Photo Credits: Cover: Earth Imaging/Getty Images; 2 John Chumack/Photo

Researchers, Inc.; 3 StockTrek/Photodisc/Getty Images; 4 Yang Liu/Corbis; 5 Scenics of

America/PhotoLink/Photodisc/Getty Images; 7 Eckhard Slawik/Photo Researchers, Inc.;
7 Exkhard Slawik/Photo Researchers, Inc.; 8 D. Van Ravenswaay/PhotoGRADE 6
Researchers,
Inc.; 9 SOHO/ESA/NASA/Photo Researchers, Inc.; 10 Frank Zullo/Photo OL Researchers,
Book 1
Inc.; 12 NASA/Science Source/Photo Researchers, Inc.; 13 NASA/Photo Researchers,
Inc.; 14 Corbis; 15 Corbis; 16 Rev. Ronald Royer/Photo Researchers,WORD
Inc.; 17COUNT
John
Chumack/Photo Researchers, Inc.; 18 Time Life Pictures/Getty Images; 19 350
Corbis.

GENRE
Expository Nonfiction
LEVEL
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 What a Ride!
Earth is always moving. It travels through space
at 170,000 kilometers per hour! You can’t feel it,
but you are rushing through space every moment.
Earth moves two ways. It makes circles around
the sun. This movement is called revolution. Each
trip around the sun takes about 365 days. A year is
the amount of time it takes Earth to travel around
the sun.

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]ZVi!VcYZcZg\n
Vh^igZkdakZh
VgdjcYi]Zhjc#

2
HX^Zci^hihiV`Ze]didhd[
:Vgi][gdbheVXZI]^h
]ZaehjhjcYZghiVcY
:Vgi]ÉhbdkZbZci#

Earth moves
another way. It
spins like a top. This
movement is called rotation.
The invisible line that Earth spins around is called its
axis. The axis goes through the middle of Earth. It
goes all the way from the North Pole to the South
Pole. Earth is a little tilted on its axis. It takes 24
hours for Earth to complete a rotation. Each
rotation is one day.
Earth spins like a top on its axis. At the same
time, it travels in large circles around the sun.
B6>C>9:66C99:I6>AH CVbZildlVnhi]Vi
:Vgi]bdkZh#

3
 Passing Time
Here on Earth, day turns to night. Summer
turns to winter. How do these changes happen?
You know that Earth makes one rotation every
day. It takes 24 hours to
complete a rotation. For 12
>bV\^cZ^[i]ZhjcY^YcÉi
hours, half of the Earth is
hZi#>i]VeeZchVihdbZ
facing the sun. It is daytime
eaVXZh;dgVWdji]Va[i]Z there. The other half of the
nZVg!i]ZhjcYdZhcÉihZiVi Earth is not facing the sun. It
i]ZCdgi]EdaZ#>iX^gXaZh^c is nighttime there. Earth
i]Zh`n#I]Zdi]Zg]Va[d[ never stops spinning on its
i]ZnZVg!i]ZhjcYdZhcÉi axis. Soon, day turns to
hZiVii]ZHdji]EdaZ# night and night to day.

L]Zc^iÉhYVni^bZ^c;adg^YV!
^iÉhc^\]ii^bZ]ZgZ^c8]^cV
6hi]ZYVnhiVgih^c8]^cV!V
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4
 >iÉhVlVgbVcYhjccn
?janYVn^c;adg^YV#7ji
^iÉhl^ciZg^c6jhigVa^V

Many places on Earth have four yearly

seasons—spring, summer, fall, and winter. The
temperature changes each season. It drops in
winter. Summer has the highest temperature.
What causes these changes?
As Earth revolves around the sun, it tilts on its
axis. In the part of Earth that is tilted toward the
sun, it is summer. In the part of Earth that is tilted
away from the sun, it is winter.
Since Earth stays tilted at the same angle, each
place on Earth has the same seasons every year.
B6>C>9:66C99:I6>AH L]VibV`ZhhZVhdchX]Vc\Z
dc:Vgi]4

5
 Our Moon
The moon moves, too. Like Earth, it makes
revolutions. But the moon doesn’t revolve around
the sun. It revolves around Earth!
As the moon revolves, its shape seems to
change. Some nights, the moon looks round and
bright. Other nights, it is shaped like a banana.
Some nights, you can’t see the moon at all.
The moon does not make its own light. As the
moon circles Earth, light from the sun hits different
sides of it. The moon reflects the sunlight. From
Earth, you see the side of the moon that is lit.
The moon looks as if it changes shapes. Really,
you can see only the part of the moon that is lit by
the sun. Each shape is a moon phase. The moon
follows a pattern of phases. This pattern is a
lunar cycle.
What if sunlight hits the moon from the front?
You see the moon in its full phase. It is round and
bright. When sunlight hits
a side of the moon, you see
only that side. When light >c&.+.!VhigdcVjihWgdj\]i
hits the far side of the bddcgdX`hWVX`id:Vgi]#
moon, you can’t see the HX^Zci^hihjhZYi]ZhZgdX`h
light—or the moon! idÒ\jgZdji]dldaYi]Z
H:FJ:C8: :meaV^c]dli]Z
bddclVh#>iÉhVWdji)#*
bddcX]Vc\Zhe]VhZh# W^aa^dcnZVghdaY

6
 I]^hbddc^h^c^ih
lVm^c\XgZhXZci
e]VhZ#>i^hdc^ih
lVnidWZ^c\V
[jaabddc#

I]^hbddc^h^c
^ihlVc^c\
XgZhXZcie]VhZ#
Hddc!ndjldcÉi
WZVWaZidhZZ
i]ZbddcViVaa

7
 Anybody Out There?
Earth is a planet. A planet is a large body of
rock or gas in space. The sun is a star. Stars are
hot balls of glowing gases that give off energy.
The Earth revolves around the sun. The sun
gives us energy, heat, and light. The other planets
also revolve around the sun. Like Earth, many of
these planets have moons. The sun, the planets
and their moons, and the small objects that
revolve around it make up our solar system.

I]Zhjc^hi]ZXZciZgd[djg
hdaVghnhiZb#6aa^iheaVcZih
gZkdakZVgdjcY^i#

8
 I]Zhjc\^kZhd[[hd
bjX]]ZVii]VilZXVc
[ZZa^i]ZgZdc:Vgi]#

The sun is not the only star in the universe.

There are too many stars to count! You can see
many stars at night. All are made of hot gas, but
the sun is our closest star. Without the sun, Earth
would have no warmth and no light.
8DBE6G:6C98DCIG6HI 8dbeVgZi]Zhjciddi]Zg
hiVgh^ci]Zjc^kZghZ#

9
 Searching for Stars
When you look in the night sky you might see
many stars.
To us, the sun looks yellow
and appears large. Compared
to other stars, the sun is
>ci]Z&-%%h!
medium in its size and its
ZhXVeZYhaVkZh
temperature. As we know
jhZYi]Z7^\
from living near the sun, stars
9^eeZgid]Zae
give off heat and light.
i]ZbÒcYi]Z^g
lVnCdgi]#

10
 Constellations are patterns of stars that form
imaginary pictures or designs in the sky. In ancient
times, people gave names to constellations. These
patterns of stars do not change.
At night the stars seem to move across the sky
just like the sun does during the day. The part of
the sky we see changes as Earth rotates.
Because Earth orbits the sun, we see a different
part of space during each season. At different
times of the year, we see different constellations.
Most of the stars you can see in summer are on
the other side of the sun in winter.
86JH:6C9:;;:8I L]nYdhiVghhZZbidbdkZVXgdhh
i]Zh`n4

I]Z7^\9^eeZg^hZVhnid
gZXd\c^oZ#>i^heVgid[
i]ZXdchiZaaVi^dcJghV
BV_dgÅi]Z\gZViWZVg#

11
 Our Neighbors
The four planets that are closest to the sun are
known as the inner planets. Earth is one of the
inner planets.
Mercury is closest to the sun. It is so hot, that
metal would melt on its surface. Mercury is the
smallest planet.
Venus is the next planet from the sun. Scientists
believe that Venus once had oceans. They are not
there anymore.

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YdZhcdiadd`a^`Z:Vgi]
>icdadc\Zg]VhlViZg#

1212
 I]Zhjg[VXZd[BVghadd`hV
a^iiaZa^`ZaVcYdc:Vgi]#>i
^hbdhiangdX`h#

Earth is the third planet from the sun. It is the

only planet that has water on its surface.
Mars comes after Earth. Mars has ice, like Earth.
However, most of the ice on Mars is far below its
surface.
The inner planets are smaller than the other
planets. Since they are closer to the sun, they are
much warmer than the other planets. The inner
planets also have rocky surfaces.
B6>C>9:66C99:I6>AH CVbZi]Z[djg^ccZgeaVcZih#

13
 Spaced Out!
The next four planets are far from the sun. They
are much larger than the inner planets. They also
are much colder! Many are just balls of frozen
gases. The outer planets tend to have many moons.
Mars is the last inner planet. The first outer
planet comes right after Mars. It is Jupiter. Jupiter
is the largest planet. It has more than 60 moons!
Next comes Saturn. Many people know Saturn
because of its rings. They look beautiful through
a telescope.

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e^ZXZhd[^XZ#HdbZd[i]Z
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1414
CZeijcZ^hi]Z
[Vgi]ZhieaVcZi[gdb
:Vgi]#>iÉhhjg[VXZ^h
bVYZd[[gdoZc\VhZh#

The last two planets are

Uranus and Neptune. Both are ?je^iZg]VhV<gZViGZY
made from frozen gases. Hedii]VindjXVchZZ
Neptune is the farthest from i]gdj\]ViZaZhXdeZ#
the sun. I]Zhedi^hgZVaanV
The farther a planet is from hidgbÅl^i]a^\]ic^c\
the sun, the longer it takes to VcY\gZVi]jgg^XVcZ
revolve around the sun. Uranus l^cYh#I]Zhidgb]Vh
takes 84 Earth years to make aVhiZYdkZg(%%nZVgh
one trip around the sun.
Neptune has the longest year.
One year on Neptune lasts about 165 Earth years!
8DBE6G:6C98DCIG6HI 8dbeVgZVcYXdcigVhii]Z
^ccZgeaVcZihVcYi]ZdjiZgeaVcZih#

15
 ThereÕs More Out There!
Planets, stars, and moons make up most of our
solar system. But there are other small objects
circling our sun.
Asteroids are like mini-planets that revolve
around the sun. Asteroids can fall out of their loop
and crash into a planet. A large asteroid could
cause big changes on a planet. An asteroid hit
Earth near the end of the time of the dinosaurs.
Scientists think it changed Earth so much that the
dinosaurs died out.
Meteors are small bits of rock and metal. They
fall off course and hit Earth. When they do, you
see “falling stars.”
86JH:6C9:;;:8I L]VildjaY]VeeZcid:Vgi]^[V
aVg\ZVhiZgd^YXgVh]ZY^cid^i4

NZVganbZiZdgh]dlZgh
hiVgi^c?jan#NdjXVc
d[iZchZZi]ZhZÆ[Vaa^c\
hiVghÇ^c6j\jhi#

1616
 Have you ever seen the Milky Way? The Milky
Way is a galaxy, or a huge group of stars, gases,
and dust. Almost everything in the nighttime sky
is part of the Milky Way Galaxy. Our solar system
is in the Milky Way Galaxy. We are near the edge
of the galaxy.
There are billions of galaxies. Each galaxy has its
own shape. The Milky Way Galaxy is a spiral
309sRUHL GALAXY

I]ZB^a`nLVn<VaVmn
^h_jhidcZ\VaVmn^c
heVXZ#

17
 The Universe
The universe is everything that exists in space. It
includes small pieces of dust and galaxies with
billions of stars. We still don’t know the size of the
universe. Scientists think it’s growing all the time.
B6>C>9:66C99:I6>AH =dlVgZVhdaVghnhiZb!V
\VaVmn!VcYi]Zjc^kZghZgZaViZY4

NdjXVcaZVgcVWdji
dW_ZXih^ci]Zh`nVi
VeaVcZiVg^jb#

18
H^miZZccVi^dchVgZldg`^c\
id\Zi]ZgidWj^aYi]Z
>ciZgcVi^dcVaHeVXZHiVi^dc#
>i^hWZ^c\XdchigjXiZY
cdlÅ^cheVXZL]Zc^i^h
Òc^h]ZY!VhigdcVjihXVca^kZ
i]ZgZ[dgjeidh^mbdci]h

Summary
Earth is hurtling
through space! It
rotates on its axis as it
revolves around the
sun. Night changes HeVXZkZ]^XaZh[gdb
:Vgi]]VkZaVcYZYdc
into day as Earth i]ZbddcVcYBVgh#
rotates. As Earth
revolves around the
sun, our seasons change. During the night, we can
see the moon that circles Earth, reflecting sunlight.
As our moon goes through its lunar cycle, we see
different phases. Earth is one of the planets that
revolve around the sun. Mercury, Venus, Earth, and
Mars are the inner planets. Jupiter, Saturn, Uranus,
and Neptune are the outer planets. These planets,
their moons, and the sun are part of our solar
system.

19
 Glossary
axis (AKssis) A line you cannot see that goes from
the top of the Earth, through the center, to the
bottom (3, 4, 5, 19)
constellation (kahnsSTUHsLAYsSHUHN A group of
stars that appear to form the shape of an animal, a
person, or an object (11)
lunar cycle (LOONser SYskuhl) The pattern of
phases of the moon (6, 19)
moon phases (MOON FAYZsuhz) The different
shapes that the moon seems to have in the sky
when the moon is observed from Earth (6, 7, 19)
planet (PLANsit) A large body of rock or gas in
space (8, 12, 13, 14, 15, 16, 19)
revolution (revsuhsLOOsshuhn) The movement of
Earth one time around the sun (2, 6)
rotation (rohsTAYsshuhn) The spinning of Earth
on its axis (3, 4, 19)
solar system (SOHsler SISstuhm) The sun, the
planets and their moons, and the small objects
that revolve around the sun (8, 16, 17, 19)
stars (STARZ) Hot balls of glowing gases that give
off energy (3, 8, 9, 10, 11, 16)

20
Think and Write
1. Name the inner and outer planets of the
solar system.
2. SEQUENCE Explain how night changes into
day every 24 hours.
3. MAIN IDEA AND DETAILS What makes up
our solar system?
4. Narrative Writing Suppose you are an
astronaut of the future. Choose a planet. Write
about your trip to the planet.

Hands-On Activity
Work together as a class. Make a model of the eight
planets in our solar system, along with the sun. Be
sure to place them in their correct order.

School-Home Connection
Make a calendar of the next month. Each night,
look at the moon. On your calendar, draw the phase
you see. Take your completed calendar to school to
compare with those of your classmates.
 GRADE 3
OL Book 10
WORD COUNT
1025
GENRE
Expository Nonfiction
LEVEL
See TG or go online

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