Earth rotates on a tilted axis and orbits the Sun. - ClassZone

Sunshine State
STANDARDS
SC.E.1.3.1: The student
understands the vast
size of our Solar
System and the relationship
of the planets
and their satellites.
SC.H.1.3.5: The student
knows that a change in
one or more variables
may alter the outcome
of an investigation.
SC.H.2.3.1: The student
recognizes that patterns
exist within and
across systems.
VOCABULARY
axis of rotation p. 708
revolution p. 709
season p. 710
equinox p. 710
solstice p. 710
KEY CONCEPT
<strong>Earth</strong> <strong>rotates</strong> on a tilted
axis and orbits the Sun.
BEFORE, you learned
• Stars seem to rise, cross the sky,
and set because <strong>Earth</strong> turns
• The Sun is very large and far
from <strong>Earth</strong>
• <strong>Earth</strong> orbits the Sun
EXPLORE Time Zones
NOW, you will learn
• Why <strong>Earth</strong> has day and night
• How the changing angles of
sunlight produce seasons
What time is it in Iceland right now?
PROCEDURE
1
2
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?
MATERIAL
time zone map
<strong>Earth</strong>’s rotation causes day and night.
When astronauts explored the Moon, they felt the Moon’s gravity
pulling them down. Their usual “down”—<strong>Earth</strong>—was 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 <strong>Earth</strong>?
If you both pointed down, you would be pointing toward each other.
<strong>Earth</strong>’s gravity pulls objects toward the center of <strong>Earth</strong>. No matter where
you stand on <strong>Earth</strong>, the direction of down will be toward <strong>Earth</strong>’s center.
There is no bottom or top. Up is out toward space, and down is toward
the center of the planet.
As <strong>Earth</strong> 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 <strong>Earth</strong>?
Chapter 20: <strong>Earth</strong>, Moon, and Sun 707

The globe and the flat
map show the progress
of daylight across <strong>Earth</strong>
in two ways. This location
is experiencing sunrise.
What causes day and night?
708 Unit 6: Space Science
The directions north, south, east, and west are based on the way the
planet <strong>rotates</strong>, or turns. <strong>Earth</strong> <strong>rotates</strong> 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 <strong>Earth</strong> turns. If you
extend your right thumb and
pretend its tip is the North Pole,
then your fingers curve the way
<strong>Earth</strong> <strong>rotates</strong>.
At any one time, about half
of <strong>Earth</strong> is in sunlight and half
is dark. However, <strong>Earth</strong> turns on
its axis in 24 hours, so locations
move through the light and
darkness in that time. When a
location is in sunlight, it is day-
time 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 <strong>Earth</strong>?
In this model the lamp represents the Sun, and your head represents <strong>Earth</strong>.
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
2
3
4
noon
night moves
westward
Rotation
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.
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?
midnight
SKILL FOCUS
Making models
MATERIALS
lamp
TIME
15 minutes

<strong>Earth</strong>’s tilted axis and orbit cause seasons.
Just as gravity causes objects near <strong>Earth</strong> to be pulled toward <strong>Earth</strong>’s
center, it also causes <strong>Earth</strong> and other objects near the Sun to be pulled
toward the Sun’s center. Fortunately, <strong>Earth</strong> does not move straight
into the Sun. <strong>Earth</strong> moves sideways, at nearly a right angle to the
Sun’s direction. Without the Sun’s gravitational pull, <strong>Earth</strong> would
keep moving in a straight line out into deep space. However, the
Sun’s pull changes <strong>Earth</strong>’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 <strong>Earth</strong> to rotate once on its axis,
a year is the time it takes <strong>Earth</strong> 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.
<strong>Earth</strong>’s rotation and orbit do not quite line up. If they did, <strong>Earth</strong>’s
equator would be in the same plane as <strong>Earth</strong>’s orbit, like a tiny hoop
and a huge hoop lying on the same tabletop. Instead, <strong>Earth</strong> <strong>rotates</strong> at
about a 23˚ angle, or tilt, from this lined-up position.
orbit
23°
23°
<strong>Earth</strong>’s axis points in a constant direction
as <strong>Earth</strong> orbits the Sun. <strong>Earth</strong> 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.
Not to scale
As <strong>Earth</strong> moves, its axis always points in the same direction in space.
You could model <strong>Earth</strong>’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.
<strong>Earth</strong>’s orbit is not quite a perfect circle. In January, <strong>Earth</strong> 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 <strong>Earth</strong>. However, the combination of <strong>Earth</strong>’s motion
around the Sun with the tilt of <strong>Earth</strong>’s axis does cause important
changes of temperature. Turn the page to find out how.
reading tip
Use the second vowel in
each word to help you
remember that an object
<strong>rotates</strong> on its own axis,
but revolves around
another object.
January
153,000,000 km
148,000,000 km
July
Not to scale
<strong>Earth</strong>’s orbit is almost a
circle. <strong>Earth</strong>’s distance
from the Sun varies by
only about 5,000,000
km—about 3%—during
a year.
Chapter 20: <strong>Earth</strong>, Moon, and Sun 709

VOCABULARY
Remember to put each
new term into a frame
game diagram.
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.
710 Unit 6: Space Science
Seasonal Patterns
Most locations on <strong>Earth</strong> 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 <strong>Earth</strong>’s axis.
Look at the diagram on page 711 to see how the constant direction of
<strong>Earth</strong>’s tilted axis affects the pattern of sunlight on <strong>Earth</strong> at different
times of the year. As <strong>Earth</strong> 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 <strong>Earth</strong> 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 hemisphere
and a minimum in the other hemisphere. Equinoxes and solstices happen
on or around the 21st days of certain months of the year.
1
2
3
4
solstice
September Equinox When <strong>Earth</strong> 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.
December Solstice Three months later, <strong>Earth</strong> 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 After another quarter of its orbit, <strong>Earth</strong> 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.
June Solstice This position is opposite the December solstice.
<strong>Earth</strong>’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 In what month does winter begin in the Southern Hemisphere?

Seasons
<strong>Earth</strong>’s orbit and steady, tilted axis produce seasons.
1
2
September Equinox Half of the sunlight
is in each hemisphere. The strongest
sunlight is on the equator.
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.
View from the Sun
If you could stand on the Sun and look at <strong>Earth</strong>, you would see different parts of <strong>Earth</strong>
at different times of year.
1
fall
spring
September Equinox
winter
summer
Not to scale
spring
The equinoxes and solstices mark the beginnings of seasons in the two hemispheres.
Warmer seasons occur when more of a hemisphere is in sunlight.
fall
Look at the poles to help you see how each hemisphere is lit.
When is the South Pole completely in sunlight?
4
3
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.
March Equinox Half of the sunlight is
in each hemisphere. The strongest sunlight
is on the equator.
summer
winter
2 December Solstice 3 March Equinox 4
June Solstice
Chapter 20: <strong>Earth</strong>, Moon, and Sun 711

RESOURCE CENTER
CLASSZONE.COM
Learn more about
seasons.
FLORIDA
Content Review
reminder
Notice how the angle
of sunlight affects the
climate of a region, which
you studied in grade 6.
Sun Height and Shadows
712 Unit 6: Space Science
Angles of Sunlight
You have seen that seasons change as sunlight shifts between hemispheres
during the year. On the ground, you notice the effects of
seasons because the angle of sunlight and the length of daylight
change over the year. The effects are greatest at locations far from the
equator. You may have noticed that sunshine seems barely warm just
before sunset, when the Sun is low in the sky. At noon the sunshine
seems much hotter. The angle of light affects the temperature.
When the Sun is high in the sky, sunlight strikes the ground at close
to a right angle. The energy of sunlight is concentrated. Shadows are
short. 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 and
produces 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 winter
the noon Sun is low and warms the ground less strongly.
check your reading How are temperatures throughout the year affected by the
angles of sunlight?
Winter Solstice, 12 P.M. Spring Equinox, 12 P.M. Summer Solstice, 12 P.M.
Winter shadows are long
because sunlight is spread out.
The Sun appears
low in the sky
even at noon.
location on <strong>Earth</strong>
Spring and fall shadows are of
medium length, and the noon
Sun appears higher
in the sky.
Summer shadows are short
because the light is concentrated
in a small area. The
noon Sun appears
high in the sky.

KEY CONCEPTS
midnight
Lengths of Days
1. What causes day and night?
2. What happens to <strong>Earth</strong>’s axis
of rotation as <strong>Earth</strong> orbits
the Sun?
3. How do the areas of sunlight
in the two hemispheres change
over the year?
6 A.M.
Seasonal temperatures depend on the amount of daylight, too. In
Chicago, for example, the summer Sun heats the ground for about
15 hours a day, but in winter there may be only 9 hours of sunlight
each 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.
CRITICAL THINKING
4. Apply If you wanted to enjoy
longer periods of daylight in
the summertime, would you
head closer to the equator or
farther from it? Why?
5. Compare and Contrast
How do the average temperatures
and the seasonal changes
at the equator differ from those
at the poles?
noon
Very close to the poles, the Sun does not set at all for six months at a
time. It can be seen shining near the horizon at midnight. Tourists often
travel far north just to experience the midnight Sun. At locations near
a pole, the Sun sets on an equinox and then does not rise again for six
months. Astronomers go to the South Pole in March to take advantage
of the long winter night, which allows them to study objects in the sky
without the interruption of daylight.
Very near the equator, the periods of daylight and darkness are
almost equal year-round—each about 12 hours long. Visitors who are
used to hot weather during long summer days might be surprised when
a hot, sunny day ends suddenly at 6 P.M.At locations away from the
equator, daylight lasts 12 hours only around the time of an equinox.
Near the pole in the summer,
the Sun stays above
the horizon, so there is
no night. This series of
photographs was taken
over the course of a day.
reading tip
6 P.M.
Equinox means “equal
night”—daylight and nighttime
are equal in length.
CHALLENGE
6. Infer If <strong>Earth</strong>’s axis were tilted
so much that the North Pole
sometimes pointed straight at
the Sun, how would the hours
of daylight be affected at
your location?
Chapter 20: <strong>Earth</strong>, Moon, and Sun 713