The Earth--

Are there in a deser t? sections 1 Earth in Did you know the Moon exerts gravitational on Earth? On Earth, the Moon’s gravita- 2 and tional attraction is evidenced as tides in Lab Comparing the of and . Even in the , the to Intensity Moon’s influences the flexing of 3 Earth’s Moon tectonic plates. Lab Identifying the Moon’s Features and Science Journal Research to discover what Landing Sites or events are affected by the Moon’s gravitational force on Earth.

184 Richard Cummins/CORBIS Start-Up Activities

Earth, Sun, and Moon Make the following Foldable to help organize what you learn about Relative of Earth, the Moon, the Earth-Moon-Sun system. and the Sun Can you picture the relative sizes of Earth, the STEP 1 a sheet of paper vertically from Moon, and the Sun? Earth is about four side to side. Make larger than the Moon in , but the the front edge about Sun is much larger than either. The Sun’s ᎏ1ᎏ 2 inch longer than diameter is about 100 times that of Earth and the back edge. about 400 times that of the Moon. In this Lab, you’ll investigate the relative sizes of all three STEP 2 Turn lengthwise objects. and fold into thirds. 1. Get permission to draw some circles on a sidewalk or paved area with chalk. You STEP 3 Unfold and cut only the top layer could also use a stick to draw circles on a along both folds to make three tabs. dirt playing . 2. Select a scale that will enable you to draw circles that will represent each object. Hint:Using 1 cm for the Moon’s diameter is a good start. 3. Use a meterstick to draw a circle with a STEP 4 Label each tab. 1-cm diameter for the Moon. 4. Now draw two more circles to represent Sun Earth Moon Earth and the Sun. 5. Think Critically In your Science Journal, Questions As you read the chapter, write what explain how the Moon and the Sun can you learn about each body under the correct tab appear to be about the same in the of your Foldable. After you read the chapter, . Think about how things look smaller note the many ways that the three affect each other. the farther they are from you.

Preview this chapter’s content and activities at

185 Richard Cummins/CORBIS Earth in Space

Reading Guide

Review Vocabulary ■ Compare and contrast Earth’s Gravity from the Earth-Moon-Sun : curved path of one object, physical characteristics with those system directly affects what it’s like such as the Moon, around another of other . to live here on Earth. object, such as Earth ■ Explain Earth’s magnetic field. ■ Describe Earth’s movement in New Vocabulary space and how occur. • • ellipse • gravity

Figure 1 Objects fall toward Earth’s Size and Shape Earth’s center. Like most people, you are aware that Earth is round like a Infer How would apples fall from trees if Earth were shaped like a cube? ball. But can you prove that this is true? If you jump up, you know that you’ll come back down, but why is this so? What is the force that brings you down? You may have used a to tell directions, but do you know how a compass works? You will learn the answers to these questions and also about many phys- ical characteristics of Earth in this section.

Ancient Measurements Earth’s shape is similar to a sphere. A sphere is a round, three-dimensional object, the surface of which is the same distance from the center in all directions. Even ancient knew that Earth is spherical in shape. We have pictures of Earth from space that show us that it is spheri- cal, but how could astronomers from long ago have learned this? They used evidence from observations. was one of these early astronomers. He made three different observations that indicated that Earth’s shape is spher- ical. First, as shown in Figure 1, no where you are on Earth, objects fall straight down to the surface, as if they are falling toward the center of a sphere. Second, Earth’s on the Moon during a lunar is always curved. If Earth weren’t spherical, this might not always be the case. For exam- ple, a disk casts a straight-edged shadow sometimes. Finally, people in different parts of the see different above their . More specifically, the pole Polaris is lower in the sky at some locations on Earth than at others.

186 CHAPTER 7 The Earth-Moon-Sun System Everyday Evidence of Earth’s Shape What have you seen, other than pictures from space, that indicates Earth’s shape? Think about walking toward someone over a hill. First, you see the top of the person’s head, and then you can see more Topic: Comparing Earth to and more of that person. Similarly, if you sail toward a light- Other Planets house, you first see the top of the lighthouse and then see more Visit for Web links to information about how and more of it as you move over Earth’s curved surface. Earth is similar to and different You can see other evidence, too. Just like ancient from other planets in the solar astronomers, you can see for yourself that objects always fall system. straight down. Today, however, we know more about gravity. Activity Make a that lists Gravity is the attractive force between two objects that depends the similarities and differences of on the of the objects and the distance between them. , , Earth, and . Astronomers think Earth formed by the accumulation of infalling objects toward a central . released in the impacts kept the growing Earth molten. Gravity caused it to form into the most stable shape, a sphere. In this shape, the pull of gravity toward the center of the is the same in all direc- tions. If a planet is massive enough, the pull of gravity could be so strong that even tall would collapse under their own . Table 1 lists some of Earth’s other properties.

How does the pull of gravity indicate that Earth’s shape is spherical?

Table 1 Earth’s Physical Properties

Diameter (pole to pole) 12,714 km

Diameter (through ) 12,756 km

Circumference (poles) 40,008 km

Circumference (equator) 40,075 km

Mass 5.98 ϫ 1024 kg

Average 5.52 g/cm3

Average distance to the Sun 149,600,000 km

Average distance to the Moon 384,400 km

Period of 23 h, 56 min

Period of revolution 365 days, 6 h, 9 min

SECTION 1 Earth in Space 187 Earth’s Magnetic Field Earth has a magnetic field that protects us from harmful radi- Van Allen Belts The mag- ation from the Sun. Scientists hypothesize that Earth’s rotation netosphere lies above the and movement of matter in the core set up a strong magnetic outer layers of Earth’s field in and around Earth. This field resembles that surrounding . Within this a bar magnet, shown in Figure 2. Earth’s magnetic field is con- are belts of centrated at two ends of an imaginary magnetic axis running charged known as from Earth’s north magnetic pole to its magnetic pole. This the Van Allen belts. They contain thin plasma com- axis is tilted about 11.5° from Earth’s geographic axis of rotation. posed of protons (inner belt) and electrons (outer Wandering Poles The locations of Earth’s magnetic poles belt) that are trapped by change slowly over time. Large-scale movements, called polar Earth’s magnetic field. wandering, are thought to be caused by movements in Earth’s Research how the magne- and upper . The magnetic north pole is carefully tosphere protects Earth remapped periodically to pinpoint its location. from the solar and why the Van Allen belts are hazardous to astro- The An area within Earth’s magnetic field, called the nauts and . Report magnetosphere, deflects harmful coming from the your findings to the class. Sun, a stream of particles called . Some of these ejected particles from the Sun produce other charged particles in Earth’s outer atmosphere. These charged particles spiral along Earth’s magnetic field lines toward Earth’s magnetic poles. There they collide with in the atmosphere. These collisions Figure 2 Like a common bar cause the atoms to emit light. This light is called the aurora magnet, Earth also has north and borealis (northern lights) in the and the south magnetic poles. The inner aurora australis (southern lights) in the . and outer shells respectively represent the positive and nega- tive Van Allen Belts. Earth the Sun Explain why the Van Allen Belts Earth orbits the Sun at an average distance of are shaped as they are. 149,600,000 km. Its orbit, like those of all the planets, , , and many , N is shaped like an ellipse. An ellipse is an elongated, closed Van Allen belts curve with two foci. The Sun is not located at the center of the ellipse, but at one of its two foci. This means the dis- tance from Earth to the Sun varies during the . Earth is closest to the Sun—about 147 million km away— around January 3 and is far- thest from the Sun—about Magnetic axis 152 million km away— S around July 4 of each year.

188 CHAPTER 7 The Earth-Moon-Sun System Earth as a is a planet, just as Venus, Mars, and are planets. However, Earth is the only planet whose characteristics make it possible for as we know it to survive. Earth resembles Venus more than any other planet. Earth and Venus are nearly the same size, and both have that contain dioxide, although in greatly different amounts. Earth’s oceans, shown in Figure 3, absorbed much of the in Earth’s early atmosphere. Also, Venus’s atmosphere is much denser than Earth’s, with pressures as high as those encountered by submarines in Earth’s oceans at depth of 900 m. Another difference is the surface . On Earth, you can walk outside and feel how or warm it is. However, tem- Figure 3 By absorbing carbon perature on Venus is over 450°C. This high is caused dioxide, oceans protect Earth from by the large amount of carbon dioxide in Venus’s atmosphere, experiencing a which traps energy and prevents it from escaping. Think like that on Venus. about what life on Earth might be like with more carbon dioxide in the atmosphere.

What feature of Earth’s surface has led to such a difference between Earth and Venus? Although Mars is almost half the size of Earth, its surface gravitational pull is less than two-fifths that of Earth’s. Yet con- ditions there are more like those on Earth than any other planet in the . Mars may even have frozen near its surface. Mercury is very different from Earth; it has no atmos- phere, and is cratered like Earth’s moon.

Summary Self Check Earth’s Size and Shape 1. Identify two pieces of evidence that prove Earth’s • Earth has a spherical shape. spherical shape. Gravity causes very large objects in space to 2. Define the term gravity. • form . 3. Explain what produces Earth’s magnetic field. Earth’s Magnetic Field 4. Describe how Venus and Earth are similar. Earth’s magnetic field protects life on 5. Think Critically Evidence indicates that Mars once had • Earth’s surface from harmful radiation. liquid water on its surface. Discuss some possible Interaction of the solar wind with Earth’s reasons why it has none today. • magnetic field produces the aurora. Earth Orbits the Sun 6. Calculate Earth’s circumference at the equator Earth orbits the Sun in an elliptical orbit. • is 40,075 km. If it spins once each , what is the Earth and Venus are similar in size, and both speed of spinning in km/h? • have atmospheres that contain carbon dioxide.

More Section Review SECTION 1 Earth in Space 189 NASA Time and Seasons

Reading Guide

New Vocabulary ■ Calculate the time and date in Days and are measurements of • different time zones. Earth’s movements. • rotation ■ Differentiate between revolution • revolution and rotation. Review Vocabulary • ■ Discuss what causes seasons to latitude: north or • change. south of Earth’s equator •

Measuring Time on Earth People can determine the approximate time of day by determining where the Sun is in the sky. If the Sun is near an imaginary line drawn from due north to due south, it is about 12:00 . have used movements of Earth, the Moon, and the Sun to measure time for thousands of years. Around 3000 B.C., the Babylonians devised a method of timekeeping using their counting methods, which were based on 60. They noticed that the Sun appeared to take a circular path through the sky. Because their counting methods were based on 60, they divided a circle into 360 parts called degrees. The sym- bol for degree (º) was taken from their symbol for the Sun.

Earth Movements Measure Time Earth spins and makes one complete turn in about 24 , as shown in Figure 4. This Figure 4 The sunlit side of Earth spinning causes the Sun to appear to move across the sky from is day; the shadow side is . to . It takes 24 hours from when the Sun is highest in the sky (noon) until it is highest in the sky again (noon the next day). If Earth spins approximately 360° in 24 hours, then it spins through 15° in one . This led to the setting up of time zones on Earth that have the same time in minutes but vary in hours. A time zone is an area 15° wide in which the time is the same. Figure 5 shows the time zones in the U.S. Ideally, time zones should be equal in size and follow lines drawn from the north pole to the south pole. However, for convenience, time zones are modified to fit around city, , and country borders, and other key sites.

How many degrees does Earth spin in one hour?

190 CHAPTER 7 The Earth-Moon-Sun System The Date Line You can see that a problem would quickly arise if you just kept dropping back an hour earlier for each 15°. Eventually, you would come around Earth and it would be 24 hours earlier. It cannot be two different days at the same spot, so a day is added to the time at the . If it is Monday to the east of the date line, then it is the same hour on Tuesday to the west of the date line. This line is drawn down through the Pacific (around , such as New Zealand) directly opposite the Prime , the starting point for this worldwide system of measuring time. The Prime Meridian is an imaginary line drawn on Earth that passes through Greenwich, England. Time based on this method is called Coordinated (UTC). In some areas, this time is modified in so that there are more hours of in the . This is referred to as Daylight Saving Time (DST). Some areas apply local modifications to this sys- tem as well.

Figure 5 The is divided into 24 time zones. Lines of roughly determine the locations of time zone boundaries. Notice that each successive time zone to the west is one hour earlier. Think Critically If you leave Russia at 12:30 A.M.on Tuesday and fly east for one hour across the , what time and day will you arrive in Alaska?

90° 105° 120° 135° 150° 165° 180° 165° 150° 135° 120° 105° 90° 75° 60° 45° 30° 15° 0° 15° 30° 45° 60° 75° 90°



SOUTH International Date Line Date International AMERICA

Areas where standard time differs by half an hour or where a zone system is not followed

SECTION 2 Time and Seasons 191 Rotation Measures Days The spinning motion of Earth enables you to measure the passing hours of the day. Rotation is the spinning of Earth on its axis, an imaginary line drawn through Earth from its rotational north pole to its rotational south pole. The apparent movement of the Sun from noon one day until noon the next day is called a solar day. This period is a bit longer than the time it takes Earth to rotate on its axis, however. This is because while Earth rotates, it also moves in orbit around the Sun and must rotate a bit more each day to make the Sun reach noon. However, if you measure time based on when a certain star rises above the until it rises again, you will see a slightly shorter time period (23 h 56 m 4 s). This is called a sidereal day and is the true measure of the time it takes for Earth to rotate once on its axis.

Revolution Measures Years The motion of Earth around the Sun enables you to measure the passing of years. Revolution is the motion of Earth in its orbit around the Sun. Figure 6 shows Earth’s orbit around the Sun. As Earth revolves in its orbit, the Sun appears to move through the compared to the seemingly fixed positions of the stars. The time it takes for the Sun to make one complete trip through the sky in reference to the background of stars is the same amount of time it takes for Earth to complete one trip Figure 6 If the Sun were as faint around the Sun, or one . The apparent path of the as the stars at night, then you Sun during this year is called the ecliptic. Also, the ecliptic is would see it travel along an annual defined as the of Earth’s orbit around the Sun. The 12 con- path through the constellations of stellations (star patterns) through which we observe the Sun the Zodiac. moving during this year is called the zodiac, shown in Figure 6. Think Critically What causes this apparent motion of the Sun? What is the ecliptic?

Sagittarius Capricornus Libra Virgo Aquarius

Sun November January Earth Earth Leo

Aries Pisces Cancer Taurus Gemini

192 CHAPTER 7 The Earth-Moon-Sun System Why do seasons change? Recall that Earth’s orbit around the Sun is an ellipse. This means that Earth is closer to the Sun at one time than it is at other times. Is this the cause of seasonal changes on Earth? Because Earth is closest to the Sun in January, you would expect this to be the warmest . However, you know this isn’t true in the northern hemisphere; something else must be causing the change. These seasonal changes are caused by Earth’s rotation, its revolution, and the tilt of its axis. Seasons change on Earth because the number of hours of daylight each day varies and also because the angle at which sunlight strikes Earth’s surface varies at different times of the year. Earth’s axis is tilted 23.5° from a line drawn perpendicular to the plane of its orbit, or ecliptic. Because of this tilt, Earth’s north geographic pole points toward Polaris throughout the year. Later, you will learn how this tilt, along with Earth’s revo- lution, causes the seasons.

Changing Angle of Sunlight During the summer, the Sun is higher in the sky, and sunlight hits Earth’s surface at a higher angle. As the year progresses, the Sun is lower and lower in the sky, and sunlight strikes Earth’s surface at lower . When striking Earth’s surface at higher angles, approaching 90º, sun- light is more intense and warms Earth’s surface more than when it strikes the surface at lower angles. Because Earth remains tilted in the same direction as it revolves, different hemispheres are tilted toward the Sun at different times of the year. As shown in Figure 7, the hemisphere tilted toward the Sun receives sun- light at higher angles than the hemisphere tilted away from the Figure 7 The Sun’s rays strike Sun. The greater intensity of sunlight is one reason why summer Earth’s surface at higher angles in is warmer than , but it is not the only reason. Another fac- the northern hemisphere when the tor is involved. north pole is tilted toward the Sun.

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SECTION 2 Time and Seasons 193 More Hours of Daylight in Summer During the summer, the Sun is above the horizon for more hours than it is when school begins in the fall. As the year progresses, the number of hours of daylight each day becomes fewer and fewer until it reaches a minimum around December 21 for the northern hemisphere. When do you think the number of hours of day- light would be at a maximum in the northern hemisphere? This happens six later, around June 21. As shown in Figure 8, the hemisphere of Earth that is tilted toward the Sun receives more hours of daylight each day than the hemisphere tilted away from the Sun. This longer period of daylight is the second rea- son why summer is warmer than winter.

During which month does Earth’s northern hemisphere experience more hours of daylight?

Figure 8 During the for the northern hemisphere, the Sun’s rays strike Earth perpendicular at the , while the area within the circle remains in darkness. During the , the Sun’s rays are perpendicular to Earth at the , while the area within the remains in sunlight.

194 CHAPTER 7 The Earth-Moon-Sun System and Because of the tilt of Earth’s axis, the Sun’s position relative to Earth’s equator constantly changes. Most of the time, the Sun is north or south of the equator, but two times during the year, the Sun is directly over the equator. Figure 8 shows that the Sun reaches an equinox when it is Modeling the Sun’s directly above Earth’s equator, and the number of daylight hours Rays at Solstice equals the number of nighttime hours all over the world. The Procedure term equinox is derived from two words meaning “equal” and 1. Use a globe with the “night.” At that time, neither the northern nor the southern equator, the Tropic of hemisphere is tilted toward the Sun. In the northern hemi- Cancer, and the Tropic of sphere, the Sun reaches the equinox on March 20 or 21, Capricorn indicated. and the fall equinox on September 22 or 23. In the southern 2. Set up a light source, such as a flashlight or goose- hemisphere, the equinoxes are reversed. neck lamp, so light shines The solstice is the point at which the Sun reaches its greatest vertically at Earth’s equator. distance north or south of the equator the Tropic of Cancer and 3. Tilt the globe 23.5° from Tropic of Capricorn, respectively. The term solstice is derived vertical so that first the from the Sun’s name, Sol, and a word meaning “standing”; northern and then the that is, it appears to stand, or stop moving, north or south in the southern hemisphere is tilted toward the light. sky. In the northern hemisphere, the Sun reaches the summer solstice on June 21 or 22, and it reaches the winter solstice on Analysis December 21 or 22. Just the opposite is true for the southern 1. When the globe was tilted 23.5° toward and away hemisphere. When the Sun is at the summer solstice, there are from the light, what lati- more hours of daylight than during any other day of the year, tudes received vertical rays? and the Sun’s rays strike at a higher angle. When it’s at the win- 2. What areas of Earth never ter solstice, on the shortest day of the year, the most nighttime received vertical rays? hours occur and the Sun’s rays strike at the lowest angle.

Summary Self-Check Measuring Time on Earth 1. Determine how long it takes Earth to make one com- Humans use movements of Earth, the Moon, plete turn on its axis. • and the Sun to measure time. 2. Explain why each time zone contains 15° of longitude. Earth’s rotation is used to measure days. If it is 4:15 one time zone, what is the time two • time zones to the west? Earth’s revolution is used to measure years. • 3. Explain how a solar day differs from a sidereal day. The ecliptic is the Sun’s apparent yearly path • through the zodiac. 4. Compare and contrast rotation and revolution. 5. Think Critically Why does Earth’s surface become warmer in summer than it does in winter? Why do seasons change? Earth’s seasonal changes are caused by its tilt, • rotation, and revolution. 6. Use Decimals It takes Earth about 365.25 days to • Equinoxes occur when the Sun is directly over make one trip around the Sun. As it does this, Earth Earth’s equator. Solstices occur when the Sun travels 360° in its orbit. On average, how many reaches it greatest distance north or south of degrees does Earth travel each day? the equator.

More Section Review SECTION 2 Time and Seasons 195 Comparing the Angle of Sunlight to Intensity Earth is warmed differently depending on the 4. Place T-1 in the pocket and lay them on a angle at which sunlight strikes it. desktop. Turn on the lamp. Position the lamp so that light strikes the pocket at an Real-World Problem angle of 75°. 5. How can you a model the angle at which sun- Record the temperature of T-1 at 10 min light strikes Earth’s surface? and at 20 min. 6. Repeat steps 3, 4, and 5 using T-2, but aim Goals the lamp at an angle of 20°. ■ Model different angles at which sunlight strikes Earth’s surface. ■ Compare and contrast the amount of heat Conclude and Apply generated by light striking at different angles. 1. Compare and contrast the temperature readings of each thermometer. Materials 2. Infer which angle models the Sun’s position 75-W bulb in a gooseneck lamp during the summer and during the winter. alcohol thermometers (2) 3. Explain sheets of construction paper, one color (2) how changes in the angle at which protractor sunlight strikes Earth’s surface are one cause * unshaded 75-W lamp of Earth’s changing seasons. * books to change the angle of the thermometers *Alternate materials Data Table Safety Precautions Original Temperature Temperature Thermometer Temperature at 10 min at 20 min T-1 WARNING: Do not touch lamp or lightbulb T-2 without safety gloves.They stay hot after being turned off.Handle thermometers carefully.

Procedure Compile your classmates’ data. Find the 1. Copy the data table shown on this page. average temperatures—original, at 2. Label the thermometers T-1 and T-2 and 10 min, and at 20 min—for T-1 and T-2. record their temperatures in the data table. Compare and contrast your results with the 3. Fold the construction paper to form a pocket class averages. that will conceal the thermometer’s bulb.

196 CHAPTER 7 The Earth-Moon-Sun System Matt Meadows Earth’s Moon

Reading Guide

Review Vocabulary ■ Describe how tides on Earth are The Moon is our nearest neighbor : molten rock caused by the Moon. in space and affects Earth in many ■ Explain how the Moon’s phases ways. New Vocabulary depend on the relative positions of the Sun, the Moon, and Earth. • moon phase ■ Compare and contrast solar and • lunar eclipses. • ■ • Analyze what surface features of • maria the Moon reveal about its history. •

Movement of the Moon You have seen the Moon move across the sky from east to west, just like the Sun. This is an apparent movement like the Sun’s, caused by Earth’s rotation. But, the Moon actually does move in another way. If you look at the Moon each day at the same time over a period of a few days, you will see that it moves toward the east. Figure 9 The face of the “” is always facing Earth. Rotation and Revolution This eastward movement of the Explain why the same side of the Moon is an actual movement that is caused by the Moon’s rev- Moon always faces Earth. olution in its orbit. It takes 27.3 days (a sidereal month) for the Moon to revolve once around Earth and line up with the same Moon's rotation star again. Because Earth also revolves around the Sun, it takes more than two more days for the Moon to line up with Earth and the Sun again. This means that a complete cycle takes 29.5 days, known as a synodic month. Many people think the Moon does not rotate because it always keeps the same side facing Earth. This is not true. As North pole shown in Figure 9, the Moon keeps the same side facing Earth because it takes 27.3 days to rotate once on its axis—the same amount of time that it takes to revolve once around Earth. You can observe this by having a friend move the ball around you while keeping the same side of it facing you. You will see only Moon's orbit one side.

SECTION 3 Earth’s Moon 197 How does the Moon affect Earth? The Moon affects Earth in many ways, some obvious and others less so. If you have ever been to a beach for vacation, you realized that the water is not always at the same location on the beach. Sometimes the water comes farther up the beach than at other times. Have you ever placed your towel on the beach and come back later to find it wet because the tide came in? You also may have noticed that the Moon doesn’t look the same each evening. Sometimes you can see all of the side facing Earth, while at other times, you can barely see any of it. Let’s take a look at these and some other effects of the Moon on Earth.

Solve a Simple Equation

TIDES The Moon rises an average of 52.7 min later each day. If the time of high tide is known for one day, this formula can be used to determine when high tide will occur on the next day or any successive day. ϭ ϩ ϫ TN T0 N 52.7 min

In this formula, T0 is the original time of high tide on a given day and TN is the time of high tide on any successive day. N is the number of days later for which you wish to determine the time of high tide. If the tide is high at 1:00 P.M., find out what time it will be high in 7 days. IDENTIFY known values and unknown values Identify the known values: ϭ ϭ T0 original time of high tide 1:00 P.M. N ϭ the number of days later for which you wish to determine high tide ϭ 7 52.7 min ϭ how much later the Moon rises each day Identify the unknown value: ϭ TN the time of high tide on N number of days

SOLVE the problem Substitute the known values into the equation for time. ϭ ϩ ϫ ϭ ϩ TN 1:00 P.M. 7 52.7 min 1:00 P.M. 369 min ϭ ϩ ϭ TN 1:00 P.M. 6 h 09 min 7:09 P.M.

Low tide is the best time to hunt for seashells. If you see that the tide is low at noon on , when during the day will it be low on the following Sunday? For more practice problems, go to page 879 and visit Math Practice at .

198 CHAPTER 7 The Earth-Moon-Sun System Tides Think again of the beach and how the level of the rose and fell during the day. This rise and fall in is called a tide.Atide on Earth is caused by a giant wave produced by the gravitational pulls of the Sun and the Moon. This wave has a wave height of only 1 or 2 m, but it has a wavelength of thou- sands of kilometers. As the crest of this large wave approaches the shore, the level of the water in the ocean rises. This rise of sea level is called high tide. About six hours later, as the trough of the wave approaches, sea level drops, causing a low tide. Earth and the Moon both revolve around their common cen- ter of mass located about 1,700 km below Earth’s surface. Because Earth is much more massive, the Moon does most of the moving, and it seems to us as though the Moon were revolving around Earth. This , in turn, revolves around the Sun. This is why Earth, Moon, Sun are considered as a three-body system. As Earth rotates and the Moon revolves, different locations Figure 10 Earth’s tides are an on Earth’s surface pass through the high and low tides. Although example of how the Sun, Moon, the Sun is much more massive than the Moon, it also is much and Earth pull on each other and farther away. Because of this, the Moon has a greater effect on operate as a three-body system. Earth’s tides than does the Sun. However, the Sun does affect The Sun, Earth, and Moon are Earth’s tides: it can strengthen or weaken the tidal effect. When in alignment during spring tide. the Moon and the Sun pull together, when they are lined up, The Sun, Earth, and Moon form high tides are much higher and low tides are much lower. This a right angle during neap tide. is called a spring tide, as shown in Figure 10. However, when the Identify whether high tide is higher during spring tide or neap two are at right angles to each other, the high tide is not as high tide. and the low tide not as low, producing a neap tide. A What happens to the sea level at spring tide? Moon Moonlight The Moon shines because it reflects sunlight from its surface. Just as half of Earth experiences day as the other half experi- Sun Earth ences night, half of the Moon is lighted while the other half is Tidal dark. As the Moon revolves around Earth, different portions of bulge large the side facing Earth are lighted, causing the Moon’s appearance to change. Moon phases are the changing appearances of the Moon as seen from Earth. The phase you see depends on the rel- B Moon ative positions of the Moon, Earth, and the Sun.

Phases of the Moon A occurs when the Moon is between Earth and the Sun. During a new moon, the side of the Earth Moon facing away from Earth is lighted and the side of the Moon facing Earth receives no light from the Sun. The Moon is Sun in the sky, but it cannot be seen, except for a special alignment Tidal you will learn about later. bulge large

SECTION 3 Earth’s Moon 199 Waxing Phases After a new moon, the moon’s phases are said to be waxing—the lighted portion that we see appears larger each night. The first phase we see after a new moon is called the waxing . About a after a new moon, we Modeling Phases and see one-half of the Moon’s lighted side, or one-quarter of the Eclipses Moon’s surface. This phase is the first-quarter. The moon is in the waxing gibbous phase from the first Procedure quarter up until . A full moon occurs when we see all of the Moon’s lighted side. At this time, the Moon is on the side 1. Turn on a lamp with no of Earth opposite from the Sun. shade and place a small, white, plastic-foam ball on the end of a pencil. Waning Phases After a full moon, the lighted portion that 2. Stand facing the lamp and we see begins to appear smaller. The phases are said to be wan- hold the white plastic- ing. When only half of the side of the Moon facing Earth is foam ball between your lighted, the third-quarter phase occurs. The waning crescent head and the lamp. occurs before another new moon. Only a small slice of the side 3. Slowly move the ball coun- of the Moon facing Earth is lighted. terclockwise around your head and observe how The word month is derived from the same root word as Moon. much of the side of the ball The complete cycle of the Moon’s phases, shown in Figure 11, facing you is lighted at dif- takes about 29.5 days, or one synodic month. Recall that it takes ferent positions. about 27.3 days for the Moon to revolve around Earth. The dis- 4. Note positions where the crepancy between these two numbers is due to Earth’s revolution ball blocks light from the around the Sun. It takes the Moon a little over two days to “catch lamp—or moves into the shadow cast by your head. up” with Earth’s advancement around the Sun. Analysis 1. Describe what happened to the ball as it was moved around your head, and identify the moon phases at various positions. 2. At which positions 1st qtr. (phases) was the ball Sunlight blocking the light or falling Waxing gibbous Waxing crescent into shadow? At which phase(s) can eclipses occur?

Earth Full New Figure 11 When viewed from Earth’s north pole, the Moon has a counterclockwise orbit. Infer In this figure, the Sun’s rays are coming from the right. Why are the Moon’s waning phases showing Waning gibbous Waning crescent sunlight on the left? 3rd qtr. 200 CHAPTER 7 The Earth-Moon-Sun System Eclipses If you knew nothing about what the Sun is or why it produces so much light and heat, wouldn’t you be concerned if suddenly it darkened? Think of how humans from long ago must have reacted when the Moon passed in front of the Sun and the source of light and heat was blocked, as it is during an eclipse. Figure 12 The solar can In the year 585 B.C., a battle was raging between the armies be seen as a pale glow around the of the Lydians and the Persians when suddenly, the Sun was lunar disk. Sunlight shining eclipsed by the Moon. The two armies were so stunned by the through lunar valleys produces a event that they put down their weapons and stopped fighting. -ring effect. Now we understand what causes eclipses of both the Sun and the Moon. We know that for the Moon to block out the Sun, it must appear to be the same size. In fact, both the Sun and the Moon have apparent that are almost the same, about 0.5°. If it weren’t for this, a total eclipse of the Sun might never Early happen. Because the Sun is about 400 times larger than the Celestial objects were Moon, it also must be about 400 times farther from Earth for a studied by early civiliza- total solar eclipse to occur. tions. Some hypotheses Eclipses occur when Earth or the Moon temporarily blocks proposed by the sunlight from reaching the other object. Sometimes, during a new Babylonians and the early Greeks were close to reali- moon, a shadow cast by the Moon falls on Earth, causing a solar ty, but other ideas were eclipse. During a full moon, a shadow of Earth can be cast on the wrong. Research how Moon, resulting in a lunar eclipse. Eclipses can occur only when some early civilizations the Sun, the Moon, and Earth are lined up perfectly. Because the explained eclipses and Moon’s orbit is tilted about 5° from the plane of Earth’s orbit other astronomical obser- around the Sun, eclipses happen only a few times each year. vations and report your findings to your class using drawings and Solar Eclipses A solar eclipse occurs when the Moon moves diagrams. directly between the Sun and Earth and casts a shadow on part of Earth. The darkest portion of the Moon’s shadow is called the umbra. A person standing within the umbra experiences a total Figure 13 A total solar eclipse solar eclipse. As shown in Figure 12, the only portion of the Sun that appears only within the Moon’s is visible during a total eclipse is part of its atmosphere, which umbra. appears as a pearly white glow around the edge of the eclipsing Moon. This is the only time the entire disk of the new moon Area of total eclipse phase can be photographed—it appears black against the Sun. As shown in Figure 13, surrounding the umbra is a Umbra lighter shadow on Earth’s surface called the penumbra. Persons standing in the penumbra experience a partial solar eclipse. WARNING: Regardless of where you are standing, never look directly at a solar eclipse. The light can permanently damage your eyes. Penumbra

How are Earth, the Moon, and the Sun Area of partial eclipse aligned during a solar eclipse?

SECTION 3 Earth’s Moon 201 Roger Ressmeyer/CORBIS Lunar Eclipses When Earth’s shadow falls on the Moon, a lunar eclipse occurs. A lunar eclipse begins when the Moon moves into Earth’s penumbra. As the Moon continues to move, it enters Earth’s umbra, and you see a curved shadow on the Moon’s surface, as shown in Figure 14. It was this shadow that led Aristotle to conclude that Earth is spherical. When the Moon moves completely into Earth’s umbra, a total lunar eclipse occurs, as shown in Figure 15. The Moon sometimes becomes red during an eclipse because light from the Sun is scattered and refracted by Earth’s atmosphere. Longer wavelength red light is affected less than shorter wavelengths, so more red light falls on Figure 14 Sometimes during the Moon. apartial lunar eclipse, Earth’s A partial lunar eclipse occurs when only a portion of the curvature can be seen silhouetted Moon moves into Earth’s umbra. The remainder of the Moon is on the Moon. The red coloration in Earth’s penumbra and, therefore, receives some direct sun- is caused by the refraction of light. A partial lunar eclipse also occurs when the Moon is par- sunlight passing through Earth’s tially or totally within Earth’s penumbra. However, this can be atmosphere before reaching the difficult to see because some direct sunlight falls on the Moon, Moon’s surface. making it appear only slightly dimmer than usual. A total solar eclipse can occur as often as twice a year, yet most people live their entire without witnessing one. You may never see a total solar eclipse, but it is almost certain you will have a chance to see a total lunar eclipse. The reason why it is so rare to view a total solar eclipse is that only those people in the small where the Moon’s umbra strikes Earth can see one and, even then, there must be clear skies. In contrast, the opportunities to witness lunar eclipses are much more frequent, and anyone on the night side of Earth can see them.

How are Earth, the Moon, and the Sun aligned during a lunar eclipse?

Figure 15 A total lunar eclipse occurs when the Moon is entirely within Earth’s umbra. Umbra is the Latin word for “shadow.” Just as a peninsula is almost an , a Penumbra penumbra is almost a shadow. Research What does umbrella mean? Umbra

202 CHAPTER 7 The Earth-Moon-Sun System NASA Kennedy Space Center The Moon’s Surface When you look at the Moon, as shown in Figure 16, you can see many of its larger surface features. Craters, rays, mountains, and maria can easily be seen through a small or a pair of binoculars. What are these different features, how did they form, and what do they tell us about the Moon’s history and interior?

Craters, Maria, and Mountains Many depres- sions on the Moon were formed by , aster- oids, and comets, which strike the of planets and their satellites. These depressions, which are called craters, formed early in the Moon’s history. Surrounding many craters are ray patterns produced by lighter-colored material from just below the lunar surface that was blasted out on impact and settled on top of the darker surface material Figure 16 Notice the light- around the craters. During the impact, when these large basins colored material radiating outward formed, cracks may have formed in the Moon’s crust, allowing from the large crater near the base lava from the still-molten interior to reach the surface and fill in of the Moon. the basins, forming maria. Identify What are the flat, dark- Maria are the dark-colored, relatively flat on the colored regions called? What are the Moon’s surface, shown in Figure 16. The igneous rocks of the light-colored areas radiating from maria are 3 to 4 old. They are the youngest rocks the craters called? found on the Moon so far. This indicates that the craters formed after the Moon’s surface originally cooled. However, the maria formed early enough in the Moon’s history that molten rock material still remained in the Moon’s interior. Surrounding the large depressions that later filled with lava are areas that were thrown upward in the original collision and formed mountains. The largest ranges on the Moon surround the large, flat, dark-colored maria.

Regolith When NASA scientists started to plan for crewed to on the Moon, they were concerned about whether the lunar surface would be able to support the craft? To find out, unmanned Surveyor spacecraft were landed on its sur- face. One Surveyor craft actually bounced a few times as it landed on the side of a crater. What was this material that the spacecraft had landed on? Impacts on the Moon thoughout its history led to the accu- mulation of known as regolith. On some areas of the Moon, this regolith is almost 40 m thick, while in other loca- tions, it is only a few centimeters thick. Some regolith is coarse, but some is a fine dust. If you watch astronauts walking on the Moon, you will notice that they often kick up a lot of dust.

SECTION 3 Earth’s Moon 203 NASA Space Center The Moon’s Interior Upper mantlemantle mantle Upper Upper The presence of maria on Crustmantle mantle Lower Lower Lower mantleCrust Crust the Moon’s surface tells us something about its interior. If cracks did form when the large depressions were produced by impacts, and lava did flow onto the lunar surface, then the inte- Core Core Core rior of the Moon just below its surface must have been molten at that time. It is believed that this was the case and that before the Moon cooled to what it is like today, its interior separated into layers. Other information about the Moon’s interior comes from Figure 17 The Moon’s crust is seismographs left on the Moon by Apollo astronauts. Just as the thinnest on the side nearest to study of allows scientists to map Earth’s interior, Earth. the study of moonquakes helps them study the Moon’s interior Infer Why is the Moon’s crust and has led to the model shown in Figure 17. This model shows thinnest on the side facing Earth? that the Moon’s crust is about 60 km thick on the side facing Earth and about 150 km thick on the side facing away. Below the crust, a solid mantle may extend to a depth of 1,000 km. A partly molten zone of the mantle extends farther down. Below this is an -rich, solid core.

Where is the Moon’s crust thickest?

Exploring the Moon More than 20 years after the ended, the spacecraft was placed in . Clementine com- piled a detailed map of the Moon’s surface, including the South Pole-Aitken Basin. This is the oldest identifiable impact feature on the Moon’s surface. It is also the largest and deepest impact basin or Clementine Topic: and found so far anywhere in the solar system, measuring 12 Lunar Missions Visit for Web km in depth and 2,500 km in diameter. Because the angle of sun- links to information about the light is always low near the poles, much of this depression remains missions of the Clementine and in shadow throughout the Moon’s rotation. This location provides spacecraft. a cold area where deposits from impacting comets may have col- Activity Make a table that lists lected. The Clementine spacecraft, and later the Lunar Prospector, the major goals of each mission both collected evidence that supports the hypothesis that water-ice and whether or not it succeeded. has accumulated in South Pole-Aitken Basin. See Figure 18 to learn more about this.


Figure 18

stronomers long believed that the Moon was a cold, dry place without atmosphere. But a few scientists hypothesized that water could exist on the Moon under certain Aconditions. This hypothesis was proven correct in the late 1990s by data from the spacecrafts Clementine and Lunar Prospector.

HOW DID IT GET THERE? Throughout its history, the Moon has been bombarded by comets and meteorites, most of which contain water-ice. Upon impact, some of the water would have quickly vaporized and be lost to space. However, some was deposited in the bottom of deep polar craters. Temperatures in these craters never exceed about –173°C. At these temperatures, ice could persist for billions of years.

HOW MUCH IS THERE? Estimates of how much water-ice exists vary, but some estimates are as high as 6 tril- lion kg. Ice might be buried several meters below the surface, either in solid blocks or as ice crystals mixed with lunar regolith. Water is impor- tant for many reasons. It would be needed for the survival of humans at any future lunar bases. Also, it could be split using solar power into and to make rocket fuel.

SECTION 3 Earth’s Moon 205

(t)NASA/JPL/USGS, (c)NASA/CORBIS, (b)Julian Baum/Photo Researchers Mars-size body

Primitive Earth

A large, Mars-sized body collided with primitive The violent collision melted and vaporized some of Earth approximately 4.6 billion years ago. Earth’s crust and mantle and hurled it into space.

Figure 19 According to the Clementine and Lunar Prospector Data from Clementine giant impact theory, the Moon confirmed that the crust on the side facing Earth is much thin- formed after Earth was struck a ner than on the far side. Data also found that the crust thins glancing blow by a body that was under impact basins and showed the location of mascons, con- more massive than Mars. For mil- centrations of mass that are located under impact basins. What lions of years after the Moon’s do you think they might be? Clementine also provided informa- birth, stray rock fragments ejected tion on the mineral content of Moon rocks. In fact, this part of by the original impact continued to its mission explains the spacecraft’s name. Clementine was the pelt its surface, creating the craters daughter of a miner in the ballad “My Darlin’ Clementine.” that now blanket the Moon. What are mascons? In 1998, the Lunar Prospector spacecraft orbited the Moon, taking photographs of the lunar surface. Maps made using these photographs confirmed the Clementine data. Also, data from Lunar Prospector confirmed that the Moon has a small, iron-rich core about 600 km in diameter. Lunar Prospector also conducted a detailed study of the Moon’s surface searching for clues as to its origin and structure. Prior to the data obtained from the Apollo space missions, there were three theories about the Moon’s origin. The first was that the Moon was captured by Earth’s gravity (the capture the- ory). It had formed elsewhere and wandered near Earth. The sec- ond theory was that the Moon condensed from the same loose material that Earth formed from during the early formation of the solar system (the binary theory). The third theory was that a glob of molten material was ejected from Earth while Earth was still molten (the fission theory). Ironically, the goal of one Apollo mission was to help determine which of these theo- ries was correct. Instead, the mission showed that none of the three theories can explain the Moon’s composition.

206 CHAPTER 7 The Earth-Moon-Sun System Some material fell back to Earth, some escaped Solid particles eventually condensed from the into interplanetary space, and some orbited Earth cooling gas and the Moon began to accumulate. as a ring of hot gas and debris.

Giant Impact Theory Data gathered by the Apollo missions led many scientists to form a new giant impact theory, which has gained wide acceptance among astronomers. According to this theory, the Moon formed about 4.6 billion years ago when a Mars- sized object collided with Earth. After colliding, the cores of the two bodies combined and settled toward the center of the larger object. Gas and other debris were thrown into orbit. Some fell back to Earth, but the remainder condensed into a large mass, forming the Moon. This sequence is shown in Figure 19. This the- ory helps to explain how the Moon and Earth are similar, yet not similar enough to have formed from the same condensing mass. If the core of the Mars-sized body was added to the core of Earth, this explains why the Moon’s composition is like Earth’s mantle and why the Moon has a much smaller central core than expected.

Summary Self Check How does the Moon affect Earth? 1. Compare and contrast solar and lunar eclipses. • The Moon and the Sun affect Earth’s tides. 2. Explain tides in Earth’s oceans. As the Moon revolves, different amounts of 3. Diagram the positions of Earth, the Moon, and the Sun • the side facing Earth are lighted by the Sun, during a full moon. causing the changing phases. 4. Describe how lunar maria might have formed. • The alignment of Earth, the Moon, and the 5. Think Critically Why is it so important to future space Sun produces eclipses. if the Moon has water-ice near its surface? The Moon’s Surface and Interior The Moon’s surface has craters, mountains, • and maria. 6. Calculate An estimate of the amount of water frozen Maria are the dark-colored, relatively flat at the Moon’s south pole is 100,000 m3. If this deposit • regions on the Moon. was spread over an area measuring 160 m by 125 m, The Clementine and Lunar Prospector found how many meters deep would the deposit be? • evidence of water-ice on the Moon.

More Section Review SECTION 3 Earth’s Moon 207 Identifying the M##n’s Surface Features and APOLLO Landing Sites

Goals Real-World Problem ■ Identify prominent When you look at a full moon in the , you can see light and surface features on the dark areas. When you look through binoculars or a small telescope, Moon. you can see many craters and the large, dark-colored maria. Many ■ Determine the rela- craters are named after great philosophers and scientists. The maria tive ages of features on are named for what early scientists thought they saw there; for exam- the Moon’s surface. ple, Oceanus Procellarum means “ocean of .” Can you tell the ■ Locate the Apollo difference between an old crater and one that has formed more landing sites on the recently? If craters are seen on a maria, which of the features is older? Moon. If one crater partially covers another, which one formed first? Materials large-scale maps or Procedure of the Moon 1. Obtain a large-scale map or globe of the Moon. individual, smaller maps 2. Familiarize yourself with some of the more prominent surface fea- of the Moon tures of the Moon.

208 CHAPTER 7 The Earth-Moon-Sun System Bettmann/CORBIS 3. Look for examples of younger craters (those with sharp sides and peaks in the center) and examples of older craters (those whose sides are worn down or missing). 4. Using a large-scale, labeled map or globe of the Moon, locate, identify, and label the following prominent surface features of the Moon and Apollo landing sites on a copy of the Moon’s surface. (If an unlabeled Moon map is not avail- able, draw one and illustrate and label the features listed below.)

Features on the Moon Maria Craters Mountain Ranges Apollo Landing Sites Mare Crisium Alphonsus Alps 11-Mare Tranquillitatus Mare Frigoris Aristarchus Apennine 12-Oceanus Procellarum Mare Imbrium Clavius 14-Fra Mauro Mare Serenitatis Copernicus Jura 15-Mt. Hadley Mare Tranquillitatus Fra Mauro Mt. Hadley 16-Descartes Oceanus Procellarum Grimaldi 17-Taurus Littrow Kepler Plato Tycho

Analyze Your Data 1. Describe the specific lunar features you studied and what you learned from them. 2. Identify one specific observation that helped you decide which of two features formed first.

Conclude and Apply 1. Infer from your study of the large-scale map or globe whether Copernicus Crater or Grimaldi Crater is older. Do the same for Fra Mauro Crater and Tycho Crater. Explain your answers. 2. Research the Apollo missions and explain Compare your map of the Moon with the why there is no landing site for . maps labeled by other students in your class. Discuss why individual maps may be labeled differently or why map illustrations might look different.


“If I have been able to see further, it was Even only because I stood Great on the shoulders of .” Scientists —Sir Isaac Newton Make Mistakes

oday, scientists know that sometimes lem this dispersal of light causes. Newton light behaves like a wave and at other thought the opposite was true. He tried Ttimes it behaves like a . However, to eliminate chromatic by chang- early scientists believed it had to be one way ing or adding lenses and using prisms, but or the other. Sir Isaac Newton, 1642–1727, was unable to correct the problem. believed in the particle of light. Based Another mistake made by Newton was on his observations and a few erroneous that he assumed light particles begin to assumptions, he eventually invented what is refract before they contact matter. Because of now called the Newtonian reflecting tele- these assumptions, he concluded that there scope. was no way to fix the problem. So, he gave up on refracting and used a Where did Newton go wrong? curved mirror to light. This worked, One erroneous assumption involved the because light does not disperse when it is behavior of light as it passes through matter reflected. His incorrect assumptions led to such as glass, a property known as chromatic the invention of the most-used instrument in aberration. White light is composed of many , the reflecting telescope. different wavelengths that can disperse when Sometimes the greatest discoveries in sci- refracted. This also happens with telescopes ence are based on incorrect assumptions. because lenses act But, that’s okay—it is called science. like a combination of many prisms Newtonian Telescope and disperse white Eye piece Primary mirror light into the col- ors of the rainbow. We now know that Incoming light the larger the lens, the less of a prob- Flat secondary mirror

Try it yourself Experiment with shining light through a prism and reflecting it from a mirror. Try using several For more information, visit different prisms with different angles.

(t)Bill Sanderson/Photo Researchers, (b)Ronald Royer/Science Photo Library Earth in Space 4. The Sun reaches its greatest distance north of the equator on the summer solstice for 1. The fact that Earth always casts a curved the northern hemisphere and on the winter shadow, as shown here, is evidence of its solstice for the southern hemisphere. spherical shape. 2. When Earth was molten, Earth’s Moon the force of gravity pulling equally in all directions 1. Tides in Earth’s oceans are affected by the caused Earth to form into a gravity of the Moon and the Sun. The spherical shape. Moon’s gravity has a greater effect because 3. Earth’s rotation and movement of matter in the Moon is much closer to Earth. its core cause Earth’s magnetic field. 2. Changing positions of the Moon in relation 4. Earth is different than other planets. These to the Sun cause it to go through a lunar differences enable life to flourish on Earth. phase cycle every 29.5 days. 3. Solar eclipses occur during a new moon, Time and Seasons and lunar eclipses occur during a full moon. 1. Earth’s rotation and revolution are used to 4. Craters, like this one, are depressions on the measure time in days and years. Moon’s surface. Some large depressions 2. During the year, the Sun appears to move may have filled with lava, forming maria. on the ecliptic through a background of constellations called the zodiac. 3. Earth experiences changing seasons because locations on Earth receive sunlight for varying amounts of time each day and at varying angles throughout the year, as shown here.

Use the foldable that you made at the begin- ning of this chapter to review the Earth-Moon-Sun system.

Interactive Tutor CHAPTER STUDY GUIDE 211 (t)NASA Kennedy Space Center, (b)NASA/CORBIS 12. During winter solstice in the northern hemisphere, the Sun is directly over which ecliptic p. 192 revolution p. 192 part of Earth? ellipse p. 188 rotation p. 192 A) equator equinox p. 195 solar eclipse p. 201 B) pole gravity p. 187 solstice p. 195 lunar eclipse p. 202 sphere p. 186 C) Tropic of Cancer maria p. 203 tide p. 199 D) Tropic of Capricorn moon phase p. 199 time zone p. 190 regolith p. 203 13. Which movement causes lunar phases? A) Earth’s revolution Match the correct vocabulary word or phrase B) Earth’s rotation with each definition given below. C) the Moon’s revolution D) the Moon’s rotation 1. dark-colored, relatively flat areas on the Moon 14. Which eclipse do you experience if you are 2. Earth spinning on its axis standing in the Moon’s umbra? 3. a large wave in Earth’s oceans caused by A) partial lunar C) total lunar the gravity of the Moon and the Sun B) partial solar D) total solar

4. a round, three-dimensional object, the sur- 15. Which phase occurs when the Moon is on face of which is the same distance from the the opposite side of Earth from the Sun? center in all directions A) B) C) D) 5. Earth moving in orbit around the Sun 6. eclipse that occurs during a new moon 7. 15°-wide area on Earth’s surface in which the time is the same 16. Which material may have been found on the Moon by the Clementine spacecraft? 8. occurs when the Sun is directly above A) atmosphere Earth’s equator B) dark-colored rocks 9. attractive force between two objects C) light-colored rocks 10. yearly path of Earth around the Sun D) water-ice

17. On average, how many degrees of longi- tude are contained in one time zone? A) 0.5° C) 23.5° Choose the word or phrase that best answers B) 15° D) 30° each question. 11. How long is a month of lunar phases? 18. Which occurs a few days after a full moon? A) 14 days A) waning crescent B) 27.3 days B) waxing crescent C) 29.5 days C) waning gibbous D) 365 days D) waxing gibbous

212 CHAPTER REVIEW Vocabulary PuzzleMaker 19. Which begins around December 21 25. Explain why a lunar base would best in Australia? be built on a that is always in A) spring C) fall sunlight. B) summer D) winter 26. Form a hypothesis about why during crescent 20. Which forms when small meteorites crash phases we can often see a dim of into the Moon? the rest of that side of the Moon. Hint: A) craters C) mountains Recall the arrangement of Earth, the Moon, B) maria D) cracks and the Sun during crescent phases. 27. Form a hypothesis about how the thickness of Interpreting Graphics the Moon’s crust might play a part in the fact that the side of the Moon facing Earth 21. Make an illustration showing how magnetic has more maria than the side facing away. force lines surrounding Earth are similar to those surrounding a bar magnet. Use the data in the table below to answer question 22. Use the illustration below to answer questions Earth’s Physical Properties 28–29. Diameter (pole to pole) 12,714 km Diameter (through equator) 12,756 km Near side crust Far side crust Circumference (poles) 40,008 km (approx. 65 (approx. 150 km thick) km thick) Circumference (equator) 40,075 km Average distance to the Sun 149,600,000 km Average distance to the Moon 384,400 km

To Earth 22. What is the difference between the diame- ter of Earth through the poles compared to the equator? How many times farther from Earth is the Sun, compared to the Moon? 28. Model to Scale If you are making a scale model of the Moon (diameter approx. 3,500 km), what scale should 23. Infer why more craters are present on the you use to obtain a model that is about Moon’s surface than on Earth’s. Hint: 35 cm in diameter? Consider gravity and the presence of an 29. Calculate Using your scale, what would atmosphere. the thicknesses be, in centimeters, for 24. Infer how seasons would be affected if the near-side crust and the far-side Earth had no tilt instead of the 23.5° tilt crust? that it has.

More Chapter Review CHAPTER REVIEW 213 Record your answers on the answer sheet provided by your teacher or on a sheet of paper.

4. Which is a way that Venus and Earth are similar? A. atmospheric density Use the illustration below to answer question 1. B. liquid water oceans C. rocky nature D. surface temperature

Use the illustration below to answer question 5.

Sun November January Earth Earth 1. Which theory explaining the Moon’s origin is widely accepted by astronomers? A. binary accretion theory B. capture theory 5. Which is a group of constellations through which the Sun appears to move? C. fission theory A. ecliptic D. giant impact theory B. equinox 2. How far is Earth’s magnetic axis tilted from C. solstice its geographic axis? D. zodiac A. 5° B. 11.5° 6. Which contains bands of charged particles known as the Van Allen belts? C. 15° A. D. 23.5° B. 3. On which number did the Babylonians base C. magnetosphere their counting methods? D. A. 10 B. 60

C. 100 Caution Read each question carefully for full understanding. D. 360

214 STANDARDIZED TEST PRACTICE 7. During which month of the year is Earth farthest from the Sun? A. January 11. B. April What may have caused the great difference in the percentage of CO2 found in Earth’s C. July atmosphere compared to those of Venus D. September and Mars? 12. What causes the ?

8. If a synodic month is 29.5 days long and a sidereal month is 27.3 days long, how Use the illustration below to answer question 13. much longer is a synodic month?

Use the illustration below to answer question 9.

165º 150º 135º 120º 105º 90º 75º 60º 45º

San Francisco

New York

9. If it is 9:00 A.M. in New York city, what time is it in ? 10. If the collision of two planetary-sized 13. PART A The Lunar Prospector discovered objects (Earth and another object) formed possible concentrations of water- the Moon, why is the Moon’s iron core so ice in the area of the South Pole- small compared to Earth’s? Aitken Basin on the Moon. How does location and geographic ter- affect the possibility of find- ing water there?

PART B How might this discovery affect the future of ?