Sunshine StateSTANDARDSSC.E.1.3.1: The studentunderstands the vastsize of our SolarSystem and the rela-tionship of the planetsand their satellites. SC.H.1.3.5: The studentknows that a change inone or more variablesmay alter the outcomeof an investigation.SC.H.2.3.1: The studentrecognizes that pat-terns exist within andacross systems.

Chapter 20: Earth, Moon, and Sun 707

VOCABULARYaxis of rotation p. 708revolution p. 709season p. 710equinox p. 710solstice p. 710

BEFORE, you learned

• Stars seem to rise, cross the sky,and set because Earth turns

• The Sun is very large and farfrom Earth

• Earth orbits the Sun

NOW, you will learn

• Why Earth has day and night• How the changing angles of

sunlight produce seasons

KEY CONCEPT

Earth rotates on a tiltedaxis and orbits the Sun.

EXPLORE Time Zones

What time is it in Iceland right now?

PROCEDURE

Find your location and Iceland on the map.Identify the time zone of each.

Count the number of hours between your location and Iceland. Add or subtract that number of hours from the time on your clock.

WHAT DO YOU THINK?• By how much is Iceland’s time earlier or later

than yours?• Why are clocks set to different times?

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Earth’s rotation causes day and night.When astronauts explored the Moon, they felt the Moon’s gravitypulling them down. Their usual “down”—Earth—was up in theMoon’s sky.

As you read this book, it is easy to tell which way is down. But isdown in the same direction for a person on the other side of Earth? If you both pointed down, you would be pointing toward each other.Earth’s gravity pulls objects toward the center of Earth. No matter whereyou stand on Earth, the direction of down will be toward Earth’s center.There is no bottom or top. Up is out toward space, and down is towardthe center of the planet.

As Earth turns, so do you. You keep the same position with respect to what is below your feet, but the view above your head changes.

check your reading In what direction does gravity pull objects near Earth?

MATERIALtime zone map

The directions north, south, east, and west are based on the way theplanet rotates, or turns. Earth rotates around an imaginary line runningthrough its center called an The ends of the axis arethe north and south poles. Any location on the surface moves from west

to east as Earth turns. If youextend your right thumb andpretend its tip is the North Pole,then your fingers curve the wayEarth rotates.

At any one time, about halfof Earth is in sunlight and halfis dark. However, Earth turns onits axis in 24 hours, so locationsmove through the light anddarkness in that time. When alocation is in sunlight, it is day-

time there. When a location is in the middle of the sunlit side, it isnoon. When a location is in darkness, it is night there, and when thelocation is in the middle of the unlit side, it is midnight.

check your reading If it is noon at one location, what time is it at a location directlyon the other side of Earth?

axis of rotation.

708 Unit 6: Space Science

What causes day and night?In this model the lamp represents the Sun, and your head represents Earth. The North Pole is at the top of your head. You will need to imagine locations on your head as if your head were a globe.

PROCEDURE

Face the lamp and hold your hands to your face as shown in the photograph. Your hands mark the horizon. For a person located at your nose, the Sunwould be high in the sky. It would be noon.

Face away from the lamp. Determine what time it would be at your nose.

Turn to your left until you see the lamp along your left hand.

Continue turning to the left, through noon, until you just stop seeing the lamp.

WHAT DO YOU THINK?• What times was it at your nose in steps 2, 3, and 4?

• When you face the lamp, what time is it at your right ear?

CHALLENGE How can a cloud be bright even when it is dark on the ground?

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RotationRotationSKILL FOCUSMaking models

MATERIALSlamp

TIME15 minutes

mid

nig

ht

no

on

night moves westward

The globe and the flatmap show the progressof daylight across Earthin two ways. This locationis experiencing sunrise.

Earth’s tilted axis and orbit cause seasons.Just as gravity causes objects near Earth to be pulled toward Earth’scenter, it also causes Earth and other objects near the Sun to be pulledtoward the Sun’s center. Fortunately, Earth does not move straight into the Sun. Earth moves sideways, at nearly a right angle to the Sun’s direction. Without the Sun’s gravitational pull, Earth would keep moving in a straight line out into deep space. However, the Sun’s pull changes Earth’s path from a straight line to a round orbitabout 300 million kilometers (200,000,000 mi) across.

Just as a day is the time it takes Earth to rotate once on its axis,a year is the time it takes Earth to orbit the Sun once. In astronomy,a is the motion of one object around another. The wordrevolution can also mean the time it takes an object to go around once.

Earth’s rotation and orbit do not quite line up. If they did, Earth’sequator would be in the same plane as Earth’s orbit, like a tiny hoopand a huge hoop lying on the same tabletop. Instead, Earth rotates atabout a 23˚ angle, or tilt, from this lined-up position.

As Earth moves, its axis always points in the same direction in space.You could model Earth’s orbit by moving your right fist in a circle ona desktop. You would need to point your thumb toward your leftshoulder and keep it pointing that way while moving your handaround the desktop.

Earth’s orbit is not quite a perfect circle. In January, Earth is about5 million kilometers closer to the Sun than it is in July. You may besurprised to learn that this distance makes only a tiny difference intemperatures on Earth. However, the combination of Earth’s motionaround the Sun with the tilt of Earth’s axis does cause importantchanges of temperature. Turn the page to find out how.

revolution

Chapter 20: Earth, Moon, and Sun 709

Earth’s orbit is almost a circle. Earth’s distancefrom the Sun varies byonly about 5,000,000km—about 3%—duringa year.

reading tip

Use the second vowel ineach word to help youremember that an objectrotates on its own axis, but revolves around another object.

23°

23°

orbit

Earth’s axis points in a constant directionas Earth orbits the Sun. Earth is tilted23˚ from its orbit.

Not to scale

Not to scale

Use your thumb to represent the North Pole.Keep it steady as you move your hand in acounterclockwise circle on a tabletop.

153,000,000 km

148,000,000 km

July

January

710 Unit 6: Space Science

Seasonal PatternsMost locations on Earth experience patterns of temperaturechanges and other weather trends over the course of a year. Near theequator, the temperatures are almost the same year-round. Near thepoles, there are very large changes in temperatures from winter tosummer. The temperature changes occur because the amount ofsunlight at each location changes during the year. The changes in the amount of sunlight are due to the tilt of Earth’s axis.

Look at the diagram on page 711 to see how the constant direction ofEarth’s tilted axis affects the pattern of sunlight on Earth at differenttimes of the year. As Earth travels around the Sun, the area of sunlight ineach hemisphere changes. At an (EE-kwuh-NAHKS), sunlightshines equally on the northern and southern hemispheres. Half of eachhemisphere is lit, and half is in darkness. As Earth moves along its orbit,the light shifts more into one hemisphere than the other. At a (SAHL-stihs), the area of sunlight is at a maximum in one hemisphereand a minimum in the other hemisphere. Equinoxes and solstices hap-pen on or around the 21st days of certain months of the year.

September Equinox When Earth is in this position, sunlight shinesequally on the two hemispheres. You can see in the diagram that theNorth Pole is at the border between light and dark. The Septemberequinox marks the beginning of autumn in the Northern Hemi-sphere and of spring in the Southern Hemisphere.

December Solstice Three months later, Earth has traveled a quarterof the way around the Sun, but its axis still points in the samedirection into space. The North Pole seems to lean away from thedirection of the Sun. The solstice occurs when the pole leans as faraway from the Sun as it will during the year. You can see that theNorth Pole is in complete darkness. At the same time, the oppositeis true in the Southern Hemisphere. The South Pole seems to leantoward the Sun and is in sunlight. It is the Southern Hemisphere’ssummer solstice and the Northern Hemisphere’s winter solstice.

March Equinox After another quarter of its orbit, Earth reachesanother equinox. Half of each hemisphere is lit, and the sunlight iscentered on the equator. You can see that the poles are again at theborder between day and night.

June Solstice This position is opposite the December solstice.Earth’s axis still points in the same direction, but now the North Poleseems to lean toward the Sun and is in sunlight. The June solsticemarks the beginning of summer in the Northern Hemisphere.In contrast, it is the winter solstice in the Southern Hemisphere.

check your reading In what month does winter begin in the Southern Hemisphere?

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solstice

equinox

seasons,

reading tip

The positions and lightingcan be hard to imagine, soyou might use a model aswell as the diagram on the next page to help youunderstand.

VOCABULARY Remember to put eachnew term into a framegame diagram.

Chapter 20: Earth, Moon, and Sun 711

Seasons

Earth’s orbit and steady, tilted axis produce seasons.

The equinoxes and solstices mark the beginnings of seasons in the two hemispheres. Warmer seasons occur when more of a hemisphere is in sunlight.

If you could stand on the Sun and look at Earth, you would see different parts of Earth at different times of year.

September Equinox1

September Equinox Half of the sun-light is in each hemisphere. The strongestsunlight is on the equator.

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December Solstice Less than half ofthe Northern Hemisphere is in sunlight. Thestrongest sunlight is south of the equator, sothe Southern Hemisphere grows warmer.

2

March Equinox Half of the sunlight isin each hemisphere. The strongest sunlightis on the equator.

3

June Solstice More than half of theNorthern Hemisphere is in sunlight. Thestrongest sunlight is north of the equator,so the Northern Hemisphere grows warmer.

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View from the Sun

Look at the poles to help you see how each hemisphere is lit. When is the South Pole completely in sunlight?

Not to scale

spring

fall

summer

winter

winter

summer

fall

spring

December Solstice2 March Equinox3 June Solstice4

Angles of SunlightYou have seen that seasons change as sunlight shifts between hemi-spheres during the year. On the ground, you notice the effects ofseasons because the angle of sunlight and the length of daylightchange over the year. The effects are greatest at locations far from theequator. You may have noticed that sunshine seems barely warm justbefore sunset, when the Sun is low in the sky. At noon the sunshineseems much hotter. The angle of light affects the temperature.

When the Sun is high in the sky, sunlight strikes the ground at closeto a right angle. The energy of sunlight is concentrated. Shadows areshort. You may get a sunburn quickly when the Sun is at a high angle.When the Sun is low in the sky, sunlight strikes the ground at a slant.The light is spread over a greater area, so it is less concentrated andproduces long shadows. Slanted light warms the ground less.

Near the equator, the noonday Sun is almost overhead every day,so the ground is warmed strongly year-round. In the middle latitudes,the noon Sun is high in the sky only during part of the year. In winterthe noon Sun is low and warms the ground less strongly.

check your reading How are temperatures throughout the year affected by theangles of sunlight?

712 Unit 6: Space Science

RESOURCE CENTERCLASSZONE.COM

Learn more about seasons.

Sun Height and Shadows

Winter Solstice, 12 P.M. Spring Equinox, 12 P.M. Summer Solstice, 12 P.M.

Winter shadows are longbecause sunlight is spread out.The Sun appearslow in the skyeven at noon.

Summer shadows are shortbecause the light is concentratedin a small area. Thenoon Sun appearshigh in the sky.

Spring and fall shadows are ofmedium length, and the noonSun appears higherin the sky.

location on Earth

reminder

Notice how the angle of sunlight affects the climate of a region, whichyou studied in grade 6.

Content ReviewFLORIDA

Chapter 20: Earth, Moon, and Sun 713

KEY CONCEPTS1. What causes day and night?

2. What happens to Earth’s axisof rotation as Earth orbits the Sun?

3. How do the areas of sunlightin the two hemispheres changeover the year?

CRITICAL THINKING4. Apply If you wanted to enjoy

longer periods of daylight inthe summertime, would youhead closer to the equator orfarther from it? Why?

5. Compare and ContrastHow do the average tempera-tures and the seasonal changesat the equator differ from thoseat the poles?

CHALLENGE6. Infer If Earth’s axis were tilted

so much that the North Polesometimes pointed straight atthe Sun, how would the hoursof daylight be affected at your location?

reading tip

Equinox means “equalnight”—daylight and night-time are equal in length.

Lengths of DaysSeasonal temperatures depend on the amount of daylight, too. InChicago, for example, the summer Sun heats the ground for about 15 hours a day, but in winter there may be only 9 hours of sunlighteach day. The farther you get from the equator, the more extreme the changes in day length become. As you near one of the poles,summer daylight may last for 20 hours or more.

Very close to the poles, the Sun does not set at all for six months at atime. It can be seen shining near the horizon at midnight. Tourists oftentravel far north just to experience the midnight Sun. At locations neara pole, the Sun sets on an equinox and then does not rise again for sixmonths. Astronomers go to the South Pole in March to take advantageof the long winter night, which allows them to study objects in the skywithout the interruption of daylight.

Very near the equator, the periods of daylight and darkness arealmost equal year-round—each about 12 hours long. Visitors who areused to hot weather during long summer days might be surprised whena hot, sunny day ends suddenly at 6 P.M. At locations away from theequator, daylight lasts 12 hours only around the time of an equinox.

Near the pole in the sum-mer, the Sun stays abovethe horizon, so there is no night. This series ofphotographs was takenover the course of a day.

6 P.M.noon6 A.M.midnight