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)
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... Test Your Hypothesis Make a Model Place the tennis ball on the 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. 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? 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. Step Step Materials tennis ball fabric square string graph paper SI.12 Use data and information gathered to develop an 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
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 What is gravity? 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
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
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) sunrise sunlight light side (day) axis 426
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
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 1 4 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. autumn begins 428
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 Earth is closer to the Sun in January than in July. Quick Check Fact and Opinion Write a fact about why Earth s seasons change. Critical Thinking When the season in the Northern Hemisphere is fall, what season is it in the Southern Hemisphere? 429
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 Leo Lyra
Visual Summary 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. An object stays in orbit because of gravity and inertia. Because of Earth s rotation the Sun, stars, and Moon appear to move from east to west across the sky. Seasons change because of Earth s revolution around the Sun and because Earth s axis is tilted about 23 degrees. 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 24 hours long. Is this statement a fact or an opinion? Explain. Fact Opinion Critical Thinking Do all locations on Earth experience night at the same time? Explain. Test Prep Which keep Earth in its orbit around the Sun? A lift and pressure B thrust and inertia C gravity and inertia D pressure and gravity 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 Calculating Earth s Mass 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? Health Link Weightlessness 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 Sun Mercury 87.96 Jupiter 4,331.98 Earth 365.25 Venus 224.68 Mars 686.98 Uranus 30,707.41 Saturn 10,760.56 Try It Make a chart with titles like the one shown. Record the revolution data from the diagram on your chart. Use Numbers Calculate the Earthplanet ratio for all of the planets. If you were 6 years old in Earth years, how old would you be in Mars years? Apply It Use Numbers Now calculate how old you would be if you lived on each of the planets. On which planet would you be the oldest in that planet s years? On which planet would you be the youngest? 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)