a planet seen in the sky
C.  The actual backwards motion of some planets orbiting the Sun
D.  The moon’s motion relative to the stars, not the Sun
E.  A secret play the 49ers will use in the Superbowl
Reading Reminders
•  Here is a neat moonrise video.
•  http://www.theatlantic.com/video/
archive/2013/01/full-moonsilhouettes/272673/
•  -Jason
Your Questions via JiTT2
1) How does angular size tell us distance? (several people)
•  What is retrograde motion?
•  What are the heliocentric and
geocentric models of the solar
system?
•  What is stellar parallax?
•  What were Galileo’s main
discoveries?
2) How does precession of the earth's axis work, or what
does it mean? (several people)
3) Can longitude/latitude of the sky be covered a bit
more? (several people)
4) More about equinoxes/solstices, celestial equator
(perhaps with demo)? (several)
5) More about the moon phases, and what must be known
for the class regarding phases? (several)
6) What is the celestial sphere, and how does it affect our
view of the night sky?
7) Can retrograde motion be described a bit more?
Outline for Next Three Lectures
•  Stars – recap
•  Seasons
•  Lunar Phases
•  Eclipses
•  Precession
•  Planetary motion
•  The main point is linking observations to
mental models … don’t just memorize
•  We’ve seen the value of a good mental
model… what if there are TWO????
Seasons
Motions of the Sun
•  What causes the seasons?
•  How is the Earth oriented relative to the Sun
for different seasons?
•  How high does the sun get at different times of
the year?
•  Where does the sun rise & set at different
times of the year?
Excellent Student Question
•  I had a question relating to our
discussion of the seasons. I was
wondering why, if the axis is roughly
at the same angle, and both
hemispheres are exposed to the same
amount of direct sunlight, Fall is a
colder season, in general, than Spring
is in Colorado?
Around the fall equinox, where is the
Sun directly overhead at noon?
A: all latitudes
B: 23.5 degrees north
C: only on the equator
D: 23.5 degrees south
E: not at any latitude
Where will the Sun be directly overhead on 21 June?
a. 
b. 
c. 
d. 
At noon at all latitudes
At noon at 23° N
At noon on the equator
At noon at 23° S
Around the fall equinox, where is the
Sun directly overhead at noon?
A: all latitudes
B: 23.5 degrees north
C: only on the equator
D: 23.5 degrees south
E: not at any latitude
Where will the Sun be directly overhead on 21 June?
a. 
b. 
c. 
d. 
At noon at all latitudes
At noon at 23° N
At noon on the equator
At noon at 23° S
Where will the Sun be directly overhead on 21 March?
a. 
b. 
c. 
d. 
At noon at all latitudes
At noon at 23° N
At noon on the equator
At noon at 23° S
Where will the Sun be directly overhead on 21 March?
a. 
b. 
c. 
d. 
At noon at all latitudes
At noon at 23° N
At noon on the equator
At noon at 23° S
a. 
b. 
c. 
d. 
Here’s the Sun as we see in December at noon.
How will it be different at noon in June?
a. 
b. 
c. 
d. 
Here’s the Sun as we see in December at noon.
How will it be different at noon in June?
Larger because we’re closer
Higher in the sky
Larger and higher in the sky
No significant change
Larger because we’re closer
Higher in the sky
Larger and higher in the sky
No significant change
Rules for rising and setting
•  Only on the equinoxes (March 21 or
Sept. 21) does the sun sit right on
the celestial equator.
•  In December, the Sun is south of the
celestial equator, north in June
•  Rising and setting directions depend
only on and objects position north,
south or of the celestial equator
Motions of the Sun – wrap-up
•  The date (season) determines whether the sun
rises NE, E or SE
•  The location determines the path of rising/setting,
which affects how high an object will appear
What if…
•  Imagine Earth was shaped like a soup
can, surrounded by a tube of stars
far overhead.
–  Will stars change as you travel?
–  How can you prove Earth isn’t like this?
At equator!
At 23.5 deg N!
Imagine “Irth” was shaped like a cylinder,
surrounded by a tube of stars far overhead.
•  How can you prove Earth isn’t like this?
Lunar Phases
–  The failing answer: Earth is a sphere, not a
cylinder
–  The “C” answer: Stars above “Irth” would move
differently from ours
–  The “B” answer: Circumpolar stars would be
different on Irth
–  The “A” answer: Irth would have no circumpolar
stars at any location
•  Expect an exam question where a different model leads
to different observations (or vice-versa). Full-credit
answers depend on specific observations
•  What causes phases of the Moon?
•  How are the Moon, Earth and Sun positioned for
different phases?
•  How long does the cycle of phases take?
•  When do the different phases of the moon rise,
set and appear near overhead?
How well do you understand lunar
phases?
Why does the Moon undergo phases
over a month?
•  As in this case, there can be multiple correct approaches
A. 
B. 
C. 
D. 
Pretty well
OK, I guess
Pretty shaky
Hardly at all
A: Earth’s orbital motion around the Sun
B: Earth’s rotation
C: The moon’s orbital motion around the Earth
D: More than one of the above is very important
E: Trick question: moon goes through its cycle of
phases each day
Hint: Which motion(s) would you need to turn off to
stop the cycle of phases?
Why does the Moon undergo phases
every month?
A: Earth’s orbital motion around the Sun
B: Earth’s rotation
C: The moon’s orbital motion around the Earth
D: More than one of the above is very
important
E: Trick question: moon goes through its cycle
of phases each day
What time does the full moon set?
A: Noon
B: sunset
C: midnight
D: dawn
E: any of the above - depends on time of year
What time does the full moon set?
A: Noon
B: sunset
C: midnight
D: dawn
E: any of the above - depends on time of year
Lunar Phases
The Essential Diagram
Great Practice: Choose two of three and find the
third!
•  Phase: full, new, first/last quarter, crescent
•  Time of Day: noon, midnight, sunrise, sunset
•  Observation: rise, set, overhead, not visible
Lunar Phases
The Essential Diagram
•  Draw the Earth, Sun & Moon as seen from above
the north pole
•  Add orbital & rotational motions
•  Shade in night side
•  Place moon in position for first quarter, shade
night side
•  What time of day does this phase rise?
•  Draw person watching moonrise
Putting it All Together
I need some volunteers!
•  We live on a rotating sphere…
… which orbits the Sun
… with its axis slightly tilted (at Polaris)
•  The stars lie at great distance
•  The moon orbits the Earth
…keeping the same face to the Earth
Eclipses
•  Why do they happen? What phases of the
moon make eclipses possible
•  Do eclipses happen every month? If not,
why not?
•  How would eclipses be different if the
objects were different sizes or
distances?
Do solar eclipses occur every month?"
A.  Yes, but some are partial and we don’t
even notice them any more
B.  Yes, but at different locations on Earth
so they are pretty rare here
C.  No, the new moon doesn’t always line up
with the Sun
D.  No, sometimes the moon is so far away
it doesn’t block the Sun
Do solar eclipses occur every month?"
A.  Yes, but some are partial and we don’t
even notice them any more
B.  Yes, but at different locations on Earth
so they are pretty rare here
C.  No, the new moon doesn’t always line up
with the Sun
D.  No, sometimes the moon is so far away
it doesn’t block the Sun
About how far apart should the globe and ball be
placed to represent the Earth-Moon system?
A) 
B) 
C) 
D) 
E) 
About how far apart should the globe and ball be
placed to represent the Earth-Moon system?
A) 
B) 
C) 
D) 
E) 
10 centimeters (or less)
1 meter
10 meters
100 meters
1 km (or more)
Good logic? Or not?
10 centimeters (or less)
1 meter
10 meters
100 meters
1 km (or more)
Precession:
Earth’s Tilt - with a twist
•  How does it affect Earth’s motions?
•  How does it affect the sky, seasons, etc.?
Gyroscope Demo
•  Spinning things obey rules not seen in
everyday life
•  Earth represented by spinning gyroscope
–  What happens as I move & rotate?
•  Moving the weight represents the extra
gravitational ‘tugs’ by the Sun & Moon on
our equatorial bulge
–  What happens as these tugs become
important?
Precession
•  Precession changes the direction of Earth’s tilt
over 26,000 years
•  Precession does NOT change the amount of the
tilt - always 23.5 degrees
Discussion questions:
•  So what? How does precession affect our sky?
•  How does precession affect the seasons????
•  What if precession took 1 year, keeping pole
towards Sun???
How does precession affect Earth over
its 26,000 year cycle?
A: it dramatically changes the intensity of seasons
B: it changes which star (if any) is the North Star
C: it changes which way the sun rises on warm
summer days (southeast vs. northeast)
D: more than one of the above
E: none of the above