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?