the cycles of the moon
TRANSCRIPT
The Cycles of the Moon
Chapter 3
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.
Guidepost
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
Outline
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
Outline (continued)
The Phases of the Moon (1)• The Moon orbits Earth in a sidereal period of
27.32 days.
27.32 days
EarthMoon
Fixed direction in space
The Phases of the Moon (2)
• The Moon’s synodic
period (to reach the
same position relative
to the sun) is 29.53
days (~ 1 month).
Fixed direction in
space
Earth
Moon
Earth orbits around Sun =>
Direction toward Sun
changes!
29.53 days
The Phases of the Moon (3)
From Earth, we
see different
portions of the
Moon’s surface
lit by the sun,
causing the
phases of the
Moon.
The Phases of the Moon (4)
New Moon First Quarter Full Moon
Evening Sky
The Phases of the Moon (5)
Full Moon Third Quarter New Moon
Morning Sky
The TidesCaused by the
difference of the Moon’s
gravitational attraction
on the water on Earth
2 tidal maxima
Excess gravity pulls
water towards the
moon on the near sideForces are
balanced at the
center of the Earth
12-hour cycle
Excess centrifugal
force pushes water
away from the moon
on the far side
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
The Tidally-Locked Orbit of the Moon
The Earth also exerts tidal forces on the moon’s 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.
Acceleration of the Moon’s Orbital
Motion
Earth’s tidal bulges are
slightly tilted in the direction
of Earth’s rotation.
Gravitational force
pulls the moon slightly
forward along its orbit.
Lunar EclipsesEarth’s
shadow
consists of a
zone of partial
shadow, the
Penumbra,
and a zone of
full shadow,
the Umbra.
If the moon passes through Earth’s full shadow
(Umbra), we see a lunar eclipse.
If the entire surface of the moon enters
the Umbra, the lunar eclipse is total.
A Total Lunar Eclipse (1)
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 Earth’s
atmosphere.
Lunar Phases
(SLIDESHOW MODE ONLY)
Lunar Eclipses: 2002-2012
Typically, 1 or
2 lunar
eclipses per
year.
Small Angle Formula
(SLIDESHOW MODE ONLY)
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.
Solar Eclipses: 2002-2012
Approximately 1 total solar eclipse per year
Total Solar Eclipse
Prominences
Chromosphere and Corona
Solar Atmosphere Revealed
Diamond Ring Effect
Earth and Moon’s Orbits Are
Slightly Elliptical
Sun
Earth
Moon
(Eccentricities greatly exaggerated!)
Perihelion = position
closest to the sun
Aphelion
= position
furthest
away
from the
sun
Perigee = position
closest to Earth
Apogee =
position furthest
away from Earth
Annular Solar Eclipses
The angular
sizes of the
moon and the
sun vary,
depending on
their distance
from Earth.
When Earth is near perihelion, and
the moon is near apogee, we see
an annular solar eclipse.
Perigee Apogee Perihelion Aphelion
Annular Solar Eclipses (2)
Almost total, annular eclipse of May 30, 1984
Conditions for Eclipses (1)
A solar eclipse can only
occur if the moon passes a
node near new moon.
The moon’s orbit is inclined against the ecliptic by ~ 50.
A lunar eclipse can only
occur if the moon passes a
node near full moon.
Conditions for Eclipses (2)
Saros cycle: 18 years, 11 days, 8 hours
Eclipses occur in a cyclic pattern.
The Saros Cycle
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
prominence
diamond ring effect
annular eclipse
perigee
apogee
node
eclipse season
line of nodes
eclipse year
saros cycle
New Terms
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 moon’s orbit
were not tipped with respect to the plane of Earth’s
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?
Discussion Questions
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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