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For Wednesday, Feb. 4Reading: Section 2.6
Assignments: Mini-Project #1 (due today)
Homework #1 (due today)
Mini-Project #2 (due Wed. Feb. 11)
QUIZ #1 FRIDAY FEB. 6
PLANETARIUM SHOWS:Tues. Feb. 3: NOON, 1 PMWed. Feb. 4: NOON, 1 PM, 3 PMThur. Feb. 5: NOONROOFTOP TELESCOPE VIEWING (Tues. 7 pm; PA Building)
Mon. Feb. 9: 3 PMTues. Feb. 10: 1 PM, 3 PMWed. Feb. 11: 1 PM
The Moon and Orbits
What can we learn about how orbits work by carefully observing Moon’s behavior?
• direction of Moon’s motion• time to orbit (“period”)• shape of orbit (“eccentricity”)• tilt of orbit (“inclination”)
VIEW FROM ABOVE NORTH POLE
MOON
EARTH
SUNLIGHT
Reflecting LightSearches for planets
around other stars:Moon phases:
(IMAGINARY VIEW FROM EARTH)
•How bright is the Moon going to be?
•Is the Moon going to interfere with observations?
•When is it best to try to detect an extrasolar planet?
Thought Question:What would the Moon’s
phase look like from the northern hemisphere of Earth (the part facing you in the figure)?
YOUR VIEW:A B C D
Moon Phases• As Moon orbits Earth,
we see different amounts of lit and dark Moon…
STEPS:1. What direction is sunlight
coming from?
2. What side of Moon is lit?
3. What part of lit side is visible to you?
VIEW FROM ABOVE NORTH POLE
MOON
EARTH
SUNLIGHT
The PhasesThe Phases
about 1 week for each quarter (like newfirst quarter)
Moon PhasesVIEW FROM ABOVE NORTH POLE
LESS THAN HALF FULL
(CRESCENT)
EARTH
MORE THAN HALF FULL (GIBBOUS)
GETTING MORE FULL (WAXING)
EARTH
GETTING LESS FULL (WANING)
Thought Question:The diagram below shows Earth and the Sun as well as 5
different possible positions of the Moon. Which of the positions best corresponds to the phase of the Moon in shown in the box?
A.
B.
C.
D.
E.
Thought Question:If the Moon moves around its orbit as shown
below, what will happen?
A. It will rise earlier night after night.
B. It will rise later night after night.
C. It will rise at the same time night after night.
Directions in the Solar System
All of these…• Earth rotation (and most other planet
spins)• Moon’s orbit of Earth (and most other
moon orbits)• all planet orbits around Sun
have counterclockwise direction as seen from above Earth’s N pole
…came from spin of gas cloud that formed the solar system!
…CONSERVATION OF ANGULAR MOMENTUM
SUN
MOON
Sidereal and Synodic Months
SUN
• synodic month: time for Moon to make one circle of sky compared to Sun
• sidereal month: time for Moon to make one circle of sky compared to stars
SUN
Which type of month corresponds to exactly one Moon orbit?
Which type of month corresponds to one cycle of Moon phases?
27.3 d 29.5 d
• Angular speed of Moon to east (relative to stars):
• Angular speed of Moon to east (relative to the Sun): • Moon rises later by
Rising Later…
Moon moves from W to E around celestial sphere
Moon Rise and Set
Imagine your head in Earth’s place:
• Is your head turned in the correct direction to see it?
VIEW FROM ABOVE NORTH POLE
MOON
SUNLIGHT
EARTH
Time of DayVIEW ABOVE N. POLE
NOON (Sun high
overhead)
SUNSET (must
look west to see Sun)
SUNRISE (must look east to see
Sun)
MIDNIGHT (Sun on opposite
side of Earth)
SUNLIGHT
ROTATION DIRECTION
NP
East West
E W
EW
EW
EW
Thought Question:If the Moon is positioned in its orbit as shown below,
at what time would it be highest overhead?
A. 9 am
B. 3 pm
C. 9 pm
D. 3 am
E. None of the above
Moonrise
• MOONRISE:
about 9 am (in this example)
about 6 hours before meridian
VIEW FROM ABOVE NORTH POLE
SUNLIGHT
MOON
HORIZON
E
WE WS
VIEW FROM EARTH
Moonset
• MOONSET:
about 9 pm (in this example)
about 6 hours after meridian
VIEW FROM ABOVE NORTH POLE
MOON
SUNLIGHTHORIZON
E
W
E WS
VIEW FROM EARTH
For Friday, Feb. 6
QUIZ #1
Assignments: Mini-Project #2 (due Wed. Feb. 11)
PLANETARIUM SHOWS:Wed. Feb. 4: NOON, 1 PM, 3 PMThur. Feb. 5: NOON
Mon. Feb. 9: 3 PMTues. Feb. 10: 1 PM, 3 PMWed. Feb. 11: 1 PM
Solar EclipsesSolar Eclipses
The Moon and Orbits
What can we learn about how orbits work by carefully observing Moon’s behavior?
• direction of Moon’s motion• time to orbit (“period”)• shape of orbit (“eccentricity”)• tilt of orbit (“inclination”)
VIEW FROM ABOVE NORTH POLE
MOON
EARTH
SUNLIGHT
Thought Question:A total lunar eclipse just occurred. How long will it be before
another lunar eclipse can occur?
A. 1/2 month
B. 1 month
C. 3 months
D. 6 months
E. 12 months
Orbits and Inclination
Sun
Orbits are flat (they can fit in a flat plane)
BUT
they are usually tilted relative to each other…
inclination (i): angle between Earth’s and object’s orbit planes
SIDE VIEW:
Earth orbit
Inclination i
Moon orbit
planet orbits are only inclined by a few degrees:
Tilt of Moon’s Orbit
For an eclipse: Moon must be new (solar eclipse) or full (lunar eclipse), AND
Moon must be crossing the ecliptic (near a “node”)
• Moon’s orbit is tilted 5º from ecliptic
Thought Question:How far can the Moon’s shadow miss Earth’s center?
(Earth’s radius is 6380 km; Moon averages 384,400 km from Earth)
Enter the number of Earth radii (rounded to the nearest whole number).
to Sun
Moon’s umbra
plane of Moon orbit
Earth SIDE VIEW
?5º
VIEWS FROM SUN:
3 Months later: eclipses possible
Moon closest to Sun, but below Earth level
Moon farthest from Sun, but above Earth level
Annular Eclipses
• Moon’s distance from Earth changes…
5 pictures superimposed
Angular Size
angle A
distance d size D
If A is small,
From Earth, Moon and Sun have nearly same angular size:
the fraction of a circle the object covers
Thought QuestionEarth’s Moon can almost perfectly eclipse the Sun. Can
Jupiter’s moon Ganymede totally eclipse the Sun as seen from Jupiter’s cloud tops? (Jupiter is about 5 times farther from the Sun than Earth is, Ganymede is about 50% larger than our Moon, and Ganymede’s orbit is about 3 times larger than Moon’s orbit.)
A. Yes, total eclipses would happen even if Ganymede was the size of the Moon.
B. Yes, total eclipses happen, but only because Ganymede is larger than the Moon.
C. No, total eclipses do not happen
perigee
(closest to Earth)
apogee
(farthest from Earth)
Not a perfect circle!
”eccentricity”
Top View Top View of Moon’s of Moon’s
OrbitOrbit
Top View Top View of Moon’s of Moon’s
OrbitOrbit
perigee
(closest to Earth)
apogee
(farthest from Earth)
Thought Question
A) B) C) D)
Draw the sun as a white circle, and the moon as a dark circle
A
B
CD
What would you see looking back toward the Sun?
Thought Question:
A. A more-than-total eclipse
B. An exactly total eclipse
C. A less-than-total eclipse (annular)
D. A partial eclipse.
C
What would things look like from “C”?
ShadowsSIDE VIEWS:
total shadow: UMBRA
partial shadow: PENUMBRA
lower edge of Sun blocked from view
upper edge of Sun blocked from view
Thought Question
A) B) C) D)
Draw the sun as a white circle, and the moon as a dark circle
A
B
CD
What would you see looking back toward the Sun?