1
What You Will Learn…
26 CHAPTER 2
SECTION
Earth and the
Sun’s Energy
You live in Chicago and have just won an exciting prize—a trip
to Australia during winter vacation in January. As you prepare for
the trip, your mother reminds you to pack shorts and a swimsuit.
You are confused. In January you usually wear winter sweaters and
a heavy jacket.
Why is the weather so different in Australia?
Building Background Seasonal differences in weather are
an important result of Earth’s constant movement. As the planet
moves, we experience changes in the amount of energy we receive
from the sun. Geographers study and explain why different places on
Earth receive differing amounts of energy from the sun.
Earth’s Movement
Energy from the sun helps crops grow, provides light, and warms
Earth. It even influences the clothes we wear, the foods we eat,
and the sports we play. All life on Earth requires
solar energy,
or energy from the sun, to survive. The amount of solar energy
places on Earth receive changes constantly. Earth’s rotation, rev-
olution, and tilt, as well as latitude, all affect the amount of solar
energy parts of the planet receive from the sun.
Rotation
Imagine that Earth has a rod running through it from the North
Pole to the South Pole. This rod represents Earth’s axis—an
imaginary line around which a planet turns. As Earth spins on
its axis, different parts of the planet face the sun. It takes Earth
24 hours, or one day, to complete this rotation.
A rotation is one
complete spin of Earth on its axis. As Earth rotates during this
24-hour period, it appears to us that the sun moves across the
sky. The sun seems to rise in the east and set in the west. The
1. Earth’s movement affects
the amount of energy we
receive from the sun.
2. Earth’s seasons are caused
by the planet’s tilt.
Main Ideas
Earth’s movement and the
sun’s energy interact to create
day and night, temperature
changes, and the seasons.
The Big Idea
Key Terms
solar energy, p. 26
rotation, p. 26
revolution, p. 27
latitude, p. 27
tropics, p. 29
If YOU lived there...
Use the graphic organizer online
to take notes on Earth’s movement
and the seasons.
TEKS 3B, 3C, 6A, 6C, 21B
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North
Pole
Sun’s Rays
South
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PLANET EARTH 27
Earth’s tilt and rotation cause
changes in the amount of energy we
receive from the sun. As Earth rotates
on its axis, energy from the sun cre-
ates periods of day and night. Earth’s
tilt causes some locations, especially
those close to the equator, to receive
more direct solar energy than others.
Solar Energy
AnAlyzing VisuAls
Is the region north or south of the
equator receiving more solar
energy? How can you tell?
AnAlysis
skill
sun, however, does not move. It is actually
Earth’s rotation that creates the sense of
the sun’s movement.
Earth’s rotation also explains why day
changes to night. As you can see in the
illustration, solar energy strikes only the
half of Earth facing the sun. Warmth and
light from the sun create daytime. At the
same time, the half of the planet facing
away from the sun experiences the cooler
temperatures and darkness of night. Earth’s
rotation causes regular shifts from day to
night. As a result, levels of solar energy on
Earth constantly change.
Revolution
As Earth spins on its axis, it also follows
a path, or orbit, around the sun. Earth’s
orbit around the sun is not a perfect circle.
Sometimes the orbit takes Earth closer to
the sun, and at other times the orbit takes
it farther away. It takes 365¼ days for Earth
to complete one
revolution , or trip around
the sun. We base our calendar year on the
time it takes Earth to complete its orbit
around the sun. To allow for the fraction
of a day, we add an extra day—February
29—to our calendar every four years.
Tilt and Latitude
Another factor affecting the amount of
solar energy we receive is the planet’s tilt.
As the illustration shows, Earth’s axis is not
straight up and down. It is actually tilted
at an angle of 23½ degrees from vertical.
At any given time of year, some locations
on Earth are tilting away from the sun, and
others are tilting toward it. Places tilting
toward the sun receive more solar energy
and experience warmer temperatures.
Those tilting away from the sun receive
less solar energy and experience cooler
temperatures.
A location’s
latitude , the distance north
or south of Earth’s equator, also affects the
amount of solar energy it receives. Low-
latitude areas, those near the equator like
Hawaii, receive direct rays from the sun all
year. These direct rays are more intense and
produce warmer temperatures. Regions
with high latitudes, like Antarctica, are
farther from the equator. As a result, they
receive indirect rays from the sun and have
colder temperatures.
ReAding CheCk
Finding Main Ideas What
factors affect the solar energy Earth receives?
ACAdeMiC
VOCABulARy
factor cause
VIDEO
Secrets of the
Sun
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North Pole
South Pole
June 21
SUMMER
March 21
SPRING
December 21
WINTER
September 22
FALL
The Seasons
Does the thought of snow in July or 100-
degree temperatures in January seem odd
to you? It might if you live in the Northern
Hemisphere, where cold temperatures are
common in January, not July. The planet’s
changing seasons explain why we often
connect certain weather with specific times
of the year, like snow in January. Seasons are
periods during the year that are known for
a particular type of weather. Many places
on Earth experience four seasons—winter,
spring, summer, and fall. These seasons are
based on temperature and length of day. In
some parts of the world, however, seasons
are based on the amount of rainfall.
Winter and Summer
The change in seasons is created by Earth’s
tilt. As you can see in the illustration below,
while one of Earth’s poles tilts away from
the sun, the other tilts toward it. During
winter part of Earth is tilted away from the
sun, causing less direct solar energy, cool
temperatures, and less daylight. Summer
occurs when part of Earth is tilted toward
the sun. This creates more direct solar
energy, warmer temperatures, and longer
periods of daylight.
Because of Earth’s tilt, the Northern and
Southern hemispheres experience opposite
seasons. As the North Pole tilts toward the
sun in summer, the South Pole tilts away
28 CHAPTER 2
As Earth orbits the sun, the tilt of its axis
toward and away from the sun causes
the seasons to change. Seasons in the
Northern Hemisphere change at about
the same time every year.
ANALYZING VISUALS As the
Northern Hemisphere experiences
winter, what season is it
in the Southern
Hemisphere?
The Seasons: Northern Hemisphere
Winter and Spring The North
Pole tilts away from the sun in win-
ter, causing cooler temperatures. In
the spring, temperatures gradually
rise as the North Pole begins to
point toward the sun.
Summer and Fall Summer’s
warm temperatures are the result
of the North Pole’s tilt toward
the sun. As we move away from
the sun in the fall, temperatures
slowly decline.
FOCUS ON
READING
The prefix hemi-
means half. What
does the word
hemisphere
mean?
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ONLINE QUIZ
FOCUS ON CULTURE
PLANET EARTH 29
from it. As a result, the Southern Hemi-
sphere experiences winter. Likewise, when
it is spring in the Northern Hemisphere, it
is fall in the Southern Hemisphere.
Spring and Fall
As Earth orbits the sun, there are periods
when the poles tilt neither toward nor
away from the sun. These periods mark
spring and fall. During the spring, as part
of Earth begins to tilt toward the sun, solar
energy increases. Temperatures slowly start
to rise, and days grow longer. In the fall the
opposite occurs as winter approaches. Solar
energy begins to decrease, causing cooler
temperatures and shorter days.
Rainfall and Seasons
Some regions on Earth have seasons marked
by rainfall rather than temperature. This
is true in the
tropics , regions close to the
equator. At certain times of year, winds
bring either dry or moist air to the tropics,
creating wet and dry seasons. In India, for
example, seasonal winds called monsoons
bring heavy rains from June to October
and dry air from November to January.
Reading CheCk
Identifying Cause and
Effect What causes the seasons to change?
SUmmary and Preview Solar energy is
crucial for all life on the planet. Earth’s
position and movements affect the
amount of energy we receive from the
sun and determine our seasons. Next,
you will learn about Earth’s water supply
and its importance to us.
Reviewing Ideas, Terms, and Places
1. a. Identify What is solar energy, and how does it
affect Earth?
b. Analyze How do rotation and tilt each affect
the amount of solar energy that different parts of
Earth receive?
c. Predict What might happen if Earth received less
solar energy than it currently does?
2. a. Describe Name and describe Earth’s seasons.
b. Contrast How are seasons different in the North-
ern and Southern hemispheres?
c. Elaborate How might the seasons affect
human activities?
Critical Thinking
3. Identifying Cause and Effect Use your notes and
the diagram to identify the causes of seasons.
Section 1 Assessment
The Midnight Sun
Can you imagine going to sleep late at night with the sun
shining in the sky? People who live near the Arctic and
Antarctic circles experience this every summer, when they
can receive up to 24 hours of sunlight a day. The time-lapse
photo below shows a typical sunset during this period—
except the sun never really sets! This phenomenon is known
as the midnight sun. For locations like Tromso, Norway, this
means up to two months of constant daylight each summer.
People living near Earth’s poles often use the long daylight
hours to work on outdoor projects in preparation for winter,
when they can receive 24 hours of darkness a day.
Predicting How might people’s daily lives be affected by the
midnight sun?
Cause
Cause
Effect: Earth’s changing seasons
4. Describing the Seasons What are the seasons like
where you live? In your notebook, jot down a few
notes that describe the changing seasons.
Focus on Writing
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