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Edu
catio
n.
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.
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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.
Earth’s Moon The Sun-Earth-Moon System 571
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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.
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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.
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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
I found this on page .
I found this on page .
The Moon’s Motion
I found this on page .
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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:
I found this on page .
Phases of the Moon
I found this on page .
I found this on page .
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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