For Wednesday, Feb. 4
Reading: 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:
Mon. Feb. 9: 3 PM
Tues. Feb. 3: NOON, 1 PM
Tues. Feb. 10: 1 PM, 3 PM
Wed. Feb. 4: NOON, 1 PM, 3 PM
Wed. Feb. 11: 1 PM
Thur. Feb. 5: NOON
ROOFTOP TELESCOPE VIEWING
(Tues. 7 pm; PA Building)
The Moon and Orbits
VIEW FROM ABOVE NORTH
POLE
What can we learn about how
orbits work by carefully
observing Moon’s behavior?
MOON
SUNLIGHT
EARTH
•
•
•
•
direction of Moon’s motion
time to orbit (“period”)
shape of orbit (“eccentricity”)
tilt of orbit (“inclination”)
Reflecting Light
Moon phases:
Searches for planets
around other stars:
(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)?
A
YOUR VIEW:
B
C
D
Moon Phases
•
VIEW FROM ABOVE
NORTH POLE
As Moon orbits Earth,
we see different
amounts of lit and dark
Moon…
MOON
STEPS:
SUNLIGHT
1.
2.
3.
EARTH
What direction is sunlight
coming from?
What side of Moon is lit?
What part of lit side is
visible to you?
The Phases
about 1 week for each
quarter (like
newfirst quarter)
Moon Phases
VIEW FROM ABOVE NORTH POLE
MORE THAN
HALF FULL
(GIBBOUS)
LESS THAN
HALF FULL
(CRESCENT)
GETTING MORE FULL (WAXING)
EARTH
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.
E.
B.
D.
C.
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
SUN
MOON
have counterclockwise direction as
seen from above Earth’s N pole
…came from spin of gas cloud that
formed the solar system!
Sidereal and Synodic Months
• sidereal month: time for
Moon to make one circle of
sky compared to stars
SUN
• synodic month: time for
Moon to make one circle
of sky compared to Sun
SUN
27.3 d
29.5 d
Which type of month corresponds to exactly one Moon orbit?
Which type of month corresponds to one cycle of Moon phases?
Rising Later…
Moon moves from
W to E around
celestial sphere
• Angular speed of Moon to east (relative to stars):
360
» 13 / day
27.3 days
• Angular speed of Moon to east (relative to the Sun):
360
» 12 / day
29.5 days
• Moon rises later by
24 hr
60 min
´
» 49 min / day
29.5 days 1 hr
Moon Rise and Set
VIEW FROM ABOVE NORTH
POLE
Imagine your head in
Earth’s place:
MOON
SUNLIGHT
EARTH
• Is your head turned
in the correct
direction to see it?
MIDNIGHT (Sun
on opposite side
of Earth)
Time of Day
E
E
W
ROTATION
DIRECTION
E
West
NP
W
East
E
SUNRISE
(must look
east to see
Sun)
W
W
VIEW ABOVE N. POLE
SUNSET
(must look
west to see
Sun)
NOON (Sun
high
overhead)
SUNLIGHT
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
VIEW FROM ABOVE NORTH
POLE
MOON
• MOONRISE:
 about 9 am (in this
example)
 about 6 hours before
meridian
VIEW FROM EARTH
SUNLIGHT
HORIZON
E
S
W
Moonset
VIEW FROM ABOVE NORTH
POLE
MOON
• MOONSET:
 about 9 pm (in this
example)
 about 6 hours after
meridian
VIEW FROM EARTH
HORIZON
SUNLIGHT
E
S
W
For Friday, Feb. 6
QUIZ #1
Assignments: Mini-Project #2 (due Wed. Feb. 11)
PLANETARIUM SHOWS:
Wed. Feb. 4: NOON, 1 PM, 3 PM
Thur. Feb. 5: NOON
Mon. Feb. 9: 3 PM
Tues. Feb. 10: 1 PM, 3 PM
Wed. Feb. 11: 1 PM
Solar Eclipses
The Moon and Orbits
VIEW FROM ABOVE NORTH
POLE
MOON
SUNLIGHT
EARTH
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”)
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
SIDE VIEW:
Sun
Inclination i
Earth orbit
Moon orbit
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
planet orbits are
only inclined by a
few degrees:
Tilt of Moon’s Orbit
• Moon’s orbit is tilted 5º from ecliptic
For an eclipse:  Moon must be new (solar eclipse)
or full (lunar eclipse), AND
 Moon must be crossing the ecliptic (near a “node”)
Thought Question:
How far can the Moon’s shadow miss Earth’s center?
Moon’s umbra
?
5º
to Sun
Earth
(Earth’s radius is 6380 km;
SIDE VIEW
Moon averages 384,400 km from Earth)
Enter the number of Earth radii (rounded to the nearest whole number).
VIEWS FROM SUN:
Moon farthest from Sun, but above Earth level
Moon closest to Sun, but below Earth level
3 Months later: eclipses possible
Annular
Eclipses
• Moon’s distance
from Earth
changes…
5 pictures
superimposed
Angular Size
angle A
D
tan A =
d
If A is small,
A
D
=
360° 2pd
distance d
size D
the fraction of a circle
the object covers
From Earth, Moon and Sun have nearly same angular size:
0.5°
Thought Question
Earth’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.
B.
C.
Yes, total eclipses would happen even if Ganymede was
the size of the Moon.
Yes, total eclipses happen, but only because Ganymede
is larger than the Moon.
No, total eclipses do not happen
apogee
perigee
(farthest from Earth)
(closest to Earth)
Not a perfect circle!
”eccentricity”
Top View
of Moon’s
Orbit
apogee
perigee
(farthest from Earth)
(closest to Earth)
Top View
of Moon’s
Orbit
Thought Question
What would you see looking back toward the Sun?
A
D
C
B
A)
B)
C)
D)
Draw the sun as a white circle, and the moon as a dark circle
Thought Question:
What would things look like from “C”?
C
A. A more-than-total eclipse
B. An exactly total eclipse
C. A less-than-total eclipse (annular)
D. A partial eclipse.
Shadows
SIDE VIEWS:
lower edge of Sun blocked
from view
upper edge of Sun blocked
from view
total shadow: UMBRA
partial shadow: PENUMBRA
Thought Question
What would you see looking back toward the Sun?
A
D
C
B
A)
B)
C)
D)
Draw the sun as a white circle, and the moon as a dark circle