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Seeing the MoonWhy does the Moon shine? Why does its shape seem to

change? The Moon does not give off light. Unlike the Sun, the Moon is a solid object. You see the Moon because it reflects light from the Sun. Data about the Moon’s mass, size, and distance from Earth are shown in the table below.

Moon Data

Mass Diameter

Average

distance

from Earth

Time for

one

rotation

Time for

one

revolution

1.2% of

Earth’s

mass

27% of

Earth’s

diameter

384,000

km27.3 days 27.3 days

The Moon’s Formation The most widely accepted idea about how the Moon formed is

the giant impact hypothesis. The giant impact hypothesis states that shortly after Earth formed, an object about the size of the planet Mars crashed into Earth. The impact caused a ring of vaporized rock to form around Earth. Eventually, the material in the ring cooled, clumped together, and formed the Moon.

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before

column if you agree with the statement or a D if you disagree. After

you’ve read this lesson, reread the statements to see if you have

changed your mind.

Before Statement After

3. The Moon was once a planet that orbited

the Sun between Earth and Mars.

4. Earth’s shadow causes the changing

appearance of the Moon.

Key Concepts

• How does the Moon

move around Earth?

• Why does the Moon’s

appearance change?

Make an Outline As you

read, make an outline to

summarize the lesson. Use

the main headings in the

lesson as the main headings

in the outline. Complete the

outline with the information

under each heading. Use

the completed outline to

review the lesson.

Reading Check

1. Name the hypothesis

that best explains how the

Moon formed.

The Sun-Earth-Moon SystemEarth’s Moon

CHAPTER 18

LESSON 2

Earth’s Moon The Sun-Earth-Moon System 569

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The Moon’s Surface

Early in the Moon’s history, various features formed on its surface. The main features on the Moon are craters, maria (MAR ee uh), and highlands.

Craters The Moon’s craters formed when objects from space, such as large rocks, crashed into the Moon. Light-colored streaks called rays run outward from some craters.

Most of the Moon’s craters formed about 3.5 billion years ago. This was long before dinosaurs lived on Earth. Earth also was hit by many large objects during this time. Wind, liquid water, and plate tectonics erased these craters on Earth.

The Moon has no atmosphere, no liquid water, and no plate tectonics. Without these forces, Moon’s craters, formed billions of years ago, have not changed much.

Maria The large, dark, flat areas on the Moon are called maria. The maria (MAR ee uh) formed long after most impacts on the Moon’s surface had ended. Maria formed when lava flowed through the Moon’s crust and hardened. The hardened lava covered large portions of the Moon’s surface, including many of its craters and other features.

Highlands The light-colored areas on the Moon are the highlands. Highlands were not covered by the lava that formed the maria because they were too high for the lava to reach. Highlands are older than the maria and are covered with craters.

The Moon’s MotionAs Earth revolves around the Sun, the Moon revolves around

Earth. The pull of Earth’s gravity causes the Moon to move in an orbit around Earth. Recall from Lesson 1 that if the pull of the Sun’s gravity ended, Earth would fly in a straight line into space. The same would be true for the Moon if the pull of Earth’s gravity ended. The Moon makes one revolution around Earth every 27.3 days.

Like Earth, the Moon also rotates as it revolves. One complete rotation of the Moon also takes 27.3 days. Notice that this is the same amount of time it takes the Moon to make one complete revolution around Earth.

The same side of the Moon always faces Earth because the Moon takes the same amount of time to orbit Earth and make one rotation. This side of the Moon that faces Earth is called the near side. The side of the Moon that cannot be seen from Earth is the far side of the Moon.

Reading Check

2. Explain How were

maria produced?

Key Concept

Check

3. Restate What

produces the Moon’s

revolution around Earth?

4. Compar e Earth’s

revolution and the Moon’s

revolution.

570 The Sun-Earth-Moon System Earth’s Moon


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Phases of the MoonThe Sun is always shining on half the Moon, just as it is always

shining on half of Earth. As the Moon moves around Earth, usually only one part of the Moon’s near side is lit. The portion of the Moon or a planet reflecting light as seen from Earth is called a phase.

The motion of the Moon around Earth causes the phase of the Moon to change. The phases follow a regular pattern that is called the lunar cycle. One lunar cycle takes 29.5 days, or slightly more than four weeks, to complete.

Waxing Phases

The waxing phases occur during the first half of the lunar cycle. During the waxing phases, more of the Moon’s near side is lit each night.

Week 1—First Quarter The lunar cycle begins. A sliver of light appears on the Moon’s western edge. Each night, the lit part grows larger. By the end of the first week, the Moon reaches its first quarter phase. The entire western half of the Moon is now lit.

Week 2—Full Moon During the second week in the lunar cycle, more and more of the near side of the Moon becomes lit. By the end of the second week, the Moon’s near side is completely lit. It is at its full moon phase.

Waning Phases

After the Moon waxes, it is said to wane. During the waning

phases, less of the Moon’s near side is lit each night. As seen from Earth, the lit part is now on the Moon’s eastern side.

Week 3—Third Quarter During the third week in the lunar cycle, the lit part of the Moon becomes smaller. By the end of the third week, only the eastern half of the Moon is lit. This is the third quarter phase.

Week 4—New Moon During the fourth week in the lunar cycle, less and less of the near side of the Moon is lit. When the Moon’s near side is completely dark, it has reached the new moon phase. The entire lunar cycle is summarized in the figure on the next page.

Reading Check

6. Name What are the

waning phases of the

Moon?

Key Concept

Check

5. Identify What

produces the phases of the

Moon?

Make a bound book to

organize information about

the lunar cycle. Each page

should represent one week.



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Visual Check

7. Circle the name of the

moon phase that would be

seen at the end of Week 2

of the lunar cycle.

572 The Sun-Earth-Moon System Earth’s Moon


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Mini Glossary

maria (MAR ee uh): the large, dark, flat areas on the Moon

phase: the lit part of the Moon or a planet that can be seen from Earth

waning phase: the part of the lunar cycle in which less of the Moon’s near side is lit each night

waxing phase: the part of the lunar cycle in which more of the Moon’s near side is lit each night

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes the difference between waxing and waning.

2. Fill in the graphic organizers below to summarize the features of the Moon’s surface and the phases of the Moon.

craters

Features of the

Moon’s Surface

Moon’s Phases

Week 1

Week 2

full moon

Week 3

Week 4

3. How did making an outline help you understand what you read?

Reread the statements at the beginning of the

lesson. Fill in the After column with an A if you

agree with the statement or a D if you dis-

agree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.



574 The Sun-Earth-Moon System

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Scan Lesson 2 in your book. In your Science Journal, write three questions you have about the Moon. Try to answer your questions as you read.

Lesson 2 Earth’s Moon

Seeing the Moon

I found this on page .

Explain why you can see the Moon.

Sequence events in the Moon’s formation.

1. A collision between

occurred.

2. was ejected into space,

and a ring formed .

3. Material in the ring

and formed .

Describe three features of the Moon’s surface.

Th e Moon’s Surface

Craters Maria Highlands

Identify details about the 2 motions of the Moon.

Revolution Rotation

Period in days

Description

of motion

The Moon’s Formation



The Moon’s Motion


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The Sun-Earth-Moon System 575

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Lesson 2 | Earth’s Moon (continued)

Explain why the same side of the Moon always faces Earth.

Organize information about the Moon’s phases.

Phases of the Moon

Definition: Caused by: Length of a

complete lunar

cycle:

Categorize information about the phases of the Moon.

Phase Name Description

Waxing

phases

Week 1:

First Quarter

Week 2:

Waning

phases

Week 3:

Week 4:


Phases of the Moon



If you could live on the Moon through one lunar cycle, how would you experience the phases of the Moon? Would you see Earth going through phases? Explain.

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