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Earth in SpaceMany observations on Earthcan be attributed to Earth’splace in space.

SECTION 1Earth’s Motion andSeasonsMain Idea Earth’s tilt andrevolution around the Suncause seasons to occur.

SECTION 2Earth’s MoonMain Idea Many visualcharacteristics of the Moonresult from its surface condi-tions and its motion relativeto Earth and the Sun.

SECTION 3Our Solar SystemMain Idea Our solar systemis composed of the Sun, theplanets, and other objects inorbit around the Sun.

Light Up the NightFor thousands of years humans have looked at the night sky, butonly during the past 50 years have we been able to view Earthfrom space. This photo combines views of Earth taken fromthe International Space Station.

Describe how our view of Earth has changed in the past 50 years.Science Journal

NASA

177177

Earth and the Moon Make thefollowing Foldable to help yousee how Earth and the Moonare similar and different.

Fold a verticalsheet of paper inhalf from top tobottom.

Fold in half from sideto side with the foldat the top.

Unfold the paperonce. Cut only thefold of the top flapto make two tabs.

Turn the paper verti-cally and draw on thefrom tabs as shown.

Compare and Contrast Before you read thechapter, list the ways you think Earth andthe Moon are alike and different under theappropriate tabs. As you read, correct or addto this information.

STEP 4

STEP 3

STEP 2

STEP 1

Model Earth’s ShapeCould you prove that Earth is round? Whatcan you see from Earth’s surface that provesEarth is round?

1. Cut a strip of cardboard about 8 cm longand 8 cm tall into a triangle. Fold upabout 2 cm on one side and tape it to abasketball so that the peak of the trianglesticks straight up resembling a sailboat.

2. Place the basketball on a table at eyelevel so that the sailboat sticks out hori-zontally, parallel to the table, pointingaway from you.

3. Roll the ball toward you slowly so that thesail comes into view over the top of theball.

4. Think Critically Write a paragraph thatexplains how the shape of the basketballaffected your view of the sailboat. Howdoes this model show how Earth’s shapeaffects your view over long distances.

Start-Up Activities

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Alike

Different

NASA

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178 A CHAPTER 7 Earth in Space

Apply It! Before you read, skimthe questions in the Chapter Review. Choosethree questions and predict the answers.

Learn It! A prediction is an educated guess based on whatyou already know. One way to predict while reading is to guess what youbelieve the author will tell you next. As you are reading, each new topicshould make sense because it is related to the previous paragraph or passage.

Practice It! Read the excerpt below from Section 1.Based on what you have read, make predictions about what you will readin the rest of the lesson. After you read Sections 1 and 3, go back to yourpredictions to see if they were correct.

The spherical shape of Earth and otherplanets is because of . Gravity is a forcethat attracts all objects toward each other. Thefarther away the objects are, the weaker thepull of gravity is. Also, the more massive anobject is, the is.Large objects in space, such as

often are spherical because of how theyformed. At first particles collide and sticktogether randomly. However, as the massincreases, gravity plays a role. Particles areattracted to the center of the growing mass,making it spherical.

—from page 179

moonsplanets and

stronger its gravitational pull

gravity

Think about how you areaffected by gravity.

Predict what might hap-pen to you if Earth weremore massive and its grav-itational pull were muchstronger. Would it be easyto stand up?

Predict which object mostlikely would be perfectlyspherical—a large, massiveplanet or a smaller moon.Upon which of theseobjects would you mostlikely find the tallestmountains?

178 B

Before You Read Statement After You ReadA or D A or D

1 In one year, Earth rotates once around its axis.

2 Summer occurs in the northern hemispherewhen Earth is closest to the Sun.

3 At an equinox, the lengths of day and night arenearly equal all over the world.

4 The Moon’s crust is thicker on the side facingEarth than it is on the far side of the Moon.

5 New moon occurs when Earth is positionedbetween the Sun and the Moon.

6 A solar eclipse can only occur at new moon.

7 More than 99 percent of all matter in the solarsystem is contained in the Sun.

8 Earth’s ozone layer protects life from the Sun’sharmful ultraviolet radiation.

9 Earth is the most volcanically active body in thesolar system.

As you read, check the

predictions you made to

see if they were correct.

Use this to focus on the main ideas as you read the chapter.

Before you read the chapter, respond to the statements

below on your worksheet or on a numbered sheet of paper.

• Write an A if you agree with the statement.

• Write a D if you disagree with the statement.

After you read the chapter, look back to this page to see if you’ve

changed your mind about any of the statements.

• If any of your answers changed, explain why.

• Change any false statements into true statements.

• Use your revised statements as a study guide.

Print out a worksheet of this page at green.msscience.com


178 CHAPTER 7 Earth in Space

Earth’s Physical Data Think about the last time you saw a beautiful sunset. Late in

the day, you may have noticed the Sun sinking lower and lowerin the western sky. Eventually, as the Sun went below the hori-zon, the sky became darker. Was the Sun actually traveling outof view, or were you?

In the past, some people thought that the Sun, the Moon,and other objects in space moved around Earth each day. Nowit is known that some of the motions of these objects, asobserved from Earth, are really caused by Earth’s movements.

Also, many people used to think that Earth was flat. Theythought that if you sailed far enough out to sea, you eventuallywould fall off. It is now known that this is not true. What gen-eral shape does Earth have?

Spherical Earth As shown in Figure 1, pictures from spaceshow that Earth is shaped like a ball, or a sphere. A sphere(SFIHR) is a three-dimensional object whose surface at allpoints is the same distance from its center. What other evidencecan you think of that reveals Earth’s shape?

Earth’s Motion andSeasons

■ Identify Earth’s shape and otherphysical properties.

■ Compare and contrast Earth’srotation and revolution.

■ Explain the causes of Earth’s seasons.

Movements of Earth regulate lifepatterns.

Review Vocabularysatellite: a natural or artificialobject that revolves aroundanother object in space

New Vocabulary

• axis • orbit

• rotation • solstice

• revolution • equinox

Figure 1 Earth’s nearly sphericalshape was first observed directlyby images taken from a spacecraft.Describe observations from Earth’ssurface that also suggest that it isspherical.

SECTION 1 Earth’s Motion and Seasons 179

Evidence for Earth’s Shape Have you ever stood on a dockand watched a sailboat come in? If so, you may have noticed thatthe first thing you see is the top of the boat’s sail. This occursbecause Earth’s curved shape hides the rest of the boat fromview until it is closer to you. As the boat slowly comes closer toyou, more and more of its sail is visible. Finally, the entire boatis in view.

More proof of Earth’s shape is that Earth casts a curvedshadow on the Moon during a lunar eclipse, like the one shownin Figure 2. Something flat, like a book, casts a straight shadow,whereas objects with curved surfaces cast curved shadows.

What object casts a shadow on the Moon duringa lunar eclipse?

Influence of Gravity The spherical shape of Earth and otherplanets is because of gravity. Gravity is a force that attracts allobjects toward each other. The farther away the objects are, theweaker the pull of gravity is. Also, the more massive an object is,the stronger its gravitational pull is. Large objects in space, suchas planets and moons often are spherical because of how theyformed. At first, particles collide and stick together randomly.However, as the mass increases, gravity plays a role. Particles areattracted to the center of the growing mass, making it spherical.

Even though Earth is round, it may seem flat to you. This isbecause Earth’s surface is so large compared to your size.

Figure 2 Earth’s spherical shapealso is indicated by the curvedshadow it casts on the Moonduring a partial lunar eclipse.


Almost a Sphere Earth’s shape is not a perfect sphere. Itbulges slightly at the equator and is somewhat flattened aroundthe poles. As shown in Figure 3, this causes Earth’s circumfer-ence at the equator to be a bit larger than Earth’s circumferenceas measured through the north and south poles. The circumfer-ence of Earth and some other physical properties are listed inTable 1.

Motions of Earth Why the Sun appears to rise and set each day and why the

Moon and other objects in the sky appear to move from east towest is illustrated in Figure 4. Earth’s geographic poles arelocated at the north and south ends of Earth’s axis. Earth’s axisis the imaginary line drawn from the north geographic polethrough Earth to the south geographic pole. Earth spins aroundthis imaginary line. The spinning of Earth on its axis, called rotation, causes you to experience day and night.

What imaginary line runs through Earth’s northand south geographic poles?

Figure 3 Earth is almost asphere, but its circumferencemeasurements vary slightly.The north-south circumferenceof Earth is smaller than the east-west circumference.

23.5°

40,008 km

40,075 kmN

S Axis

Table 1 Physical Properties of Earth

Diameter (pole to pole) 12,714 km

Diameter (equator) 12,756 km

Circumference (poles) 40,008 km(distance around Earth through N and S poles)

Circumference (equator) 40,075 km(distance around Earth at the equator)

Mass 5.98 � 1024 kg

Average density 5.52 g/cm3

(average mass per unit volume)

Average distance from the Sun 149,600,000 km

Period of rotation relative to stars (1 day) 23h, 56 min

Solar day 24 h

Period of revolution (1 year) 365 days, 6 h, 9 min(path around the Sun)


Earth’s Orbit Earth has another type of motion. As it rotateson its axis each day, Earth also moves along a path around theSun. This motion of Earth around the Sun, shown in Figure 4,is called revolution. How many times does Earth rotate on itsaxis during one complete revolution around the Sun? Just as dayand night are caused by rotation, what happens on Earth that iscaused by its revolution?

Seasons A new year has begun. As days and weeks pass, younotice that the Sun remains in the sky later and later each day.You look forward to spring when you will be able to stay outsidelonger in the evening because the number of daylight hoursgradually increases. What is causing this change?

You learned earlier that Earth’s rotation causes day andnight. Earth also moves around the Sun, completing one revolu-tion each year. Earth is really a satellite of the Sun, movingaround it along a curved path called an orbit. The shape ofEarth’s orbit is an ellipse, which is rounded like a circle butsomewhat flattened. As Earth moves along in its orbit, the wayin which the Sun’s light strikes Earth’s surface changes.

Earth’s elliptical orbit causes it to be closer to the Sun inJanuary and farther from the Sun in July. But, the total amountof energy Earth receives from the Sun changes little during ayear. However, the amount of energy that specific places onEarth receive varies quite a lot.

Figure 4 Earth’s west-to-eastrotation causes day and night.

Rotation24 hours

AxisN

S

Revolution(orbit) 365 days1

4

Topic: Earth’s Rotation andRevolutionVisit for Weblinks to information about Earth’srotation and revolution and theseasons.

Activity Build a model, perhapsusing a plastic foam sphere andtoothpicks, showing how the por-tion of the surface lit by a smallflashlight varies at different anglesof tilt.

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Earth’s Tilt You can observe one reason why the amount ofenergy from the Sun varies by moving a globe slowly around alight source. If you keep the globe tilted in one direction, asshown in Figure 5, you will see that the top half of the globe istilted toward the light during part of its orbit and tilted awayfrom the light during another.

Earth’s axis forms a 23.5-degree angle with a line perpendicu-lar to the plane of its orbit around the Sun. It always points to theNorth Star. Because of this, there are more daylight hours for thehemisphere tilted toward the Sun. Think about when it gets darkoutside at different times of the year. More hours of sunlight insummer is one reason why summer is warmer than winter.Another reason is that because of Earth’s tilt, sunlight strikes thehemisphere tilted toward the Sun at a higher angle, that is, closerto 90 degrees. Sunlight strikes the hemisphere tilted away fromthe Sun at a lower angle. This lessens solar radiation and bringswinter.

Solstices Because of the tilt of Earth’s axis, the Sun’s positionrelative to Earth’s equator changes. Twice during the year, theSun reaches its greatest distance north or south of the equatorand is directly over either the Tropic of Cancer or the Tropic ofCapricorn, as shown in Figure 6. These times are known as thesummer and winter solstices. Summer solstice, which is whenthe Sun is highest in the sky at noon, happens on June 21 or 22for the northern hemisphere and on December 21 or 22 for thesouthern hemisphere. The opposite of this for each hemisphereis winter solstice, which is when the Sun is lowest at noon.

Figure 5 When the northernhemisphere is tilted toward theSun, it experiences summer.Explain why days are longerduring the summer.

Summer for northern hemisphere

Winter for northern hemisphere


Self Check1. Explain how planets develop their spherical shapes.

2. Explain Although Earth receives the same totalamount of energy from the Sun throughout the year,why is it so much warmer in summer than it is inwinter?

3. Describe whether the shape of Earth’s orbit affects ordoes not affect the seasons.

4. Explain why the dates of the solstices and equinoxes vary.

5. Think Critically In Table 1, why is Earth’s distance fromthe Sun reported as an average distance?

SummaryEarth’s Physical Data

• Earth has a spherical shape.

• The evidence for Earth’s shape is the wayobjects appear on the horizon and its curvedshadow during a lunar eclipse.

Motions of Earth

• Earth rotates on its axis, causing day and night.

• Earth revolves around the Sun in an ellipticalorbit.

• Earth’s rotational axis is tilted at an angle of23.5°.

• The tilt of Earth’s axis as it moves around theSun causes the seasons.

• At a solstice, the Sun is directly over the Tropicof Capricorn or the Tropic of Cancer.

• At an equinox the Sun is directly over theequator.

6. Use Models Use a globe and an unshaded light sourceto illustrate how the tilt of the Earth on its axis, as itrotates and revolves around the Sun, causes changes in the number of daylight hours.

Equinoxes At an equinox,(EE kwuh nahks) when the Sunis directly above Earth’s equator,the lengths of day and night arenearly equal all over the world.During equinox, Earth’s tilt isneither toward nor away fromthe Sun. In the northern hemi-sphere, spring equinox is March21 or 22 and fall equinox isSeptember 21 or 22. As you sawin Table 1, the time it takes forEarth to revolve around the Sunis not a whole number of days.Because of this, the dates forsolstices and equinoxes changeslightly over time.

Earth’s Place in Space Earth is shaped much like a sphere.As Earth rotates on its axis, the Sun appears to rise and set in thesky. Earth’s tilt and revolution around the Sun cause seasons tooccur. In the next section, you will learn about Earth’s nearestneighbor in space, the Moon. Later, you will learn about otherplanets in our solar system and how they compare with Earth.

Figure 6 When the Sun isdirectly above the equator, dayand night have nearly equallengths.

Winter solsticefor northern hemisphere

Tropic of Cancer

Equator

Tropic of Capricorn

Spring equinox

Autumn equinox

Summer solsticefor northern hemisphere

green.msscience.com/self_check_quiz



The Moon’s Surface and Interior Take a good look at the surface of the Moon during the next

full moon. You can see some of its large surface features,especially if you use binoculars or a small telescope. You will seedark-colored maria (MAR ee uh) and lighter-colored highlandareas, as illustrated in Figure 7. Galileo first named the dark-colored regions maria, the Latin word for seas. They remindedGalileo of the oceans. Maria probably formed when lava flowsfrom the Moon’s interior flooded into large, bowl-like regionson the Moon’s surface. These depressions may have formed earlyin the Moon’s history. Collected during Apollo missions andthen analyzed in laboratories on Earth, rocks from the maria areabout 3.2 billion to 3.7 billion years old. They are the youngestrocks found on the Moon thus far.

The oldest moon rocks analyzed so far—dating to about4.6 billion years old—were found in the lunar highlands. Thelunar highlands are areas of the lunar surface with an elevationthat is several kilometers higher than the maria. Some lunarhighlands are located in the south-central region of the Moon.

■ Identify the Moon’s surfacefeatures and interior.

■ Explain the Moon’s phases.■ Explain the causes of solar and

lunar eclipses.■ Identify the origin of the Moon.

Learning about the Moon can helpyou understand Earth.

Review Vocabularydensity: the amount of matter ina given volume of a substance

New Vocabulary

• crater

• moon phase

• solar eclipse

• lunar eclipse

Earth’s Moon

Figure 7 On a clear night and especiallyduring a full moon, you can observe someof the Moon’s surface features.Describe how you can recognize themaria, lunar highlands, and craters.

Lick Observatory

SECTION 2 Earth’s Moon 185

Craters As you look at the Moon’ssurface features, you will see craters.Craters, also shown in Figure 7, aredepressions formed by large mete-orites—space objects that strike thesurface. As meteorites struck theMoon, cracks could have formed in theMoon’s crust, allowing lava flows to fillin the large depressions. Craters areuseful for determining how old partsof a moon’s or a planet’s surface arecompared to other parts. The moreabundant the craters are in a region,the older the surface is.

The Moon’s Interior During theApollo space program, astronauts leftseveral seismographs (size muh grafs)on the Moon. A seismograph is aninstrument that detects tremors, or seis-mic vibrations. On Earth, seismographsare used to measure earthquake activity. On the Moon, they areused to study moonquakes. Based on the study of moonquakes, amodel of the Moon’s interior has been proposed, as illustrated inFigure 8. The Moon’s crust is about 60 km thick on the side facingEarth and about 150 km thick on the far side. The difference inthickness is probably the reason fewer lava flows occurred on thefar side of the Moon. Below the crust, a solid layer called the man-tle may extend 900 km to 950 km farther down. A soft layer ofmantle may continue another 500 km deeper still. Below this maybe an iron-rich, solid core with a radius of about 300–450 km.

Like the Moon, Earth also has a dense, iron core. However,the Moon’s core is small compared to its total volume.Compared with Earth, the Moon is most like Earth’s outer twolayers—the mantle and the crust—in density. This supports ahypothesis that the Moon may have formed primarily frommaterial ejected from Earth’s mantle and crust.

Motions of the MoonThe same side of the Moon is always facing Earth. You can

verify this by examining the Moon in the sky night after night.You’ll see that bright and dark surface features remain in thesame positions. Does this mean that the Moon doesn’t turn onan axis as it moves around Earth? Next, explore why the sameside of the Moon always faces Earth.

Figure 8 The small size of theMoon’s core suggests the Moonformed from a part of the crustand mantle of Earth.

Crust: 60–150 km thickrock made mostly of feldsparcovered with loosedust and rockdebris

Rigid mantle:950 km thick

Non-rigid mantle:Semi-solid500 km thick

Core: Possibly iron with a radius of 300–450 km

Moon Colonies Researchfrom space probes indi-cates that conditions onsome areas of the Moonmight make mooncolonies possible some-day. Why might a mooncolony prove useful?


Revolution and Rotation of the Moon The Moonrevolves around Earth at an average distance of about384,000 km. It takes 27.3 days for the Moon to complete oneorbit around Earth. The Moon also takes 27.3 days to rotateonce on its axis. Because these two motions of the Moon takethe same amount of time, the same side of the Moon is alwaysfacing Earth. Examine Figure 9 to see how this works.

Why does the same side of the Moon alwaysface Earth?

However, these two lunar motions aren’t exactly the sameduring the Moon’s 27.3-day rotation-and-revolution period.Because the Moon’s orbit is an ellipse, it moves faster when it’scloser to Earth and slower when it’s farther away. During oneorbit, observers are able to see a little more of the eastern side ofthe Moon and then a little more of the western side.

Moon Phases If you ever watched the Moon for several daysin a row, you probably noticed how its shape and position in thesky change. You learned that the Moon rotates on its axis andrevolves around Earth. Motions of the Moon cause the regularcycle of change in the way the Moon looks to an observer on Earth.

Figure 9 Observers viewing theMoon from Earth always see thesame side of the Moon. This iscaused by two separate motionsof the Moon that take the sameamount of time.

North pole

The Moon’s orbit

Modeling theMoon’s RotationProcedure1. Use masking tape to place

a large X on a basketballthat will represent theMoon.

2. Ask two students to sit inchairs in the center of theroom.

3. Place other studentsaround the outer edgeof the room.

4. Slowly walk completelyaround the two studentsin the center while holdingthe basketball so that theside with the X alwaysfaces the two students.

Analysis1. Ask the two students in

the center whether theythink the basketball turnedaround as you circledthem. Then ask severalstudents along the outeredge of the room whetherthey think the basketballturned around.

2. Based on these observa-tions, infer whether or notthe Moon rotates as itmoves around Earth.Explain your answer.


The Sun Lights the Moon You see the Moon because itreflects sunlight. As the Moon revolves around Earth, the Sunalways lights one half of it. However, you don’t always see the entire lighted part of the Moon. What you do see are phases,or different portions of the lighted part. Moon phases, illus-trated in Figure 10, are the changing views of the Moon as seen from Earth.

New Moon and Waxing Phases New moon occurs whenthe Moon is positioned between Earth and the Sun. You can’t seeany of a new moon, because the lighted half of the Moon is fac-ing the Sun. The new moon rises and sets with the Sun andnever appears in the night sky.

Figure 10 The amount of theMoon’s surface that looks bright toobservers on Earth changes duringa complete cycle of the Moon’sphases.Explain what makes the Moon’ssurface appear so bright.

Noon

Rays fromSun

1. New5. Full

2. Crescent(waxing)

4. Gibbous(waxing)

8. Crescent(waning)

Moon phase photographsare as seen from Earth.

6. Gibbous(waning)

3. First quarter

7. Third quarter

Sunrise

Sunset

Midnight

Lick Observatory


Waxing Moon Shortly after new moon, more and more of theside facing Earth is lighted. The phases are said to be waxing, orgrowing in size. About 24 hours after new moon, a thin sliver onthe side facing Earth is lighted. This phase is called waxing cres-cent. As the Moon continues its trip around Earth, half of thatside is lighted. This phase is first quarter and occurs about aweek after new moon.

The Moon’s phases continue to wax through waxing gibbous(GIH bus) and then full moon—the phase when you can see allof the side facing Earth lighted. At full moon, Earth is betweenthe Sun and the Moon.

Waning Moon After passing full moon, the amount of theside facing Earth that is lighted begins to decrease. Now thephases are said to be waning. Waning gibbous occurs just afterfull moon. Next comes third quarter when you can see only halfof the side facing Earth lighted, followed by waning crescent, thefinal phase before the next new moon.

What are the waning phases of the Moon?

The complete cycle of the Moon’s phases takes about 29.5 days.However, you will recall that the Moon takes only 27.3 days torevolve once around Earth. Figure 11 explains the time dif-ference between these two lunar cycles. Earth’s revolutionaround the Sun causes the time lag. It takes the Moon abouttwo days longer to align itself again between Earth and the Sunat new moon.

Earth’s orbit

Day 1New moon

SunDistant

star

Day 29New moon again

2 days

Day 27Earth, Moon, and

distant star areonce again aligned.

12

13

Day 14 34

Full moon

Topic: Moon PhasesVisit for Weblinks to information about moonphases.

Activity When weather permits,take photos or make drawings ofthe Moon every three or four days.Record the date of each photo andmake a poster showing the Moon’sphases. How do your moon pic-tures correspond to what youviewed on the Web site?

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Figure 11 It takes about twodays longer for a complete Moonphase cycle than for the Moon toorbit Earth.Explain how the revolution of theEarth-Moon system around the Suncauses this difference in time.


Penumbra

Moon

Earth

Sunlight

Umbra

EclipsesYou can see other effects of the Moon’s revolution than just

the changes in its phases. Sometimes during new and full moon,shadows cast by one object will fall on another. While walkingalong on a sunny day, have you ever noticed how a passing air-plane can cast a shadow on you? On a much larger scale, theMoon can do this too, when it lines up directly with the Sun.When this happens, the Moon can cast its shadow all the way toEarth. Earth also can cast its shadow onto the Moon during afull moon. When shadows are cast in these ways, eclipses occur.

Eclipses occur only when the Sun, the Moon, and Earth arelined up perfectly. Because the Moon’s orbit is tilted at an anglefrom Earth’s orbit, the Moon’s shadow most often misses Earth,and eclipses happen only a few times each year.

Solar Eclipses During new moon, if Earth moves into theMoon’s shadow, a solar eclipse occurs. As shown in Figure 12,the Moon blocks sunlight from reaching a portion of Earth’ssurface. Only areas on Earth in the Moon’s umbra, or thedarkest part of its shadow, experience a total solar eclipse.Those areas in the penumbra, or lighter part of the shadow,experience a partial solar eclipse. During a total solar eclipse,the sky becomes dark and stars can be seen easily. BecauseEarth rotates and the Moon is moving in its orbit, a solareclipse lasts only a few minutes in any one location.

Figure 12 During a total solareclipse, viewers on Earth withinthe Moon’s umbra will see theMoon cover the Sun completely.Only the Sun’s outer atmosphereis visible as a halo.

Viewers on Earth within the Moon’s penumbra will see only a portion of theSun covered. WARNING: Never look directly at a solar eclipse. Only observesolar eclipses indirectly.

CORBIS


Lunar Eclipses A lunar eclipse, illustrated in Figure 13,occurs when the Sun, Earth, and the Moon are lined up so thatthe full moon moves into Earth’s shadow. Direct sunlight isblocked from reaching the Moon. When the Moon is in thedarkest part of Earth’s shadow, a total lunar eclipse occurs.

During a total lunar eclipse, the full moon darkens. Becausesome sunlight refracts through Earth’s atmosphere, the Moonappears to be deep red. As the Moon moves out of the umbraand into the penumbra, or lighter shadow, you can see thecurved shadow of Earth move across the Moon’s surface. Whenthe Moon passes partly through Earth’s umbra, a partial lunareclipse occurs.

Origin of the Moon Before the Apollo space program, several hypotheses were

proposed to explain the origin of the Moon. Some of thesehypotheses are illustrated in Figure 14.

The co-formation hypothesis states that Earth and the Moonformed at the same time and out of the same material. Oneproblem with this hypothesis is that Earth and the Moon havesomewhat different densities and compositions.

According to the capture hypothesis, Earth and the Moonformed at different locations in the solar system. Then Earth’sgravity captured the Moon as it passed close to Earth. The fis-sion hypothesis states that the Moon formed from materialthrown off of a rapidly spinning Earth. A problem with the fis-sion hypothesis lies in determining why Earth would have beenspinning so fast.

Figure 13 Lunar eclipses occurwhen the Sun, the Moon, andEarth line up so that Earth’sshadow is cast upon a full moon.When the entire Moon is eclipsed,anyone on Earth who can see a fullmoon can see the lunar eclipse.

Penumbra

Moon

Earth

Sunlight

Umbra

NGS TITLE

Figure 14


COLLISION A Mars-sized body collideswith the primordial Earth.The colossalimpact smashes off sufficient debris fromEarth to form the Moon.

FISSION A rapidly spinning moltenEarth tears in two.The smaller blob ofmatter enters into orbit as the Moon.

CO-FORMATION Earth and the Moon form at the same time from a vast cloudof cosmic matter that condenses into thebodies of the solar system.

Earth’s orbit

MoonSun

Earth

MoonSun

Earth

▼

▼

▼

CAPTURE Earth’s gravity captures theMoon into Earth orbit as the Moon passesclose to Earth.

Moon

SunEarth

▼

Scientists have proposed several possible explanations, or hypotheses, to account for theformation of Earth’s Moon. As shown below,

these include the co-formation, fission, capture, andcollision hypotheses.The latter—sometimes knownas the giant impact hypothesis—is the most widelyaccepted today.

VISUALIZING HOW THE MOON FORMED

(tr)NASA, (bl)Stephen Frisch/Stock Boston/PictureQuest


Self Check1. Name the phase of the Moon that occurs when Earth is

located between the Moon and the Sun.

2. Explain how maria formed on the Moon and how thisname for them originated.

3. Infer Based on what you know about the Moon’s crust,explain why there are fewer maria on the far side of theMoon.

4. Explain why the fact that Earth and the Moon have differ-ent densities favors the collision theory of lunar formation.

5. Think Critically Explain why more people observe atotal lunar eclipse than a total solar eclipse.

SummaryThe Moon’s Surface and Interior

• Surface features include highlands, maria, andcraters.

• The Moon has an outer crust, a rigid mantle, anon-rigid mantle, and possibly a small iron core.

Motions of the Moon

• The Moon revolves around Earth and rotateson its axis once every 27.3 days.

• The varying portions of the Moon’s near sidethat are lit by the Sun are called phases.

Eclipses

• A solar eclipse takes place when the Moonpasses between Earth and the Sun.

• A lunar eclipse takes place when Earth passesbetween the Sun and the Moon.

Origin of the Moon

• Evidence obtained from moon rocks supportsthe collision hypothesis of lunar formation.

6. Recognize Cause and Effect How does the Moon’sorbit around Earth cause the observed cyclical phasesof the Moon? What role does the Sun play?

7. Use a Spreadsheet to compare the four hypotheses oflunar formation in terms of modern factual evidence of the Moon’s density.

Collision Hypothesis A lot of uncertainty stillexists about the origin of the Moon. However, thecollection and study of moon rocks, shown inFigure 15, brought evidence to support one recenthypothesis. This hypothesis, summarized inFigure 14, involves a great collision. When Earthwas about 100 million years old, a Mars-sizedspace object may have collided with Earth. Such anobject would have broken through Earth’s crustand plunged toward the core. This collision wouldhave thrown large amounts of gas and debris intoorbit around Earth. Within about 1,000 years thegas and debris then could have condensed to formthe Moon. The collision hypothesis is strength-ened by the fact that Earth and the Moon have dif-ferent densities. The Moon’s density is similar to

material that would have been thrown off Earth’s mantle andcrust when the object collided with Earth.

Earth is the third planet from the Sun. Along with the Moon,Earth could be considered a double planet. In the next sectionyou will learn about other planets in the solar system. Some haveproperties similar to Earth’s—others are different from Earth.

Figure 15 Moon rocks collectedduring the Apollo space programprovide clues about how the Moonformed.

green.msscience.com/self_check_quizNASA


The position of the Moon in the sky varies asthe phases of the Moon change. Do you knowwhen you might be able to see the Moon dur-ing daylight hours? How will viewing the Moonthrough a telescope be different from viewingit with the unaided eye?

Real-World QuestionWhat features of the Moon are visible whenviewed through a telescope?

Goals■ Determine when you may be able to

observe the Moon during the day.■ Use a telescope to observe the Moon.■ Draw a picture of the Moon’s features as

seen through the telescope.

Materialstelescope drawing pencilsdrawing paper

Safety Precautions

WARNING: Never look directly at the Sun. It candamage your eyes.

Procedure1. Using your own observations, books about

astronomy, or other resource materials,determine when the Moon may be visible toyou during the day. You will need to findout during which phases the Moon is upduring daylight hours, and where in the skyyou likely will be able to view it. You willalso need to find out when the Moon will bein those phases in the near future.

2. Observe the Moon with your unaided eye.Draw the features that you are able to see.

3. Using a telescope, observe the Moon again. Adjust the focus of the telescope so that you can see as many features as possible.

4. Draw a new picture of the Moon’s features.

Conclude and Apply1. Describe what you learned about when the

Moon is visible in the sky. If a friend wantedto know when to try to see the Moon duringthe day next month, what would you say?

2. Describe the differences between howthe Moon looked with the naked eye andthrough the telescope. Did the Moon appearto be the same size when you looked at itboth ways?

3. Determine what features you were able tosee through the telescope that were not vis-ible with the unaided eye.

4. Observe Was there anything else differentabout the way the Moon looked through thetelescope? Explain your answer.

5. Identify some of the types of features thatyou included in your drawings.

The next time you notice the Moon whenyou are with your family or friends, talkabout when the Moon is visible in the skyand the different features that are visible.

LAB 193

Viewing the Mhhn

Timothy Fuller

194 CHAPTER 7

Size of the Solar System Measurements in space are difficult to make because space is

so vast. Even our own solar system is extremely large. Our solarsystem, illustrated in Figure 16, is composed of the Sun, plan-ets, asteroids, comets, and other objects in orbit around the Sun.How would you begin to measure something this large? If youare measuring distances on Earth, kilometers work fine, but notfor measuring huge distances in space. Earth, for example, isabout 150,000,000 km from the Sun. This distance is referred toas 1 astronomical unit, or 1 AU. Jupiter, the largest planet in thesolar system, is more than 5 AU from the Sun. Astronomicalunits can be used to measure distances between objects withinthe solar system. Even larger units are used to measure distancesbetween stars.

Located at the center of the solar system is a star you know asthe Sun. The Sun is an enormous ball of gas that produces energyby fusing hydrogen into helium in its core. More than 99 percentof all matter in the solar system is contained in the Sun.

Sun

Mars

Asteroid belt

Astronomical units

Jupiter

0 1.5 2.7 5.2

Figure 16 Our solar system is composed of the Sun, planets and theirmoons, and smaller bodies that revolve around the Sun, such as asteroidsand comets.

Our Solar System

■ List the important characteristicsof inner planets.

■ Identify how other inner planetscompare and contrast with Earth.

■ List the important characteristicsof outer planets.

Learning about other planets helpsyou understand Earth and the for-mation of our solar system.

Review Vocabularyatmosphere: the gaseous layerssurrounding a planet or moon

New Vocabulary

• solar system • comet

• astronomical unit • nebula

• asteroid

Evaluate

The asteroid belt is composed of rockybodies that are smaller than the planets.Evaluate About how many AU arebetween the Sun and the middle ofthe asteroid belt?

SECTION 3 Our Solar System 195

An Average Star Although the Sun is important to life onEarth, it is much like many other stars in the universe. TheSun is middle-aged and about average in the amount of lightit gives off.

The Planets The planets in our solar system can be classified as inner

or outer planets. Inner planets have orbits that lie inside theorbit of the asteroid belt. The inner planets are mostly solid,rocky bodies with thin atmospheres compared with theatmospheres of the outer planets. The outer planets haveorbits that lie outside the orbit of the asteroid belt. They arealso known as gas giants, and do not have a solid, rocky sur-face like Earth.

Inner Planets The inner planets are Mercury, Venus, Earth, and Mars.

Known as the terrestrial planets, after the Latin word terra(earth), they are similar in size to Earth and are made up mainlyof rock.

Mercury Mercury is the closest planet to the Sun. It is coveredby craters formed when meteorites crashed into its surface.The surface of Mercury also has cliffs, as shown in Figure 17,some of which are 3 km high. These cliffs may have formedwhen Mercury’s molten, iron-rich core cooled and contracted,causing the outer solid crust to shrink. The planet seems to haveshrunk about 2 km in diameter. It has no atmosphere.

Figure 17 The Discovery RupesScarp is a huge cliff that may haveformed as Mercury cooled andcontracted.

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Venus Venus, the second inner planet from the Sun, shown inFigure 18, often has been referred to as Earth’s twin, but onlybecause of their similar sizes and masses. Otherwise, the surfaceconditions and atmospheres of Earth and Venus are extremelydifferent. Thick clouds surround Venus and trap energy fromthe Sun, causing Venus’s surface temperature to reach about472°C. The process is similar to what occurs in a greenhouse.

Earth Earth is the third inner planet from the Sun. It is uniquebecause surface temperatures enable water to exist in threestates—solid, liquid, and gas. Ozone, a molecule of three oxygenatoms bound together, exists in the layer of Earth’s atmosphereknown as the stratosphere. This ozone protects life from theSun’s harmful ultraviolet radiation.

Mars Mars is the fourth inner planet from the Sun. It often iscalled the red planet. Iron oxide, the same material found inrust, exists in Mars’s weathered surface rocks, giving the planet areddish color. The Martian surface is shown in Figure 19. Therocks shown here are similar in composition to some volcanicrocks on Earth. The largest volcano in the solar system,Olympus Mons, is found on Mars.

Mars has two polar ice caps that change in size betweenMartian winter and summer. Recent data from Mars Odysseyindicate that both ice caps are made of frozen water covered bya layer of frozen carbon dioxide. Mars has two Moons, Phobos(FOH buhs) and Deimos (DI mos). Also, long channels exist onMars. The channels on Mars are hypothesized to have beencarved by flowing water sometime in the past. Mars’s atmos-phere, made up mostly of carbon dioxide with some nitrogenand argon, is much thinner than Earth’s.

Figure 19 Just as a metal toyleft outside on Earth rusts, redrocks on Mars’s surface show thatiron in the rocks has combinedwith oxygen to form iron oxide.

Planetology The spaceage has opened manynew careers. One of theseis planetology, the studyof other planets. NASAadvises those interested inthis field to start with basicchemistry, physics, andmath. Most planetary sci-entists first earn advanceddegrees in areas such asgeology, hydrology, meteor-ology, and biology and thenapply their knowledge tothe study of other planets.

Figure 18 Clouds in Venus’satmosphere are composed partlyof sulfuric acid droplets.Describe What are clouds on Earthcomposed of?

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The inner planets are small anddense and have thin atmospheres.

The outer planets are large andgaseous. Do a planet’s gravity anddistance from the Sun affect whatkinds of gases its atmosphere con-tains? Use your ability to interpreta data table to find out.

Identifying the Problem The table below lists the main

gases in the atmospheres of two innerand two outer planets. Each planet’satmosphere also contains many othergases, but these are only present insmall amounts. Looking at the table,what conclusions can you draw? Howdo you think a planet’s distance fromthe Sun and the size of the planetcontribute to the kind of atmosphereit has?

Solving the Problem 1. What gases do the atmospheres of

the inner and outer planets contain?

What is special about Earth’s atmos-phere that makes it able to supportmodern life?

2. Can you think of any reasons whythe outer planets have the gaseousatmospheres they do? Hint: Hydro-gen and Helium are the two lightestelements.


Outer Planets The outer planets are Jupiter, Saturn, Uranus, and Neptune.

They all are gaseous giant planets with dense atmospheres. Theouter planets are mainly made up of light elements such ashydrogen and helium.

Jupiter The giant planet Jupiter, shown in Figure 20, is thelargest planet in the solar system. It is the fifth planet from theSun. Jupiter’s atmosphere is made mostly of hydrogen andhelium and contains many huge storms. The largest and mostprominent of these storms is the Great Red Spot, which hasraged for more than 300 years. It is about twice the width ofEarth and rotates once every six days. With its 63 moons, Jupiteris like a miniature solar system.

Atmospheric Compositionof the Planets

Earth 78.1% nitrogen; 20.9% oxygen; traces of argon and carbon dioxide

Mars 95.3% carbon dioxide; 2.7% nitrogen; 1.6% argon; 0.13% oxygen; 0.08% carbon monoxide

Jupiter 86.1% molecular hydrogen; 13.8% helium

Uranus 82.5% molecular hydrogen; 15.2% helium; about 2.3% methane

What influences a planet’s atmosphere?


Jupiter’s Moons The four largestmoons of Jupiter are Io, Europa,

Ganymede, and Callisto. They are called the Galilean satellitesafter Galileo Galilei, who discovered them in 1610. Volcanoescontinually erupt on Io, the most volcanically active body inthe solar system. An ocean of liquid water is hypothesizedto exist beneath the cracked, frozen ice crust of Europa. Doesthis mean that life could exist on Europa? The NationalAeronautics and Space Administration (NASA) is studyinga mission to launch an orbiting spacecraft in 2008 to studythis moon.

Saturn The next outer planet is Saturn, shown in Figure 20.The gases in Saturn’s atmosphere are made up in large part ofhydrogen and helium. Saturn is the sixth planet from the Sun.It often is called the ringed planet because of its striking ringsystem. Although all of the gaseous giant planets have ringsystems, Saturn’s rings are by far the most spectacular. Saturnis known to have seven major ring divisions made up of hun-dreds of smaller rings. Each ring is made up of pieces of iceand rock.

Saturn has at least 47 moons, the largest of which is Titan.The atmosphere surrounding Titan is denser than the atmo-spheres of either Earth or Mars. The environment on Titanmight be similar to the environment on Earth before oxygenbecame a major atmospheric gas.

Saturn’s rings include seven main divisions—each of whichis composed of particles of ice and rock.

Jupiter’s atmosphere is dense because of its gravityand great distance from the Sun.

Figure 20 The four outer plan-ets—Jupiter, Saturn, Uranus, andNeptune are known also as the gasgiants.

Topic: The PlanetsVisit for Weblinks to information about theplanets.

Activity Decide which planetinterests you most and make a listof three questions you would liketo ask an astronomer about it.

green.msscience.com

(l)CORBIS, (r)Erich Karkoschka, University of AZ, and NASA



Uranus Shown in Figure 20, the seventh planet from the Sunis Uranus. The atmosphere of Uranus, made up mostly ofhydrogen, also contains helium and methane. The methanegives the planet a distinctive, bluish-green color. This is becausemethane gas reflects blue light and absorbs red light. Uranus is thought to have at least 27 moons. However, additional satel-lites might exist.

Neptune The eighth planet from the Sun is Neptune. It isthought that Neptune’s atmosphere of hydrogen, helium, andmethane gradually changes into a slushlike layer, comprised par-tially of water and other melted ices. Toward the interior, thisslushy material is thought to change into an icy solid. In turn,this icy layer may surround a central, rocky core that is about thesize of Earth.

As with Uranus, the methane in Neptune’s atmosphere givesthe planet its bluish color, as shown in Figure 20. Winds in thegaseous portion of Neptune exceed speeds of 2,400 km perhour—faster than winds on any other planet.

At least 13 natural satellites of Neptune have been discoveredso far and many more probably exist. The largest of these,Triton, has great geysers that shoot gaseous nitrogen into space.A lack of craters on Triton’s surface suggests that the surface ofTriton is fairly young.

The bluish-green color of Uranus is thought to becaused by methane in its atmosphere.

Like Uranus, the blue color of Neptune is also thoughtto be caused by methane. The Great Dark Spot was astorm system similar in size to the diameter of Earth.

Interpreting YourCreature FeatureProcedure1. Select any one of the

planets or moons in oursolar system except Earth.

2. Research its surfaceconditions.

3. Imagine a life-form thatmight have developed onyour chosen planet or moon.Be sure to indicate how yourcreature would eat, breathe,and reproduce.

Analysis1. How is this creature

protected from itsenvironment?

2. How does your creatureobtain nourishmentfor survival?

NASA/JPL

Dwarf Planets From the time of its discovery in 1930 untilAugust 2006, Pluto was known as the ninth planet in the solarsystem. In 2006, the International Astronomical Union created anew category called dwarf planets. Pluto was placed in this newcategory along with two other objects named Ceres and Eris.Pluto is shown in Figure 21 with its largest moon, Charon, andtwo smaller moons, Hydra and Nix. Scientists believe that Plutohas a small, rocky core with a frozen crust. Ceres is the smallestdwarf planet and is located in the asteroid belt. Eris is an icyobject that reaches twice as far from the Sun as Pluto when it'sin the most distant part of its orbit. Its orbit is so elliptical thatit is similar to the orbits of comets.

Other Objects in the Solar System Other objects that exist in the solar system include asteroids,

comets, and meteoroids. Asteroids are small, rocky objects thatmostly lie in a belt located between the orbits of Mars andJupiter. The asteroid belt is used by astronomers as a dividingline that separates the inner and outer planets. Jupiter’s tremen-dous gravity probably kept a planet from forming from the mat-ter contained in the asteroid belt.

Comets are made mainly of rocky particles and water ice. Astheir orbits approach the Sun, parts of comets vaporize andform tails. Comet tails, shown in Figure 22, always point awayfrom the Sun. Almost all of the solar system’s comets are locatedin the Kuiper Belt and the Oort Cloud. The Kuiper Belt islocated beyond Neptune’s orbit, and the Oort Cloud is locatedfar beyond Pluto’s orbit.

When comets break up, some of the resulting particlesremain in orbit. When asteroids collide, small pieces break off.Both of these processes produce small objects in the solar systemknown as meteoroids. If meteoroids enter Earth’s atmosphere,

they are called meteors, and when they fallto Earth, they are called meteorites.

How are meteoroidsrelated to meteorites?


Pluto

Charon

Nix

Hydra

Pluto

Nix

HydraCharon

Figure 21 Hydra and Nix aremuch smaller than Charon and areabout three times farther fromPluto than Charon is.

Figure 22 The brilliant head of a comet,called a coma, glows as its ices vaporizeupon approaching the Sun.

(t)Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility/NASA, (b)Frank Zullo/Photo Researchers


Self Check1. Explain why astronomers do not use kilometers to

measure distances in the solar system.

2. Analyze why Venus is called Earth’s twin planet.

3. Infer how 3-km-high cliffs could have formed on thesurface of Mercury.

4. Think Critically How can the composition of the plan-ets in the solar system be explained by the hypothesisdescribed on this page?

SummarySolar System

• An astronomical unit or AU is used to measuredistance in the solar system.

• The solar system includes eight planets andother objects, such as asteroids and comets.

Planets

• Mercury, Venus, Earth, and Mars are the innerplanets.

• They are similar in size to Earth.

• Jupiter, Saturn, Uranus, and Neptune are theouter planets.

Origin of the Solar System

• One hypothesis is that the Sun, planets, andother objects formed from a large cloud ofgas, ice, and dust about 5 billion years ago.

5. Calculate Ratios Research the equatorial diametersof Earth’s Moon and the four Galilean moons ofJupiter. Calculate the ratio of each moon’s diameter to that of Earth’s Moon.

Origin of the Solar System How did the solar system begin? One

hypothesis is that the Sun and all the plan-ets and other objects condensed from alarge cloud of gas, ice, and dust about 5 bil-lion years ago, as illustrated in Figure 23.This large nebula (NEB yuh luh), or cloudof material, was rotating slowly in space.Shock waves, perhaps from a nearbyexploding star, might have caused thecloud to start condensing. As it con-densed, it started rotating faster and flat-tened into a disk. Most of the condensingmaterial was pulled by gravity toward thecenter to form an early Sun. The remain-ing gas, ice, and dust in the outer areas ofthe nebula condensed, collided, and stucktogether forming planets, moons, andother components of the solar system.Conditions in the inner part of the cloudcaused small, solid planets to form,whereas conditions in the outer part werebetter for the formation of gaseous giant planets. Comets arethought to be made up of material left over from the originalcondensation of the cloud.

Figure 23 The solar system isthought to have formed from acloud of rotating gases and dustparticles.

green.msscience.com/self_check_quiz


Model and InventModel and Invent


During winter in the northern hemi-sphere, the north pole is positioned awayfrom the Sun. This causes the angle ofthe Sun’s rays striking Earth to be smallerin winter than in summer, and thereare fewer hours of sunlight. The reverseis true during the summer months. TheSun’s rays strike Earth at higher anglesthat are closer to 90°.

Real-World QuestionHow does the angle of the Sun’s rays affect Earth’s surface temperature?

Goals■ Design a model for

simulating the effect ofchanging angles of theSun’s rays on Earth’ssurface temperatures.

Materialsshallow baking pans lined

with cardboard*paper, boxes, or box lidsthermometerswood blocks*bricks or textbooksprotractorclock*stopwatch*Alternate materials

Data SourceCopy the data table intoyour Science Journal and fillit in, providing angles ofthe Sun’s rays for your area.Go to green.msscience.com tocollect this data.

Safety Precautions

Use thermometers asdirected by teacher. Donot use “shake down”lab thermometers.

WARNING: Never look dir-ectly at the Sun at any timeduring your experiment.

The Angle of the Sun’s Rays

34°79°

Verticalpole

57°

WintersolsticeDec. 22

Dallas, Texas 32°45' N 12:00 P.M.

EquinoxesMar. 21Sept. 22

SummersolsticeJune 21

34°79°

Verticalpole

57°

WintersolsticeDec. 22

Dallas, Texas 32°45' N 12:00 P.M.

EquinoxesMar. 21Sept. 22

SummersolsticeJune 21

Angle of the Sun's Raysat Noon at Your Latitude

Date Angle

December 22 (winter solstice)

January 22

February 22

March 21 (vernal equinox)

April 21

May 21

June 21 (summer solstice)

NASA/JPL


Plan the Model1. Design a model that will duplicate the angle of the

Sun’s rays during different seasons of the year.

2. Choose the materials you will need to constructyour model. Be certain to provide identical conditionsfor each angle of the Sun’s rays that you seek toduplicate.

Check the Model Plans1. Present your model design to the class in the form of

diagrams, poster, slide show, or video. Ask your class-mates how your group’s model design could beadjusted to make it more accurate.

2. Decide on a location that will provide direct sunlightand will allow your classmates to easily observe yourmodel.

Make a Model1. Create a model that demonstrates the effects different angles of the Sun’s rays

have on the temperature of Earth’s surface.

2. Demonstrate your model during the morning, when the Sun’s rays will hit theflat tray at an angle similar to the Sun’s rays during winter solstice. Measure theangle of the Sun’s rays by laying the protractor flat on the tray. Then sight theangle of the Sun’s rays with respect to the tray.

3. Tilt other trays forward to simulate the Sun’s rays striking Earth at higher anglesduring different times of the year.

Conclude and Apply1. Determine Which angle had the greatest

effect on the surface temperature of yourtrays? Which angle had the least effect?

2. Predict how each of the seasons in your areawould change if the tilt of Earth’s axis changedsuddenly from 23.5 degrees to 40 degrees.

LAB 203

Demonstrate your model for your class.Explain how your model replicated theangle of the Sun’s rays for each of the fourseasons in your area.

Wintersolstice

Equinoxes

Window Sun‘s rays

Summersolstice 79°

57°

34°

steroids have been the basis for severaldisaster movies. Can asteroids reallythreaten Earth? “Absolutely!” say manyscientists who study space. In fact, aster-

oids have hit our planet many times.Earth is scarred with about 120 recogniza-

ble craters and others may be covered by ero-sion, plant growth, and other processes.Visitors to Meteor Crater, Arizona, can see a1.2-km-wide depression caused by an asteroidthat impacted Earth about 49,000 years ago.A much older crater lies in Mexico’s YucatanPeninsula. This depression isabout 195 km wide and wascreated about 65 millionyears ago.

Some scientists believe theYucatan asteroid created agiant dust cloud that blockedthe Sun’s rays from reachingEarth for about six months andled to freezing temperatures.This would have put an endto much of Earth’s early plantlife, and may have led to the extinctionof the dinosaurs and many other species.

Can we protect ourselves from such animpact? Astronomer/geologist Eugene Shoemakerthinks so, and is responsible for alerting the worldto the dangers of asteroid impact. In 1973, he andgeologist Eleanor Helin began the first NearEarth Objects (NEO) watch at the Mount PalomarObservatory in California. But few others wereconcerned.

Then, in 1996, all that changed. An asteroid,about 0.5 km wide, came within 450,800 km of

Earth. Scientists said this was aclose call! Today, groups of sci-entists are working on creatingsystems to track NEOs. As of2003, they recorded 2,565NEOs. Of those, nearly 700were at least 1.0 km in diameter.

Don’t worry, though: thereis little chance of an asteroid

hitting Earth anytime soon.When an asteroid hits Earth’satmosphere, it is usually vapor-

ized completely. Just in case, some physicists andastronomers are thinking about ways to defendour planet from NEOs.

Meteor Crater, near Winslow,Arizona, was formed about49,000 years ago. It is about 200 m deep.

Brainstorm Working in small groups, come up with as manyways as you can to blast an asteroid to pieces or make it changecourse before hitting Earth. Present your reports to the class. For more information, visit

green.msscience.com/time

SCIENCEANDSocietySCIENCEISSUES

THAT AFFECTYOU!

Will anasteroidcollide withEarth?

(t)Kauko Helavuo/The Image Bank/Getty Images, (b)Charles & Josette Lenars/CORBIS


Copy and complete the following concept map to complete the moon phase cycle.

Earth’s Motion

1. Earth’s shape is nearly spherical.

2. Earth’s motions include rotation around itsaxis and revolution around the Sun.

3. Earth’s rotation causes day and night. Its tiltand revolution cause the seasons.

Earth’s Moon

1. Surface features on the Moon include maria,craters, and lunar highlands.

2. The Moon rotates once and revolves aroundEarth once in 27.3 days.

3. Phases of Earth’s Moon, solar eclipses, andlunar eclipses are caused by the Moon’srevolution around Earth.

4. One hypothesis concerning the origin ofEarth’s Moon is that a Mars-sized body col-lided with Earth, throwing off material thatlater condensed to form the Moon.

Our Solar System

1. The solar system includes the Sun, planets,moons, asteroids, comets, and meteoroids.

2. Planets can be classified as inner or outer.

3. Inner planets are small and rocky with thinatmospheres (or none). Outer planets aregenerally large and gaseous with thickatmospheres.

4. One hypothesis on the origin of the solarsystem states that it condensed from a largecloud of gas, ice, and dust.

CHAPTER STUDY GUIDE 205

Waxinggibbous

Waxingcrescent

New Moon

Third quarter

green.msscience.com/interactive_tutorLick Observatory


Fill in the blanks with the correct word or words.

1. A(n) is a chunk of rock that cir-cles the sun between the orbits of Mars andJupiter.

2. The motion that describes Earth’s orbitaround the Sun is called .

3. The times when the Sun reaches its greatestdistance north or south of the equator iscalled a(n) .

4. When Earth passes between the Sun and theMoon, a(n) can result.

5. The changing views of the Moon seen fromEarth are called .

Choose the word or phrase that best answers thequestion.

6. Which motion refers to Earth’s spinning onits axis?A) rotationB) waxingC) revolutionD) waning

7. What occurs twice each year whenEarth’s tilt is neither toward nor awayfrom the Sun?A) orbit C) solsticeB) equinox D) axis

8. What is the imaginary line around whichEarth spins?A) orbit C) solsticeB) equinox D) axis

9. Which moon surface features probablyformed when lava flows filled large basins?A) maria C) highlandsB) craters D) volcanoes

10. Meteorites that strike the Moon’s surfacecause which surface feature?A) maria C) highlandsB) craters D) volcanoes

11. How long is the Moon’s period ofrevolution?A) 27.3 hours C) 27.3 daysB) 29.5 hours D) 29.5 days

12. How long does it take for the Moon torotate once on its axis?A) 27.3 hours C) 27.3 daysB) 29.5 hours D) 29.5 days

Use the illustration below to answer question 13.

13. What could result from the alignmentshown above?A) lunar eclipse C) full moonB) solar eclipse D) waxing crescent

14. Which planet is most like Earth in sizeand mass?A) Mercury C) SaturnB) Mars D) Venus

15. Europa is a satellite of which planet?A) Uranus C) JupiterB) Saturn D) Mars

Sun

Moon

Earth

206 CHAPTER REVIEW

asteroid p. 200astronomical unit p. 194axis p. 180comet p. 200crater p. 185equinox p. 183lunar eclipse p. 190moon phase p. 187

nebula p. 201orbit p. 181revolution p. 181rotation p. 180solar eclipse p. 189solar system p. 194solstice p. 182

green.msscience.com/vocabulary_puzzlemaker


CHAPTER REVIEW 207

Use the table below to answer question 25.

25. How Giant Are They? Using the data in the tableabove calculate the ratios between the diame-ter of Earth and each of the gas giants.

26. Jupiter’s Orbit The orbit of Jupiter lies about4.2 AU from that of Earth. Express this distancein kilometers.

27. Martian Mass The mass of Mars is about 0.11times that of Earth. Assuming that the mass ofEarth is 5.98 � 1024 kg, calculate the mass ofMars.

Some Planet Diameters

Planet Kilometers (km)

Earth 12,700

Jupiter 143,000

Saturn 120,000

Uranus 50,800

Neptune 50,450

16. Compare and contrast the inner and outerplanets.

17. Explain why more maria are found on thenear side of the Moon than on the far side.

18. Explain why scientists hypothesize that lifemight exist on Europa.

19. Describe the Sun’s positions relative to theequator during winter and summer sol-stices and equinox.

20. Classify A new planet is found circling theSun, and you are given the job of classify-ing it. The new planet has a thick, denseatmosphere and no apparent solid surface.It lies beyond the orbit of Pluto. Howwould you classify this newly discoveredplanet?

21. Make and Use Tables Copy and complete thetable of outer planets. Show how manysatellites each planet has and what gasesare found in each planet’s atmosphere.

22. Sequence the phases of the moon startingand ending with the new moon phase, andexplain why we can see only these lightedportions of the Moon. Consider the factthat the Sun lights one half of the Moon atall times.

23. Observe the moons of Jupiter. You can seethe four Galilean moons using binocularsof at least 7� power. Check the newspa-per, almanac, or other reference to seewhether Jupiter is visible in your area andat what times. Depending on where theyare in their orbits and how clear a night itis, you may be able to see all of them.

24. Display Design a chart showing the eightplanets, using the appropriate colors foreach. Include rings and moons and anyother characteristics mentioned in thischapter.

Planetary Facts

Planet Number of Major

Satellites Atmospheric Gases

Jupiter

Saturn

Uranus

Neptune

green.msscience.com/chapter_review

Do not write in this book.


Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

Use the illustration below to answer questions 1 and 2.

1. At which point in Earth’s orbit does sum-mer begin in the southern hemisphere?A. A C. CB. B D. D

2. Which two positions indicate equinoxes,when day and night lengths are nearlyequal all over the world?A. A and C C. B and DB. B and C D. A and D

3. Which of these instruments monitorsmoonquakes to yield information aboutthe Moon’s interior?A. telescope C. seismographB. spectroscope D. centrifuge

4. Which of these is referred to as a terrestrialplanet?A. Saturn C. NeptuneB. Uranus D. Venus


5. Which letter corresponds to the moonphase waning crescent?A. D C. BB. H D. F

6. During which moon phase could a solareclipse occur?A. A C. CB. E D. G

7. Which of these planets has a large, perma-nent storm in its atmosphere known asthe Great Red Spot?A. Saturn C. JupiterB. Venus D. Neptune

8. Between the orbits of which two planets isthe asteroid belt found?A. Earth and MarsB. Mars and JupiterC. Jupiter and SaturnD. Mercury and Venus

9. Which of the following terms is used todescribe the motion of Earth as it orbitsthe Sun?A. revolution C. rotationB. ellipse D. solstice

10. Which is a satellite of Saturn?A. Charon C. PhobosB. Europa D. Titan

208 STANDARDIZED TEST PRACTICE

B

D

C A

Consider All Choices Be sure you understand the questionbefore you read the answer choices. Make special note ofwords like NOT or EXCEPT. Read and consider all the answerchoices before you mark your answer sheet.

Question 2 Be sure that both letters represent equinoxes.

Lick Observatory

A B C D E F G H

STANDARDIZED TEST PRACTICE 209

Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

11. Explain how Earth’s shape differs some-what from a perfect sphere.

12. Explain how mass and the distance of anobject from other objects affect its gravita-tional force.

13. Why does the same side of the Moonalways face Earth?


14. During what type of eclipse might youobserve something like the image in thisphotograph? What causes the dark circlein the center of the image?

15. Identify the three solar system memberswhich line up to cause this type of eclipse.Describe the order in which they align.

16. Define the unit used to describe distancesin the solar system. Why is this unit usedrather than kilometers?

17. Compare the atmospheres of Venus,Earth, and Mars.

18. Why do Neptune and Uranus appear blueor bluish-green?

19. How can lunar craters help to determinethe relative age of a particular area on theMoon’s surface?

Record your answers on a sheet of paper.

20. Explain how the tilt of Earth’s axis causeswarmer summer than winter temperaturesin the northern hemisphere.

21. Scientists have proposed several hypothesesto explain the Moon’s origin. Choose twohypotheses and describe how they differ.

22. Describe two observations which can bemade with the unaided eye from Earththat show Earth is spherical.

Use the illustration below to answer question 23.

23. The photos show Jupiter’s moons Io andEuropa, respectively. Could life exist onthese moons? Why or why not?

24. Compare and contrast the motions ofEarth and the Moon in space.

25. Imagine your spacecraft has landed some-where on the planet Mars. Describe yourobservations of the surface.

26. Explain the differences among meteoroids,meteors, and meteorites.

27. Describe the composition of comets andexplain what happens as they come closerto the Sun.

28. Describe the asteroid belt and explainwhat may have caused it to form.

green.msscience.com/standardized_test(l)CORBIS, (c)NASA/Photo Researchers, (r)NASA/Science Source/Photo Researchers


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