chapter 29 The Earth-Moon-Sun System section 3 Earth’s Moon

chapter The Earth-Moon-Sun System

29

section ●3 Earth’s Moon

Before You Read What You’ll Learn

The appearance of the Moon in the sky changes during ■ how the Moon causes
the month. Describe changes in the Moon’s shape that you Earth’s tides
have seen.
■ what causes the phases
of the Moon

■ about solar and lunar
eclipses

Read to Learn Study Coach

Movement of the Moon Two-Column Notes

You have seen the Moon move across the sky at night from Create two columns in which to
east to west. This apparent movement, just like the Sun’s, is organize notes from this section.
caused by Earth’s rotation. If you look at the Moon at the In one column, write the main
same time for several nights, you will also see that it appears ideas. Write details about the
farther east each night. main ideas in the other column.

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. How is the Moon’s rotation remarkable? ●C Gather Information

The apparent eastward movement of the Moon is real and Make the following Foldable to
is caused by the Moon’s revolution in its orbit. It takes the help you arrange what you learn
Moon 27.3 days to revolve once around Earth. This period of about the Moon and how it
time is called a sidereal month. Since Earth also revolves affects Earth.
around the Sun, it takes more than two more days for the
Moon to line up with Earth and the Sun again. This means Earth's Cause Effect
that one complete lunar cycle takes 29.5 days. This period of Moon
time is known as a synodic month.
Tides
The Moon always keeps the same side facing Earth, but the
Moon does rotate. It takes the Moon 27.3 days to rotate on its Phases
axis. This is the same amount of time it takes the Moon to
revolve once around Earth. If the Moon did not rotate at all, Eclipse
over the course of a month we would be able to see all its
sides. Because the Moon’s rotation equals its revolution, we Craters
see only one face of the Moon.

Reading Essentials 537

1. Explain What causes How does the Moon affect Earth? Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

tides? The Moon affects Earth in some obvious and some not-so-
obvious ways. You might have noticed that the level of the
Picture This ocean water reaches different heights at different times at a
particular beach. You have likely noticed that the Moon looks
2. Identify Mark the diagram different in the sky during different parts of a month.

showing spring tide with What are tides?
an X. Circle the diagram
showing neap tide. A tide on Earth is the rise and fall of the level of the sea.
Earth’s tides are caused by a huge wave produced by the
gravitational pull of the Moon and the Sun. The giant wave has
a height of only 1 to 2 m but a wavelength of thousands of
kilometers. As the crest of the wave nears shore, the level of the
water in the ocean rises. This sea-level rise is called high tide.
About 6 h later, as the wave’s trough approaches and the water
recedes from the shore, there is low tide.

Both Earth and the Moon revolve around a center of mass
that is about 1,700 km below Earth’s surface. Because Earth is
far more massive, the Moon seems to be revolving around
Earth. This center of mass inside Earth, in turn, revolves
around the Sun. That is why Earth, the Moon, and the Sun
are considered a three-body system.

As Earth rotates and the Moon revolves, different areas of
Earth pass through high and low tides. Because the Moon is
so close to Earth, it has a greater effect on tides than the Sun
does. The more massive Sun affects tides too, but to a smaller
degree because it is farther away than the Moon. However, the
Sun does strengthen or weaken the tides. When the Moon and
the Sun are lined up and pull together, high tides are much
higher and low tides are much lower than normal. These
periods are known as spring tides. When the Sun and Moon
are at right angles to Earth, the high tide is relatively low and
low tide is relatively high. This is called neap tide. These
conditions are shown in the figure below.

A B Moon

Moon

Earth

Sun Earth Sun

Tidal Tidal
bulge large bulge large

538 CHAPTER 29 The Earth-Moon-Sun System

Moonlight

Moonlight is the most obvious way the Moon affects Earth.
The Moon shines because it reflects sunlight from its surface.
As the Moon revolves around Earth, different parts of the side
facing Earth are lighted, causing the Moon’s appearance to
change. Moon phases are the changing appearance of the
Moon as seen from Earth, as shown in the figure below. The
phase of the Moon you see depends on the relative positions
of the Moon, the Sun, Earth, and you.

1st qtr. Picture This

Waxing gibbous Waxing crescent Sunlight 3. Explain What is the

position of the Moon in
relation to the Sun and
Earth during a full moon?

Earth
Full New

Waning gibbous Waning crescent

3rd qtr.

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. What are the phases of the Moon? 4. List List the phases of the

A new moon occurs when the Moon is between Earth and Moon in order starting with
the Sun. During a new moon, the side of the Moon facing the new moon.
Earth is in shadow, and the Moon cannot be seen from Earth.
Reading Essentials 539
Waxing Phases Shortly after the new moon, more of the
side of the Moon facing Earth is lighted by the Sun. As the
Moon becomes visible and its lighted side becomes larger
each night, the Moon is said to be waxing. The Moon is first
seen as a waxing crescent. About a week later, one-half of the
side of the Moon facing Earth is lighted by the Sun, and we
see one-quarter of the Moon. This is the first quarter phase of
the Moon.

Moon phases continue to grow, or wax. When more than
one-quarter of the Moon facing Earth is lighted, it is called a
waxing gibbous moon. A full moon occurs when the entire
side of the Moon facing Earth is lighted. At this time, the
Moon is on the opposite side of Earth from the Sun.

Waning Phases After the full moon, the lighted part of the
Moon we see begins to get smaller, or to wane. The waning
phases after the full moon are the waning gibbous moon, the
third quarter moon, and finally, the waning crescent moon.
Then a new moon follows. The complete cycle of the Moon’s
phases takes about 29.5 days.

5. Explain What are the Eclipses

umbra and penumbra? Eclipses occur when Earth temporarily blocks sunlight
from reaching the Moon or when the Moon temporarily
blocks sunlight from reaching Earth. The Sun, the Moon, and
Earth must be lined up perfectly for an eclipse to occur.

What is a solar eclipse?

A solar eclipse occurs when the Moon moves directly
between the Sun and Earth and casts a shadow on part of
Earth. The darkest part of the Moon’s shadow is called the
umbra. As shown in the figure below, the part of Earth that
is in the Moon’s umbra undergoes a total solar eclipse. The
lighter penumbra shadow surrounds the darker umbra. The
part of Earth that is in the penumbra undergoes a partial
solar eclipse. The only part of the Sun visible during a total
eclipse is part of its atmosphere, which appears as a pearly
white glow around the edge of the eclipsing Moon.

Area of total eclipse
Umbra

Picture This Penumbra Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Area of partial eclipse
6. Compare and Contrast
What is a lunar eclipse?
Where will a partial and a
total solar eclipse occur? A lunar eclipse occurs when Earth’s shadow falls on the
Moon. A lunar eclipse begins when the Moon moves into
Earth’s penumbra, as shown in the figure on the next page.
Earth’s penumbra is the area where Earth’s shadow is lighter.
As the Moon continues to move, it enters Earth’s umbra and a
curved shadow forms on the Moon.

When the Moon moves completely into Earth’s umbra, a
total lunar eclipse occurs. The Moon sometimes becomes red
during an eclipse because light from the Sun is bent by Earth’s
atmosphere. Red light’s longer wavelength more easily passes
through Earth’s atmosphere and falls on the Moon. A partial
lunar eclipse occurs when a part of the Moon is in Earth’s
penumbra or is in direct sunlight.

540 CHAPTER 29 The Earth-Moon-Sun System

Seeing an Eclipse It is more likely that you will have a 7. Infer During what phase
chance to see a lunar eclipse than a solar eclipse. Lunar
eclipses are easier to view because they occur more often of the Moon are you most
and anyone on the night side of Earth can see them. likely to see the Moon’s
features in the greatest
detail?

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. The Moon’s Surface 8. Describe What are the

Many of the Moon surface’s larger features are visible from maria on the Moon’s
Earth. Craters and other surface features of the Moon are surface?
visible through a small telescope or pair of binoculars.
Reading Essentials 541
What are craters, maria, and mountains?

The craters, or deep depressions, on the Moon resulted
from the impacts of asteroids, meteorites, and comets that
struck the surface. Craters formed early in the Moon’s history.
Ray-like patterns of light-colored material surround many
craters. This material was blasted from just under the Moon’s
surface during impact and settled on the dark material
surrounding the craters. Some craters might have been so
large and deep that they cracked the Moon’s surface. The
cracks allowed lava to flow from the Moon’s interior to the
surface and fill huge basins, forming large, flat regions.

Maria is the name for these dark-colored, relatively flat
regions on the Moon’s surface. The maria contain igneous
rocks, 3 to 4 billion years old, that are the youngest rocks
found on the Moon. This shows that the Moon’s craters
formed after the Moon’s surface cooled. However, the maria
formed early in the Moon’s history, when the Moon still had
molten material inside. The largest mountain ranges on the
Moon surround huge areas of flat, dark maria.

What is regolith?

When NASA first sent spacecraft to study the Moon’s surface,
scientists were concerned about the depth of the debris from
meteorites. Regolith is the layers of debris from meteorites on
the Moon’s surface. On some parts of the Moon, the regolith
is almost 40 m thick, while in other regions it is only a few
centimeters thick. So far, all astronauts to visit the Moon have
landed safely on firm ground. Astronauts who walked on the
Moon even kicked up moon dust, which formed from the
countless meteors that hit the Moon.

Picture This

9. Outline Circle the craters

shown in the figure, and
draw an outline around
the regolith regions.

10. Explain What evidence The Moon’s Interior Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

do scientists have that the The presence of maria on the Moon’s surface tells us
Moon once had a molten something about the Moon’s interior. Suppose cracks did
interior? form when the large depressions were produced by comet
impacts, and lava did flow onto the lunar surface. If these
things occurred, then the interior of the Moon just below the
surface must have been molten at the time of the impacts.
Scientists think this was the case. They think that before the
Moon cooled to its present state, its interior was molten and
consisted of layers.

Other information about the Moon’s interior comes from
seismometers left on the Moon by Apollo astronauts. Just as
the study of earthquakes on Earth allows scientists to map our
planet’s interior, the study of moonquakes helps scientists
study the Moon’s interior. These studies have led scientists to
develop a model of the Moon’s interior. This model shows
that the Moon’s crust is about 60 km thick on the side facing
Earth and about 150 km thick on the side facing away from
Earth. Below the crust, a solid mantle may extend to a depth
of 1,000 km. A partly molten zone of mantle extends farther
down. Below this layer scientists believe there lies an iron-
rich, solid core. The interior layers of the Moon are shown in
the figure on the next page.

542 CHAPTER 29 The Earth-Moon-Sun System

Upper mantle Picture This
Crust Lower mantle
11. Highlight Use a highlighter
Core
to indicate the layer of the
Moon’s surface that differs
the most from the other
layer in its thickness.

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Exploring the Moon 12. Predict If scientists find

More than 20 years after the Apollo program ended, the water on the Moon, what
Clementine spacecraft was sent into lunar orbit. Clementine might they look for next?
made a detailed map of the Moon’s surface. The map
included the South Pole-Aitken Basin. Reading Essentials 543

This basin is the oldest identifiable impact feature on the
Moon’s surface. It is also the largest and deepest impact basin
or depression found anywhere in the solar system. It measures
12 km in depth and 2,500 km in diameter. Much of this
depression stays in shadow throughout the Moon’s rotation.
The basin forms a cold area where ice deposits from impacting
comets may have collected.

Clementine Data from the spacecraft Clementine confirmed
that the crust on the side of the Moon facing Earth is much
thinner than the far side. Instruments on the spacecraft
showed that the crust is thinner under impact basins. The
instruments also found areas of greater mass under the
impact basins. These areas of greater mass are called mascons.

Clementine also provided information on the mineral
content of moon rocks. In fact, this part of its mission
explains the name of the spacecraft. Clementine was the
daughter of a miner in the ballad, “My Darlin’ Clementine.”

Lunar Prospector In 1998, the Lunar Prospector spacecraft
orbited the Moon and took photographs of the lunar
surface. The maps that were made from these photographs
agreed with the data from Clementine. Also, the data from
Lunar Prospector confirmed that the Moon has a small, iron-
rich core about 600 km in diameter. Lunar Prospector also
did a detailed study of the Moon’s surface. It was searching
for clues about the origin and structure of the Moon.

13. Identify What does the Origin of the Moon

giant impact theory state Before the studies made by spacecraft, there were three
that the Moon formed from? theories about the Moon’s origin. The capture theory stated
that the Moon had formed elsewhere, wandered near Earth,
and was then caught by Earth’s gravity. A second theory stated Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
that the Moon condensed from the same loose material that
formed Earth during the early formation of the solar system.
The third theory was that the Moon formed from a glob of
molten material thrown off by a young, molten Earth. One
goal of the space missions to the Moon was to resolve which
of these theories was correct.

What is the current theory of the Moon’s origin?

The data collected by spacecraft sent to the Moon have led
many scientists to form a new giant impact theory of the
Moon’s origin. This theory states that the Moon formed about
4.6 billion years ago when a Mars-sized object collided with
Earth. After the collision, the cores of the two bodies
combined and settled toward the center of the larger object,
Earth. The collision also caused a huge amount of gas and
other debris to be thrown into orbit. Some of this material fell
back to Earth, but the rest condensed into a large mass that
formed the Moon. This sequence of events is shown in the
figure below.

The giant impact theory helps explain why the Moon and
Earth are similar, yet not similar enough to have been formed
from the same condensing mass. The giant impact theory
explains why the Moon’s composition is like Earth’s mantle. It
also explains why the Moon has a much smaller central core
than expected.

Mars-sized body

Picture This Primitive Earth B
A
14. Highlight the debris

material orbiting Earth that
condensed to become
the Moon.

C D

544 CHAPTER 29 The Earth-Moon-Sun System

After You Read regolith: the layers of debris from meteorites on the Moon’s
surface
Mini Glossary
solar eclipse: what occurs when the Moon moves directly
lunar eclipse: what occurs when Earth’s shadow falls on the between the Sun and Earth and casts a shadow on part of
Moon Earth

maria: the dark-colored, relatively flat regions on the Moon’s tide: the rise and fall of the level of the sea
surface

moon phase: the changing appearance of the Moon as seen
from Earth

1. Explain how moon phases affect tides on Earth.

2. Each diagram below shows the Moon, Earth, and the Sun. Label each diagram and
identify what is happening in each.

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

End of
Section

Reading Essentials 545