Name
CHAPTER 28
Class
Date
Minor Bodies of the Solar System
SECTION
2 Movements of the Moon
KEY IDEAS
As you read this section, keep these questions in mind:
•
•
•
•
What is the shape of the moon’s orbit around Earth?
Why do eclipses happen?
How does the moon appear in each of its phases?
How do movements of the moon affect tides on Earth?
What Is the Earth-Moon System?
To people on Earth, the moon appears to orbit Earth.
However, if you could observe Earth and the moon from
space, you would see that Earth and the moon revolve
around each other. Together they form a single system
that moves around the sun.
The mass of the moon is only about 1/80 the mass of
Earth. Thus, the balance point within the Earth-moon system is not halfway between the two bodies. The balance
point, or barycenter, is actually located within Earth. The
barycenter follows a smooth orbit around the sun. The
barycenter is not a real place. It is an imaginary point.
THE MOON’S ELLIPTICAL ORBIT
The path of the moon’s orbit is an ellipse. The ellipse
is about 5% more elongated than a circle. Therefore, the
distance between Earth and its moon varies over time.
When the moon is farthest from Earth, the moon is at
apogee. When the moon is closest to Earth, the moon is
at perigee.
Position of
moon at
apogee
To sun
Barycenter of
Earth-moon
system
Position of
moon at
perigee
Orbit of the
Earth-moon system
around sun
READING TOOLBOX
Identify Cause and Effect
As you read this section,
underline examples of cause
and effect. Discuss each
example with a partner to
make sure you understand
the cause and the effect.
READING CHECK
1. Explain Why is the
barycenter of the Earth-moon
system located inside Earth?
Talk About It
Relate Concepts Remember
that Earth is at aphelion
when it is farthest from the
sun, and it is at perihelion
when it is closest to the sun.
With a partner, talk about
how the terms aphelion,
apogee, perihelion, and
perigee are similar in
structure and in meaning.
Together, think of a way to
remember the meaning of
each term.
Because the moon’s orbit is elliptical, the moon is not always the same distance
from Earth.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
437
Minor Bodies of the Solar System
Name
SECTION 2
Class
Date
Movements of the Moon continued
APPEARANCE OF THE MOON IN THE SKY
READING CHECK
2. Explain Why can’t people
take photos of the whole
moon from Earth?
If you watched the moon every night, you would see
the moon rises and sets 50 minutes later each night. It
appears to rise and set later because of Earth’s rotation
and the moon’s revolution. While Earth completes one
rotation each day, the moon also moves in its orbit
around Earth. It takes 1/29 of Earth’s rotation, or 50
minutes, for the horizon to catch up to the moon.
While the moon revolves around Earth and the sun, it
also rotates on its axis. The moon completes a rotation
only once during one revolution around Earth. The moon
revolves around Earth once every 27.3 days. Because
the moon’s rotation and revolution take about the same
time, observers on Earth always see the same side of the
moon. People can observe the other side of the moon
only by looking at images taken by spacecraft.
The sun always lights up half the moon. As the moon
orbits Earth, the part of the moon’s surface that is lighted,
or illuminated, by sunlight changes. Sometimes the side
of the moon facing Earth is fully illuminated. At other
times, however, the side of the moon we see from Earth
is dark. The position of the moon in its orbit determines
how much of its lighted half we can see.
What Is an Eclipse?
READING CHECK
3. Define What is an
eclipse?
Bodies that orbit the sun, including Earth and the
moon, cast long shadows into space. When one body
passes into the shadow of another body, an eclipse
occurs.
Types of Eclipses
Type of Eclipse
Description
Solar
The moon’s shadow falls on Earth.
Lunar
Earth’s shadow falls on the moon.
What Happens During a Solar Eclipse?
The shadows that Earth and the moon cast have two
parts: the umbra and the penumbra. The umbra is the
inner, cone-shaped part of the shadow. In the umbra,
sunlight is completely blocked. The penumbra is the
outer part of the shadow. In the penumbra, sunlight is
partially blocked.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
438
Minor Bodies of the Solar System
Name
SECTION 2
Class
Date
Movements of the Moon continued
LOOKING CLOSER
Umbra
Penumbra
During a solar eclipse, the moon passes between Earth and the sun.
4. Interpret Diagrams Draw
arrows on the diagram to
show the directions the sun’s
rays are coming from.
When the moon is directly between the sun and
Earth, the moon’s shadow falls on Earth, causing a
solar eclipse. During a total solar eclipse, the moon
completely blocks the sun’s light from the parts of Earth
in the umbra. Observers in the penumbra see a partial
solar eclipse.
The umbra of the moon is small. Thus, a total eclipse
covers only a small part of Earth. A total eclipse is visible
in an area of only a few hundred square kilometers. A
total solar eclipse also lasts for a short time. It lasts for
no more than seven minutes at any one location.
A total eclipse will not be visible in the United States
until 2017. However, there is a total eclipse somewhere
on Earth about every 18 months.
ANNULAR ECLIPSES
If the moon is at or near apogee when it moves
between Earth and the sun, the moon’s umbra does not
reach Earth. When the umbra does not reach Earth, a
ring-shaped eclipse occurs. This type of eclipse is called
an annular eclipse. Annulus is the Latin word for “ring.”
During an annular eclipse, the moon does not block the
sun completely. Instead, a thin ring of sunlight is visible
around the edge of the moon.
What Happens During a Lunar Eclipse?
Two things have to happen for a lunar eclipse to occur:
READING CHECK
5. Compare What is one
difference between a total
solar eclipse and an annular
eclipse?
• Earth must be between the moon and the sun.
• Earth’s shadow must cross the lighted part of the moon.
A total lunar eclipse happens only when the entire moon
passes into Earth’s umbra. The figure on the next page
shows a diagram of a total lunar eclipse.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
439
Minor Bodies of the Solar System
Name
SECTION 2
Class
Date
Movements of the Moon continued
LOOKING CLOSER
Umbra
6. Infer On the diagram,
circle the part of Earth that a
total lunar eclipse would be
visible from.
Sun’s
rays
Penumbra
During a lunar eclipse, Earth’s shadow falls on the moon.
READING CHECK
7. Explain Why does the
moon often look reddish
during a total lunar eclipse?
Critical Thinking
8. Apply Concepts If the
moon’s orbit and Earth’s
orbit were in the same plane,
how many times during
each revolution of the moon
would solar eclipses happen?
When only part of the moon passes into Earth’s umbra,
a partial lunar eclipse occurs. When the entire moon
passes through Earth’s penumbra, a penumbral eclipse
occurs. A penumbral eclipse is barely noticeable.
A lunar eclipse lasts for several hours. Although the
moon is in Earth’s shadow during a lunar eclipse, the
eclipsed moon appears reddish. The red color results
because some sunlight bends around Earth through our
atmosphere. The light that reaches the moon is mainly
red light.
Why Don’t Eclipses Happen More Often?
As many as seven eclipses may happen during one
year. Four of the eclipses may be lunar eclipses, and
three may be solar eclipses, or vice versa. However, total
eclipses of the sun and moon do not happen often.
Solar and lunar eclipses do not happen during every
lunar orbit. This is because the orbits of Earth and the
moon are not in the same plane. The moon crosses the
plane of Earth’s orbit only twice during each revolution
around Earth. A solar eclipse will occur only if this
crossing happens when the moon is between Earth and
the sun. A lunar eclipse will occur only when Earth is
between the moon and the sun during that crossing.
Why Does the Moon Appear Lighted?
Although the moon seems very bright on certain
nights, the moon does not produce its own light. The
moon reflects light from the sun. Because the moon is
a sphere, sunlight always lights half of it. As the moon
revolves around Earth, we can see different amounts of
the lighted side.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
440
Minor Bodies of the Solar System
Name
Class
SECTION 2
Date
Movements of the Moon continued
What Are the Phases of the Moon?
Recall that one side of the moon, called the near side,
always faces Earth. As the moon revolves around Earth,
different amounts of the near side of the moon are lighted.
Because different parts of the near side are lighted, the
moon appears to have different shapes on different nights.
The different shapes are called phases of the moon. The
figure below shows the moon phases. It also shows the
positions of Earth and the moon during each phase.
READING CHECK
9. Explain Why does the
moon go through phases?
First quarter
Waxing gibbous
Waxing cresent
Sun’s
rays
Full moon
New moon
Waning cresent
Waning gibbous
LOOKING CLOSER
10. Describe What are the
relative positions of the sun,
Earth, and moon during a
new moon?
Third quarter
The positions of the moon, sun, and Earth determine which phase the moon is in.
The photographs show how the moon looks from Earth at each phase.
When the moon is directly between the sun and Earth,
the sun’s rays strike only the far side of the moon. As a
result, the entire near side of the moon is dark. During
the new moon, the near side is completely dark. During
this phase, the moon is between Earth and the sun.
At full moon, Earth is between the sun and the moon.
During this phase, the sun illuminates the entire near side
of the moon.
Throughout the moon’s orbit, however, different
amounts of the near side are illuminated. As the lighted
parts of the moon’s near side change, people say the
moon is waxing or waning.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
441
Minor Bodies of the Solar System
Name
SECTION 2
Class
Date
Movements of the Moon continued
WAXING PHASES OF THE MOON
READING CHECK
11. Describe How much
of the moon’s near side
is illuminated during the
waxing-crescent phase?
Talk About It
Infer Make a prediction
about what the words
waxing and waning probably
mean. Write a sentence using
each term. Your sentences
should not be about moon
phases. Share your sentences
with a partner. Together, use
the Internet or a dictionary
to figure out whether your
predicted meanings are
correct.
As the moon moves around Earth after its new moon
phase, part of the near side becomes illuminated. When
the size of the lighted part of the moon is increasing, the
moon is waxing. A waxing-crescent phase happens when
a sliver of the moon’s near side is illuminated.
When the moon has moved through one-quarter of
its orbit after the new moon phase, the moon looks
like a half circle, or semicircle. Half of the near side of
the moon is lighted. When a waxing moon becomes a
semicircle, the moon enters its first-quarter phase.
After the first-quarter moon, the lighted part of the
moon’s near side is larger than a semicircle and still
growing. This phase is called a waxing-gibbous phase.
The lighted part of the moon continues to increase until it
appears as a full circle—the full moon.
WANING PHASES OF THE MOON
After the full moon phase, the lighted part of the near
side of the moon decreases in size. The moon is waning.
The waning-gibbous phase happens when the moon is
waning, but the lighted part is still bigger than a semicircle.
When the lighted part of the near side becomes a
semicircle, the moon enters the third-quarter phase. A
waning-crescent phase happens when only a sliver of the
near side is still lighted. After the waning-crescent phase,
the moon again moves between Earth and the sun. The
moon once again appears as the new moon, and the cycle
of moon phases begins again.
During the crescent moon phases, only part of the
moon shines brightly. However, the rest of the moon’s
near side is not completely dark, as shown below.
During the crescent
moon phases, sunlight
that reflects off Earth
(earthshine) can light
the dark part of the
moon. The darker
part of the moon in
this photo is lit by
earthshine.
LOOKING CLOSER
12. Identify On the picture,
label the part of the moon
that is lit by the sun and the
part of the moon that is lit by
earthshine.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
442
Minor Bodies of the Solar System
Name
Class
SECTION 2
Date
Movements of the Moon continued
TIME FROM NEW MOON TO NEW MOON
The moon revolves around Earth in 27.3 days.
However, the moon takes longer than that to go through
a complete cycle of phases. The period from one new
moon to the next new moon is 29.5 days. The difference
of 2.2 days is a result of the orbit of the Earth-moon
system around the sun.
As the moon orbits Earth, both bodies move slightly
farther along their orbit around the sun. Therefore, the
moon must go a bit farther to be directly between Earth
and the sun. The moon takes about 2.2 days to travel this
extra distance. When the moon is directly between Earth
and the sun, the moon is in the new-moon phase.
How Do Movements of the Moon Affect
Tides?
Math Skills
13. Calculate A typical year
is 365 days long. About how
many new moons are there
in a typical year?
Recall that tides are a periodic rise and fall of the
water level in the ocean and other large water bodies.
Tidal bulges form due to the moon’s gravitational pull
on ocean water. The moon’s gravitational pull decreases
with distance. Thus, the moon pulls more strongly on the
ocean on Earth’s near side. The moon pulls less strongly
on the ocean on Earth’s far side. Because Earth rotates,
the tides in a given area change during a day.
The sun also affects tides. The effect of the sun’s
gravitational pull is smaller, however, because the sun
is farther away. Twice each month, when the sun, moon,
and Earth are almost in line, the gravitational pull
combines. The combined gravitational pull produces
especially high tides.
Tidal bulge
Force of gravity
Point B
X
X
Point A
Dire
ction
Tidal
bulge
of moo
n’s revolution
ction of E
Dire rotationarth’
s
LOOKING CLOSER
14. Identify Are point A and
point B experiencing high
tide or low tide?
The moon’s pull on Earth is greatest at point A, on Earth’s near side. The moon’s
pull on Earth is weakest at point B, on Earth’s far side.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
443
Minor Bodies of the Solar System
Name
Class
Date
Section 2 Review
SECTION VOCABULARY
apogee in the orbit of a satellite, the point that
is farthest from Earth
eclipse an event in which the shadow of one
celestial body falls on another
lunar eclipse the passing of the moon through
Earth’s shadow at full moon
perigee in the orbit of a satellite, the point that
is closest to Earth
phase the change in the illuminated areas
of one celestial body as seen from another
celestial body; phases of the moon are caused
by the changing positions of Earth, the sun,
and the moon
solar eclipse the passing of the moon between
Earth and the sun; during a solar eclipse, the
shadow of the moon falls on Earth
1. Compare Describe three differences between total lunar eclipses and total solar
eclipses.
2. Explain Why is the moon not always the same distance from Earth?
3. Apply Concepts You observe the moon one night and notice that only a sliver is
illuminated. Two nights later, a larger portion of the moon is illuminated. What
phase was the moon in on the first night?
4. Relate Ideas During which moon phase do all solar eclipses happen? Explain your
answer.
5. Explain Why is the time between two new moons longer than the time it takes the
moon to orbit Earth once?
6. Identify What is the role of the moon in forming tides?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
444
Minor Bodies of the Solar System