Earth rotates on a tilted axis and orbits the Sun.

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KEY CONCEPT

Earth rotates on a tilted

axis and orbits the Sun.

BEFORE, you learned

NOW, you will learn

? Stars seem to rise, cross the sky,

and set because Earth turns

? The Sun is very large and far

from Earth

? Earth orbits the Sun

? Why Earth has day and night

? How the changing angles of

sunlight produce seasons

VOCABULARY

EXPLORE Time Zones

axis of rotation p. 44

revolution p. 45

season p. 46

equinox p. 46

solstice p. 46

What time is it in Iceland right now?

PROCEDURE

1

MATERIAL

time zone map

Find your location and Iceland on the map.

Identify the time zone of each.

2 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?

Earth¡¯s rotation causes day and night.

When astronauts explored the Moon, they felt the Moon¡¯s gravity

pulling them down. Their usual ¡°down¡±¡ªEarth¡ªwas up in the

Moon¡¯s sky.

As you read this book, it is easy to tell which way is down. But is

down 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 where

you 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 toward

the 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?

Chapter 2: Earth, Moon, and Sun 43

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noon

The globe and the flat

map show the progress

of daylight across Earth

in two ways. This location

is experiencing sunrise.

midnight

The directions north, south, east, and west are based on the way the

planet rotates, or turns. Earth rotates around an imaginary line running

through its center called an axis of rotation. The ends of the axis are

the north and south poles. Any location on the surface moves from west

to east as Earth turns. If you

extend your right thumb and

pretend its tip is the North Pole,

then your fingers curve the way

Earth rotates.

At any one time, about half

of Earth is in sunlight and half

night moves

is dark. However, Earth turns on

westward

its axis in 24 hours, so locations

move through the light and

darkness in that time. When a

location is in sunlight, it is daytime there. When a location is in the middle of the sunlit side, it is

noon. When a location is in darkness, it is night there, and when the

location 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 directly

on the other side of Earth?

Rotation

What causes day and night?

SKILL FOCUS

Making models

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

1

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 Sun

would be high in the sky. It would be noon.

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

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

4 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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44 Unit: Space Science

MATERIALS

lamp

TIME

15 minutes

Page 3 of 7

Earth¡¯s tilted axis and orbit cause seasons.

Just as gravity causes objects near Earth to be pulled toward Earth¡¯s

center, it also causes Earth and other objects near the Sun to be pulled

toward 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 orbit

about 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 revolution is the motion of one object around another. The word

revolution 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¡¯s

equator would be in the same plane as Earth¡¯s orbit, like a tiny hoop

and a huge hoop lying on the same tabletop. Instead, Earth rotates at

about a 23? angle, or tilt, from this lined-up position.

reading tip

Use the second vowel in

each word to help you

remember that an object

rotates on its own axis,

but revolves around

another object.

Not to scale

23¡ã

orbit

23¡ã

Earth¡¯s axis points in a constant direction

as Earth orbits the Sun. Earth is tilted

23? from its orbit.

Use your thumb to represent the North Pole.

Keep it steady as you move your hand in a

counterclockwise circle on a tabletop.

July

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 on

a desktop. You would need to point your thumb toward your left

shoulder and keep it pointing that way while moving your hand

around the desktop.

Earth¡¯s orbit is not quite a perfect circle. In January, Earth is about

5 million kilometers closer to the Sun than it is in July. You may be

surprised to learn that this distance makes only a tiny difference in

temperatures on Earth. However, the combination of Earth¡¯s motion

around the Sun with the tilt of Earth¡¯s axis does cause important

changes of temperature. Turn the page to find out how.

153,000,000 km

148,000,000 km

January

Not to scale

Earth¡¯s orbit is almost a

circle. Earth¡¯s distance

from the Sun varies by

only about 5,000,000

km¡ªabout 3%¡ªduring

a year.

Chapter 2: Earth, Moon, and Sun 45

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Page 4 of 7

VOCABULARY

Remember to put each

new term into a frame

game diagram.

Seasonal Patterns

Most locations on Earth experience seasons, patterns of temperature

changes and other weather trends over the course of a year. Near the

equator, the temperatures are almost the same year-round. Near the

poles, there are very large changes in temperatures from winter to

summer. The temperature changes occur because the amount of

sunlight 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 47 to see how the constant direction of

Earth¡¯s tilted axis affects the pattern of sunlight on Earth at different

times of the year. As Earth travels around the Sun, the area of sunlight in

each hemisphere changes. At an equinox (EE-kwuh-NAHKS), sunlight

shines equally on the northern and southern hemispheres. Half of each

hemisphere 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 solstice

(SAHL-stihs), the area of sunlight is at a maximum in one hemisphere

and a minimum in the other hemisphere. Equinoxes and solstices

happen on or around the 21st days of certain months of the year.

When Earth is in this position, sunlight shines

equally on the two hemispheres. You can see in the diagram that the

North Pole is at the border between light and dark. The September

equinox marks the beginning of autumn in the Northern Hemisphere and of spring in the Southern Hemisphere.

1

September Equinox

2

December Solstice

3

After another quarter of its orbit, Earth reaches

another equinox. Half of each hemisphere is lit, and the sunlight is

centered on the equator. You can see that the poles are again at the

border between day and night.

4

June Solstice

reading tip

The positions and lighting

can be hard to imagine, so

you might use a model as

well as the diagram on the

next page to help you

understand.

Three months later, Earth has traveled a quarter

of the way around the Sun, but its axis still points in the same

direction into space. The North Pole seems to lean away from the

direction of the Sun. The solstice occurs when the pole leans as far

away from the Sun as it will during the year. You can see that the

North Pole is in complete darkness. At the same time, the opposite

is true in the Southern Hemisphere. The South Pole seems to lean

toward the Sun and is in sunlight. It is the Southern Hemisphere¡¯s

summer solstice and the Northern Hemisphere¡¯s winter solstice.

March Equinox

This position is opposite the December solstice.

Earth¡¯s axis still points in the same direction, but now the North Pole

seems to lean toward the Sun and is in sunlight. The June solstice

marks the beginning of summer in the Northern Hemisphere.

In contrast, it is the winter solstice in the Southern Hemisphere.

check your reading

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46 Unit: Space Science

In what month does winter begin in the Southern Hemisphere?

Page 5 of 7

Seasons

Earth¡¯s orbit and steady, tilted axis produce seasons.

1

4

June Solstice More than half of the

Northern Hemisphere is in sunlight. The

strongest sunlight is north of the equator,

so the Northern Hemisphere grows warmer.

September Equinox Half of the sunlight is in each hemisphere. The strongest

sunlight is on the equator.

2

December Solstice Less than half of

the Northern Hemisphere is in sunlight. The

strongest sunlight is south of the equator, so

the Southern Hemisphere grows warmer.

3

Not to scale

March Equinox Half of the sunlight is

in each hemisphere. The strongest sunlight

is on the equator.

View from the Sun

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

at different times of year.

fall

winter

spring

summer

spring

summer

fall

winter

1 September Equinox

2 December Solstice

3 March Equinox

4 June Solstice

The equinoxes and solstices mark the beginnings of seasons in the two hemispheres.

Warmer seasons occur when more of a hemisphere is in sunlight.

Look at the poles to help you see how each hemisphere is lit.

When is the South Pole completely in sunlight?

Chapter 2: Earth, Moon, and Sun 47

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