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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