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1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
The Cycles of the Moon

• Chapter 3

3
Guidepost
In the preceding chapter, we saw how the sun
dominates our sky and determines the seasons. The
moon is not as bright as the sun, but the moon
passes through dramatic phases and occasionally
participates in eclipses. The sun dominates the
daytime sky, but the moon rules the night. As we
try to understand the appearance and motions of
the moon in the sky, we discover that what we see
is a product of light and shadow. To understand
the appearance of the universe, we must
understand light. Later chapters will show that
much of astronomy hinges on the behavior of
light. In the next chapter, we will see how
Renaissance astronomers found a new way to
describe the appearance of the sky and the
motions of the sun, moon, and planets.
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Outline
I. The Changeable Moon A. The Motion of the
Moon B. The Cycle of Phases II. The Tides A.
The Cause of the Tides B. Tidal Effects III.
Lunar Eclipses A. Earth's Shadow B. Total Lunar
Eclipses C. Partial and Penumbral Lunar Eclipses
5
Outline (continued)
IV. Solar Eclipses A. The Angular Diameter of
the Sun and Moon B. The Moon's Shadow C. Total
Solar Eclipses V. Predicting Eclipses A.
Conditions for an Eclipse B. The View From
Space C. The Saros Cycle
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The Phases of the Moon (1)

• The Moon orbits Earth in a sidereal period of
  27.32 days.

27.32 days
Earth
Moon
Fixed direction in space
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The Phases of the Moon (2)
Fixed direction in space
29.53 days
Earth
Moon
Earth orbits around Sun gt Direction toward Sun
changes!

• The Moons synodic period (to reach the same
  position relative to the sun) is 29.53 days ( 1
  month).

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The Phases of the Moon (3)
From Earth, we see different portions of the
Moons surface lit by the sun, causing the phases
of the Moon.
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The Phases of the Moon (4)
Evening Sky
New Moon ? First Quarter ? Full Moon
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The Phases of the Moon (5)
Morning Sky
Full Moon ? Third Quarter ? New Moon
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The Tides
Caused by the difference of the Moons
gravitational attraction on the water on Earth
Excess gravity pulls water towards the moon on
the near side
Forces are balanced at the center of the Earth
? 2 tidal maxima
Excess centrifugal force pushes water away from
the moon on the far side
? 12-hour cycle
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Spring and Neap Tides

• The Sun is also producing tidal effects, about
  half as strong as the Moon.
• Near Full and New Moon, those two effects add up
  to cause spring tides.
• Near first and third quarter, the two effects
  work at a right angle, causing neap tides.

Spring tides
Neap tides
13
The Tidally-Locked Orbit of the Moon
The Earth also exerts tidal forces on the moons
rocky interior. ? It is rotating with the same
period around its axis as it is orbiting Earth
(tidally locked). ? We always see the same side
of the moon facing Earth.
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Acceleration of the Moons Orbital Motion
Earths tidal bulges are slightly tilted in the
direction of Earths rotation.
Gravitational force pulls the moon slightly
forward along its orbit.
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Lunar Eclipses
Earths shadow consists of a zone of partial
shadow, the Penumbra, and a zone of full shadow,
the Umbra.
If the moon passes through Earths full shadow
(Umbra), we see a lunar eclipse.
If the entire surface of the moon enters the
Umbra, the lunar eclipse is total.
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A Total Lunar Eclipse (1)
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A Total Lunar Eclipse (2)
A total lunar eclipse can last up to 1 hour and
40 min. During a total eclipse, the moon has a
faint, red glow, reflecting sun light scattered
in Earths atmosphere.
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Lunar Phases
(SLIDESHOW MODE ONLY)
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Lunar Eclipses 2002-2012
Typically, 1 or 2 lunar eclipses per year.
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Small Angle Formula
(SLIDESHOW MODE ONLY)
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Solar Eclipses
The sun appears approx. as large in the sky (same
angular diameter 0.50) as the moon. ? When the
moon passes in front of the sun, the moon can
cover the sun completely, causing a total solar
eclipse.
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Solar Eclipses 2002-2012
Approximately 1 total solar eclipse per year
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Total Solar Eclipse
Chromosphere and Corona
Prominences
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Solar Atmosphere Revealed
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Diamond Ring Effect
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Earth and Moons Orbits Are Slightly Elliptical
Apogee position furthest away from Earth
Earth
Perihelion position closest to the sun
Moon
Perigee position closest to Earth
Sun
Aphelion position furthest away from the sun
(Eccentricities greatly exaggerated!)
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Annular Solar Eclipses
When Earth is near perihelion, and the moon is
near apogee, we see an annular solar eclipse.
The angular sizes of the moon and the sun vary,
depending on their distance from Earth.
Perigee
Apogee
Aphelion
Perihelion
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Annular Solar Eclipses (2)
Almost total, annular eclipse of May 30, 1984
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Conditions for Eclipses (1)
The moons orbit is inclined against the ecliptic
by 50.
A solar eclipse can only occur if the moon passes
a node near new moon.
A lunar eclipse can only occur if the moon passes
a node near full moon.
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Conditions for Eclipses (2)
Eclipses occur in a cyclic pattern.
? Saros cycle 18 years, 11 days, 8 hours
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The Saros Cycle
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New Terms
sidereal period synodic period spring tides neap
tides umbra penumbra total eclipse (lunar or
solar) partial eclipse (lunar or solar) penumbral
eclipse small-angle formula path of
totality photosphere corona chromosphere prominenc
e diamond ring effect 
annular eclipse perigee apogee node eclipse
season line of nodes eclipse year saros cycle
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Discussion Questions
1. If the moon were closer to Earth such that it
had an orbital period of 24 hours, what would the
tides be like? 2. How would eclipses be
different if the moons orbit were not tipped
with respect to the plane of Earths orbit? 3.
Are there other planets in our solar system from
whose surface we could see a lunar eclipse? a
total solar eclipse? 4. Can you detect the
Saros cycle in Figure 3-18?
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Quiz Questions
1. Which statement below best describes the
motion of the Moon relative to the stars? a. The
Moon moves eastward relative to the stars about 1
degree per day. b. The Moon moves westward
relative to the stars about 1 degree per day. c.
The Moon moves eastward relative to the stars
about 13 degrees per day. d. The Moon moves
westward relative to the stars about 13 degrees
per day. e. The Moon does not move relative to
the stars.
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Quiz Questions
2. How does the Moon's motion relative to the
stars compare to the Sun's motion relative to the
stars? a. The Moon moves slower than the Sun and
in the same direction. b. The Moon moves faster
than the Sun and in the same direction. c. The
Moon moves slower than the Sun and in the
opposite direction. d. The Moon moves faster than
the Sun and in the opposite direction. e. The
Moon does not move relative to the stars.
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Quiz Questions
3. How does one cycle of the Moon's motion
relative to the stars compare to one cycle of the
Moon's motion relative to the Sun? a. The two
cycles take the same amount of time. b. The cycle
relative to the stars is shorter than the cycle
relative to the Sun. c. The cycle relative to the
stars is longer than the cycle relative to the
Sun. d. The two cycles vary in length, such that
at times the star cycle is shorter and at other
times the Sun cycle is shorter. e. The Moon does
not move relative to the Sun.
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Quiz Questions
4. The time for one cycle of lunar phases is a.
about one day. b. about 24.8 hours. c. about one
year. d. the same as the time for one cycle of
the Moon relative to the stars. e. the same as
the time for one cycle of the Moon relative to
the Sun.
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Quiz Questions
5. In one cycle of lunar phases each crescent and
gibbous phase has a duration of about one a.
hour. b. day. c. week. d. month. e. year.
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Quiz Questions
6. If the moon reaches Full phase on May 6, it is
at _____ phase on May 17. a. New b. Waxing
Crescent c. Waxing Gibbous d. Waning Gibbous e.
Waning Crescent
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Quiz Questions
7. During Third Quarter phase, the Moon will rise
at about a. Sunrise b. Noon c. Sunset d.
Midnight e. There is no way to predict the time
that a particular phase rises.
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Quiz Questions
8. Which statement below is the best simple
description of the tidal force? a. It is an
attractive gravitational force due to the
presence of very high-mass celestial bodies. b.
It is a centripetal (center attracting) force due
to rapid rotation. c. It is a centrifugal (center
fleeing) force due to rapid rotation. d. It is a
differential gravitational force that acts on the
different parts of a body. e. It is a centrifugal
force due to orbital motion.
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Quiz Questions
9. The ocean tides on Earth are primarily due to
the influence of what celestial body? a. The
Sun. b. The Moon. c. Venus. d. Mars. e. Jupiter.
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Quiz Questions
10. How many tidal bulges on planet Earth are due
to the Moon's gravitational influence? a.
Zero. b. One. c. Two. d. Three. e. Four.
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Quiz Questions
11. The most extreme tides (Spring Tides) on
planet Earth occur during which phase of the
Moon? a. New b. First Quarter c. Full d. Third
Quarter e. Both a and c above. f. Both b and d
above.
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Quiz Questions
12. What changes occur in the Earth-Moon system
due to tidal forces? a. Earth's rate of rotation
is slowing. b. The same side of the Moon now
always faces Earth. c. The Earth-Moon distance is
increasing. d. Both a and c above. e. All of the
above.
46
Quiz Questions
13. The least extreme tides (Neap Tides) on
planet Earth occur during which moon phase? a.
New b. First Quarter c. Full d. Third Quarter e.
Choices a and c above. f. Choices b and d above.
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Quiz Questions
14. Which of the following is true about the
umbra of the Moon's shadow? a. It is very dark,
or a total shadow. b. It is semi-dark, or a
partial shadow. c. Its "cone" points away from
the Sun. d. Its "cone" points toward the Sun. e.
Choices a and c above. f. Choices b and d above.
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Quiz Questions
15. During a total lunar eclipse the Moon is a.
completely dark. b. a reddish color. c. in
Earth's umbra. d. in Earth's penumbra. e. Both b
and c above. f. Both a and c above.
49
Quiz Questions
16. Which type of eclipse may be visible to all
the people on one side of Earth? a. An annular
eclipse. b. A total solar eclipse. c. A total
lunar eclipse. d. None of the above. e. All of
the above.
50
Quiz Questions
17. What part of the Sun is visible during the
"peak" of a total solar eclipse? a. The
photosphere. b. The chromosphere. c. The
corona. d. Both b and c above. e. All of the
above.
51
Quiz Questions
18. If the Moon is at one of its nodes it must
be a. at New phase. b. at First Quarter
phase. c. at Full phase. d. at Third Quarter
phase. e. on the Ecliptic.
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Quiz Questions
19. A total lunar eclipse occurs when the Moon
is a. at New phase. b. near the ecliptic. c. at
Full phase. d. Both a and b above. e. Both b and
c above.
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Quiz Questions
20. A total solar eclipse occurs when the Moon
is a. at New phase. b. near the ecliptic. c. at
Full phase. d. Both a and b above. e. Both b and
c above.
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Answers
1. c 2. b 3. b 4. e 5. c 6. e 7. d 8. d 9. b 10. c
11. e 12. e 13. f 14. e 15. e 16. c 17. d 18. e 19
. e 20. d