KEY CONCEPT
Earth rotates on a tilted
axis and orbits the Sun.
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.
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
EXPLORE Time Zones
What time is it in Iceland right now?
PROCEDURE
1
MATERIAL
Find your location and Iceland on the map.
Identify the time zone of each.
time zone map
2 Count the number of hours between your
VOCABULARY
axis of rotation p. 708
revolution p. 709
season p. 710
equinox p. 710
solstice p. 710
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”—Earth—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 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 20: Earth, Moon, and Sun 707
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?
708 Unit 6: Space Science
MATERIALS
lamp
TIME
15 minutes
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°
orbit
23°
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.
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.
July
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—about 3%—during
a year.
Chapter 20: Earth, Moon, and Sun 709
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 711 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.
1
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.
2
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.
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.
September Equinox
December 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
710 Unit 6: Space Science
In what month does winter begin in the Southern Hemisphere?
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 20: Earth, Moon, and Sun 711
RESOURCE CENTER
CLASSZONE.COM
Learn more about
seasons.
FLORIDA
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.
Content Review
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.
reminder
Notice how the angle
of sunlight affects the
climate of a region, which
you studied in grade 6.
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?
Sun Height and Shadows
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.
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.
location on Earth
712 Unit 6: Space Science
midnight
6 A.M.
noon
6 P.M.
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.
Lengths of Days
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.
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.
reading tip
Equinox means “equal
night”—daylight and nighttime are equal in length.
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.
KEY CONCEPTS
CRITICAL THINKING
1. What causes day and night?
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?
2. What happens to Earth’s axis
of rotation as Earth orbits
the Sun?
3. How do the areas of sunlight
in the two hemispheres change
over the year?
5. Compare and Contrast
How do the average temperatures and the seasonal changes
at the equator differ from those
at the poles?
CHALLENGE
6. Infer If Earth’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: Earth, Moon, and Sun 713