Download Lesson 2

Lesson 2
Rotation and
Revolution
Earth has circled around the Sun for about
4.6 billion years. What has kept Earth in its
path around the Sun for so long?
422
ENGAGE
ESS.41 Explain why the Moon, Sun, and stars appear to move from east to west
across the sky (ESS-M-C1) ESS.44 Explain rotation and revolution by using models or
illustrations (ESS-M-C4)
Materials
What keeps Earth moving
around the Sun?
Form a Hypothesis
If you let go of a ball being swung in a circle, in
what direction will the ball travel? Write your
answer in the form “If I let go of a ball being swung
in a circle at a particular point, then...”
• tennis ball
• fabric square
Test Your Hypothesis
• string
Make a Model Place the tennis ball on the
• graph paper
fabric and bring the four corners of the fabric
together so they cover the ball. Tie string
around the four corners to form a pouch.
Step
Be Careful. While holding the other end of
the string, lean forward and slowly spin the
ball in a circle near your feet.
Observe Let go of the string. Watch the
path that the ball takes.
Draw a diagram to show the path the ball
took when you let it go.
Repeat the experiment, letting go of the ball
at three different spots on the circle. Where
does the ball go?
Step
Draw Conclusions
Did the experiment support your hypothesis?
Why or why not?
If this activity models the movement of Earth
around the Sun, what do you, the ball, and
the string represent?
Explore
More
What results would you expect if you repeated this
experiment using a lighter ball? Form a hypothesis,
do the experiment, record your data, and write
a report.
SI.12 Use data and information gathered to develop an
e
explanation of experimental results (SI-M-A4)
SI.14 Develop models to illustrate or explain conclusions
reached through investigation (SI-M-A5)
Also covers SI.15, SI.19
423
EXPLORE
What is gravity?
▶ Essential Question
What happens as Earth
moves around the Sun?
ESS.41, ESS.44
▶ Vocabulary
gravity, p. 424
orbit, p. 425
inertia, p. 425
rotation, p. 426
revolution, p. 428
▶ Reading Skill
Fact and Opinion
Fact
Opinion
▶ Technology
e-Glossary and e-Review online
at www.macmillanmh.com
Each planet in the solar system is drawn toward the
Sun by gravity. Gravity is a force of attraction, or pull,
between any two objects. The strength of the pull of
gravity is affected by the total mass of the two objects
and by the distances between them. The pull of gravity
decreases when the total mass of the two objects
decreases and when the objects are farther apart.
Compare the pull of gravity you feel on Earth
to the pull of gravity you would feel on the Moon.
Your mass stays the same no matter where you are.
Earth’s mass is greater than the Moon’s mass. This
means that the total mass of you and Earth is greater
than the total mass of you and the Moon. The pull of
gravity between you and Earth is stronger than the
pull between you and the Moon. In fact, the Moon’s
gravity is about one-sixth of Earth’s gravity.
Two objects do not have to touch each other to
produce a force of gravity between them. The pull of
gravity between Earth and the Sun acts across about
150 million km (93 million mi) of space. Gravity
also acts across roughly 6 billion km (4 billion mi) of
space between Pluto and the Sun. Since the distance
is farther between Pluto and the Sun, the pull of
gravity between them is weaker than the pull of
gravity between Earth and the Sun.
In this photo, you
can see the height
of astronaut John
Young’s jump on the
Moon. He can jump
higher on the Moon
than on Earth because
the Moon’s gravity
is about one-sixth of
Earth’s gravity.
424
EXPLAIN
Gravity and Inertia
Gravity and inertia
together make Earth
follow this path.
Inertia alone would move
Earth in a straight line.
Sun
Gravity alone would
pull Earth into the Sun.
Read a Diagram
Which force pulls Earth
toward the Sun?
Clue: Find the arrow that
points at the Sun.
The planets are held in their orbits
around the Sun by the force of gravity
between each planet and the Sun.
An orbit is the path one object takes
around another object.
If gravity was the only force acting
on a planet, the planet would be pulled
into the Sun. What prevents this from
happening? All objects have a property
called inertia (ih•NUR•shuh). Inertia
is the tendency of a moving object to
keep moving in a straight line.
As Earth orbits the Sun, it is pulled
toward the Sun because of gravity. At
the same time, Earth’s inertia makes it
move away from the Sun. As a result of
the effects of gravity and inertia, Earth
moves in a nearly circular orbit called
an ellipse.
The distance between Earth and
the Sun varies. When Earth is closest
to the Sun, it is about 147 million km
(91 million mi) away. When Earth is
farthest from the Sun, it is about
152 million km (94 million mi) away.
This 5 million km (3 million mi)
difference shows that Earth’s orbit is
close to, but not quite, a perfect circle.
Quick Check
Fact and Opinion Astronauts can
jump higher on the Moon than on
Earth. Is this a fact or an opinion?
Critical Thinking In what direction
would the planets travel if the Sun
suddenly disappeared? Explain.
425
EXPLAIN
What causes day and night?
You probably feel like you are sitting
still as you read this page, but you are
actually spinning in a circle at about
1,600 km/h (1,000 mph) as Earth
rotates. One rotation is a complete spin
on the axis. Earth makes one rotation
every day, or 24 hours.
At any point in time, half of Earth’s
surface faces the Sun and is in daylight.
The other half of Earth’s surface faces
away from the Sun and is in darkness.
The tilt of Earth’s axis affects the
length of the day. If the axis was not
tilted, day and night would each be
12 hours long. Instead, there are more
hours of daylight and fewer hours of
darkness during the summer. In winter,
the amount of daylight is shorter.
Shouldn’t you feel movement as
Earth rotates? You don’t feel these
motions because you are carried along
with Earth. It is as if you had your eyes
closed as you sped down a perfectly
smooth highway in a car. As you sat in
the car, you would not be able to tell
that you were moving.
If you watch objects in the sky,
such as the Sun, they appear to rise
in the east and set in the west. This is
the apparent motion of these objects,
not their real motion. As Earth rotates
from west to east, objects in the sky
such as the Sun, Moon, and stars
all appear to move in the direction
opposite Earth’s movement.
Earth’s Rotation
Earth rotates
from west to east.
dark side
(night)
sunlight
sunrise
light side
(day)
axis
426
EXPLAIN
8 P.M.
U.S. Standard Time Zones
6 P.M. 5 P.M. 4 P.M.
7 P.M.
3 P.M.
2 P.M.
9 P.M.
1 P.M.
10 P.M.
Earth rotates about 360 degrees in
24 hours, or 15 degrees per hour. U.S.
states are in one of six time zones.
International
Date Line
11 P.M.
12
midnight
12
noon
11 A.M.
Prime
Meridian
10 A.M.
Alaska
Standard Time
(AST)
1 A.M.
9 A.M.
2 A.M.
8 A.M.
3 A.M.
4 A.M.
5 A.M. 6 A.M.
7 A.M.
(HAST)
Denver
Pacific
Mountain
Standard Time Standard Time
(PST)
(MST)
Central
Standard Time
(CST)
Eastern
Standard Time
(EST)
Hawaii-Aleutian
Standard Time
(HAST)
Miami
Read a Map
Standard Time Zones
When the Sun is at its highest over
your town, it is midday. However, it is
not midday everywhere else in the world
at that same time. Earth rotates toward
the east at a rate of about 360 degrees
every 24 hours, or 15 degrees per hour.
For this reason, we separate Earth into
24 zones known as standard time zones.
A standard time zone is a vertical belt,
about 15 degrees wide in longitude, in
which all locations have the same time.
There is a 1-hour difference between
adjacent time zones. If you crossed one
time zone going east, you would need
to set your clock ahead 1 hour. If you
traveled east across 24 time zones, you
would return to the time zone in which
you started, except that the date on
your watch would be 1 day ahead of
what it should be. Why?
If it is noon in Denver, what
time is it in Miami?
Clue: Determine which time
zone each of the cities is in.
The date would be wrong because
you would have set your watch ahead
1 hour a total of 24 times during
the trip. To prevent this problem,
the International Date Line, a line
at a longitude of 180 degrees, was
established. West of this International
Date Line, it is 1 calendar day later
than it is in places east of the line.
Quick Check
Fact and Opinion The Sun rises in
the east and sets in the west. Is this
a fact or an opinion? Explain.
Critical Thinking What would
happen if you traveled west across
the International Date Line?
427
EXPLAIN
What causes seasons?
Earth travels around the Sun at a
speed of 30 kilometers per second
(19 miles per second). Earth’s orbit is
about 924 million km (574 million mi)
long. How long does it take Earth to
make one revolution? A revolution
is one complete trip around the Sun.
Earth makes this trip in one year, or in
365 41 days.
During a year, you observe seasons
changing on Earth. As Earth revolves
around the Sun, sunlight strikes different
parts of Earth at different angles.
The greater the angle, the more intense
the Sun’s rays. The Sun’s rays are
most intense when the Sun is directly
overhead. As the angle increases, the
air on Earth becomes warmer. These
changes in the angle of the sunlight
cause the seasons.
The angle at which sunlight hits Earth
changes during a year because Earth’s
axis is tilted about 23 degrees. An axis
is a straight line about which an object
rotates. Earth’s axis is an imaginary line
that runs through Earth between the
North Pole and the South Pole.
How Seasons Change in the Northern Hemisphere During a Year
spring begins
summer begins
Sun
Read a Diagram
In the beginning of summer, which
hemisphere receives more sunlight?
Clue: Find the diagram of Earth when
summer begins.
428
EXPLAIN
autumn begins
As Earth revolves around the Sun,
the tilted axis always points in the
same direction. When the Northern
Hemisphere is tilted away from the Sun,
the ground does not receive much heat
energy and temperatures are low. In the
Northern Hemisphere, this is winter.
At the same time, it is summer
in the Southern Hemisphere. The
Southern Hemisphere is angled toward
the Sun, so the heat energy of the
sunlight is more concentrated. The
ground receives more heat energy, and
temperatures are warmer.
Because the tilt of Earth’s axis
always points in the same direction, the
seasons in the Northern Hemisphere
and the Southern Hemisphere are
always opposite. In spring and fall, both
hemispheres receive equal warmth from
the Sun, making mild temperatures.
Rotation and Revolution SI.14
Make a Model Work in groups of
three students. Student 1 represents
the Sun, student 2 represents Earth,
and student 3 represents the Moon.
Student 1 should stand still, holding
a flashlight that remains turned on.
Student 2 should spin slowly like
a top. Then student 2 should walk
around student 1 while continuing to
spin.
Be Careful. If you become
dizzy while spinning, stop right away.
Student 3 should quickly walk
around student 2 in such a way as
to be always facing student 2.
Observe Describe how the light
from the flashlight falls on students
2 and 3.
winter begins
Quick Check
Fact and Opinion Write a fact about
why Earth’s seasons change.
Critical Thinking When the season
Earth is closer to the Sun
in January than in July.
in the Northern Hemisphere is fall,
what season is it in the Southern
Hemisphere?
429
EXPLAIN
Why do the stars seem
to move?
The patterns of stars in the night
sky always look the same, but the
constellations appear to change
position during the night and from
season to season.
Throughout the night, the
constellations in the northern sky circle
around Polaris. They appear to move
because Earth is rotating on its axis.
As Earth rotates, the stars appear to
complete one full circle in the sky.
Some constellations are visible
all year long, while others are not.
The constellations do not change
their position. Earth’s position in
its path around the Sun changes. A
constellation we face at night in winter
we face in the daytime in summer.
For example, Orion is only visible
in the winter in the Northern
Hemisphere. During the summer, the
daytime side of Earth is facing Orion.
During summer in the Northern
Hemisphere, the constellation called
Lyra, the harp, becomes visible. Other
constellations you can see on a
summer night in the Northern
Hemisphere include Hercules, Cygnus,
and Scorpius.
Quick Check
Fact and Opinion Give one
fact and one opinion about the
constellations.
Critical Thinking If you were on
another planet, would you see the
same constellations you see from
Earth? Explain.
Seasonal Constellations
Pegasus
autumn
summer
Orion
winter
spring
430
EXPLAIN
Leo
Lyra
Visual Summary
An object stays in orbit
because of gravity and
inertia.
Think, Talk, and Write
Vocabulary The tendency of a moving
object to keep moving in a straight line
is called
.
Fact and Opinion Earth’s day is
Because of Earth’s
rotation the Sun, stars,
and Moon appear to
move from east to west
across the sky.
24 hours long. Is this statement a fact
or an opinion? Explain.
Fact
Opinion
Critical Thinking Do all locations on
Seasons change
because of Earth’s
revolution around
the Sun and because
Earth’s axis is tilted
about 23 degrees.
Make a
Study Guide
Make a three-tab book.
Use the titles shown.
On the inside of each
tab, write a fact about
the title.
Earth experience night at the same
time? Explain.
Test Prep Which keep Earth in its orbit
around the Sun?
lift and pressure
thrust and inertia
gravity and inertia
pressure and gravity
A
B
C
D
Test Prep How much time does Earth
take to complete one revolution?
A one day
B one week
C one month
D one year
Essential Question What happens as
Earth moves around the Sun?
Math Link
Health Link
Calculating Earth’s Mass
Weightlessness
The Sun’s mass is roughly 330,000
times Earth’s mass. If you made a model
of the Sun with a mass that was 1,000
kilograms, what would Earth’s mass be?
Research and write about how human
beings in space are affected by the lack
of gravity and how astronauts deal with
these effects.
-Review Summaries and quizzes online at www.macmillanmh.com
431
EVALUATE
Inquiry Skill:
Use Numbers
When scientists use numbers, they add,
subtract, multiply, divide, count, or put numbers
in order to explain and analyze data.
The orbits of each planet in the solar system
have different radii. This means each planet
takes a different amount of time to revolve
around the Sun. As the radius of the planet’s
orbit increases, the revolution time increases.
What would your age be if you lived on a
different planet?
▶ Learn It
The diagram of the planets shows the time each
planet takes to revolve around the Sun in Earth
days or years. Scientists use numbers to compare
the revolution time of the other planets in our solar
system to that of Earth. You can do that by dividing
the revolution time of a planet by the revolution
time of Earth.
For example, it takes Earth about 365 days to
travel around the Sun. Mars takes about 687 days
to complete its revolution. If you divide the time it
takes Mars to make a revolution
by the time it takes Earth
to make a revolution,
you get 1.88. Mars takes
almost twice as long as
Earth to complete one
revolution.
If you were 62 years
old in Earth years, how
old would you be in Mars
years? The ratio of Mars’s
revolution to Earth’s is
1.88. Divide your age by
the Earth-planet ratio to
calculate your age on a
specific planet.
432
EXTEND
Neptune
60,198.50
Mercury
87.96
Sun
Jupiter
4,331.98
Mars
Earth
Venus
224.68
365.25
686.98
Uranus
30,707.41
Saturn
10,760.56
▶ Try It
▶ Apply It
Make a chart with titles like the one
shown. Record the revolution data
from the diagram on your chart.
Use Numbers Now calculate how
old you would be if you lived on
each of the planets.
Use Numbers Calculate the Earthplanet ratio for all of the planets.
On which planet would you be the
oldest in that planet’s years? On
which planet would you be the
youngest?
If you were 6 years old in Earth
years, how old would you be in
Mars years?
What can you infer about the
revolution time of the planet
and the age you would be on
that planet?
SI.7 Record observations using methods that complement investigations
(e.g., journals, tables, charts) (SI-M-A3)
433
EXTEND
Lesson 3
The Solar
System
Stonehenge, England
How many planets do you see? Mars, Saturn, and Venus
are in a triangle above the center stone. Mercury is below
them to the left. Jupiter is much higher to the right. How
far away are these planets from Earth?
434
ENGAGE
ESS.42 Differentiate among moons, asteroids, comets, meteoroids, meteors, and
meteorites (ESS-M-C2) ESS.43 Describe the characteristics of the inner and outer
planets (ESS-M-C2) ESS.45 Identify Earth’s position in the solar system (ESS-M-C5)