The Earth-Moon- Sun System Are there tides in a deser t? sections 1 Earth in Space Did you know the Moon exerts gravitational force on Earth? On Earth, the Moon’s gravita- 2 Time and Seasons tional attraction is evidenced as tides in Lab Comparing the Angle of oceans and seas. Even in the desert, the Sunlight to Intensity Moon’s gravity influences the flexing of 3 Earth’s Moon tectonic plates. Lab Identifying the Moon’s Surface Features and Apollo Science Journal Research to discover what landforms Landing Sites or events are affected by the Moon’s gravitational force on Earth. 184 Richard Cummins/CORBIS Start-Up Activities Earth, Sun, and Moon Make the following Foldable to help organize what you learn about Relative Sizes of Earth, the Moon, the Earth-Moon-Sun system. and the Sun Can you picture the relative sizes of Earth, the STEP 1 Fold a sheet of paper vertically from Moon, and the Sun? Earth is about four times side to side. Make larger than the Moon in diameter, but the the front edge about Sun is much larger than either. The Sun’s ᎏ1ᎏ 2 inch longer than diameter is about 100 times that of Earth and the back edge. about 400 times that of the Moon. In this Lab, you’ll investigate the relative sizes of all three STEP 2 Turn lengthwise objects. and fold into thirds. 1. Get permission to draw some circles on a sidewalk or paved area with chalk. You STEP 3 Unfold and cut only the top layer could also use a stick to draw circles on a along both folds to make three tabs. dirt playing field. 2. Select a scale that will enable you to draw circles that will represent each object. Hint:Using 1 cm for the Moon’s diameter is a good start. 3. Use a meterstick to draw a circle with a STEP 4 Label each tab. 1-cm diameter for the Moon. 4. Now draw two more circles to represent Sun Earth Moon Earth and the Sun. 5. Think Critically In your Science Journal, Questions As you read the chapter, write what explain how the Moon and the Sun can you learn about each body under the correct tab appear to be about the same size in the of your Foldable. After you read the chapter, sky. Think about how things look smaller note the many ways that the three affect each other. the farther they are from you. Preview this chapter’s content and activities at gpescience.com 185 Richard Cummins/CORBIS Earth in Space Reading Guide Review Vocabulary ■ Compare and contrast Earth’s Gravity from the Earth-Moon-Sun orbit: curved path of one object, physical characteristics with those system directly affects what it’s like such as the Moon, around another of other planets. to live here on Earth. object, such as Earth ■ Explain Earth’s magnetic field. ■ Describe Earth’s movement in New Vocabulary space and how eclipses occur. • sphere • ellipse • gravity Figure 1 Objects fall toward Earth’s Size and Shape Earth’s center. Like most people, you are aware that Earth is round like a Infer How would apples fall from trees if Earth were shaped like a cube? ball. But can you prove that this is true? If you jump up, you know that you’ll come back down, but why is this so? What is the force that brings you down? You may have used a compass to tell directions, but do you know how a compass works? You will learn the answers to these questions and also about many phys- ical characteristics of Earth in this section. Ancient Measurements Earth’s shape is similar to a sphere. A sphere is a round, three-dimensional object, the surface of which is the same distance from the center in all directions. Even ancient astronomers knew that Earth is spherical in shape. We have pictures of Earth from space that show us that it is spheri- cal, but how could astronomers from long ago have learned this? They used evidence from observations. Aristotle was one of these early astronomers. He made three different observations that indicated that Earth’s shape is spher- ical. First, as shown in Figure 1, no matter where you are on Earth, objects fall straight down to the surface, as if they are falling toward the center of a sphere. Second, Earth’s shadow on the Moon during a lunar eclipse is always curved. If Earth weren’t spherical, this might not always be the case. For exam- ple, a flat disk casts a straight-edged shadow sometimes. Finally, people in different parts of the world see different stars above their horizons. More specifically, the pole star Polaris is lower in the sky at some locations on Earth than at others. 186 CHAPTER 7 The Earth-Moon-Sun System Everyday Evidence of Earth’s Shape What have you seen, other than pictures from space, that indicates Earth’s shape? Think about walking toward someone over a hill. First, you see the top of the person’s head, and then you can see more Topic: Comparing Earth to and more of that person. Similarly, if you sail toward a light- Other Planets house, you first see the top of the lighthouse and then see more Visit gpescience.com for Web links to information about how and more of it as you move over Earth’s curved surface. Earth is similar to and different You can see other evidence, too. Just like ancient from other planets in the solar astronomers, you can see for yourself that objects always fall system. straight down. Today, however, we know more about gravity. Activity Make a table that lists Gravity is the attractive force between two objects that depends the similarities and differences of on the masses of the objects and the distance between them. Mercury, Venus, Earth, and Mars. Astronomers think Earth formed by the accumulation of infalling objects toward a central mass. Energy released in the impacts kept the growing Earth molten. Gravity caused it to form into the most stable shape, a sphere. In this shape, the pull of gravity toward the center of the planet is the same in all direc- tions. If a planet is massive enough, the pull of gravity could be so strong that even tall mountains would collapse under their own weight. Table 1 lists some of Earth’s other properties. How does the pull of gravity indicate that Earth’s shape is spherical? Table 1 Earth’s Physical Properties Diameter (pole to pole) 12,714 km Diameter (through equator) 12,756 km Circumference (poles) 40,008 km Circumference (equator) 40,075 km Mass 5.98 ϫ 1024 kg Average density 5.52 g/cm3 Average distance to the Sun 149,600,000 km Average distance to the Moon 384,400 km Period of rotation 23 h, 56 min Period of revolution 365 days, 6 h, 9 min SECTION 1 Earth in Space 187 Earth’s Magnetic Field Earth has a magnetic field that protects us from harmful radi- Van Allen Belts The mag- ation from the Sun. Scientists hypothesize that Earth’s rotation netosphere lies above the and movement of matter in the core set up a strong magnetic outer layers of Earth’s field in and around Earth. This field resembles that surrounding atmosphere. Within this a bar magnet, shown in Figure 2. Earth’s magnetic field is con- magnetosphere are belts of centrated at two ends of an imaginary magnetic axis running charged particles known as from Earth’s north magnetic pole to its south magnetic pole. This the Van Allen belts. They contain thin plasma com- axis is tilted about 11.5° from Earth’s geographic axis of rotation. posed of protons (inner belt) and electrons (outer Wandering Poles The locations of Earth’s magnetic poles belt) that are trapped by change slowly over time. Large-scale movements, called polar Earth’s magnetic field. wandering, are thought to be caused by movements in Earth’s Research how the magne- crust and upper mantle. The magnetic north pole is carefully tosphere protects Earth remapped periodically to pinpoint its location. from the solar wind and why the Van Allen belts are hazardous to astro- The Aurora An area within Earth’s magnetic field, called the nauts and satellites. Report magnetosphere, deflects harmful radiation coming from the your findings to the class. Sun, a stream of particles called solar wind. Some of these ejected particles from the Sun produce other charged particles in Earth’s outer atmosphere. These charged particles spiral along Earth’s magnetic field lines toward Earth’s magnetic poles. There they collide with atoms in the atmosphere. These collisions Figure 2 Like a common bar cause the atoms to emit light. This light is called the aurora magnet, Earth also has north and borealis (northern lights) in the northern hemisphere and the south magnetic poles. The inner aurora australis (southern lights) in the southern hemisphere. and outer gold shells respectively represent the positive and nega- tive Van Allen Belts. Earth Orbits the Sun Explain why the Van Allen Belts Earth orbits the Sun at an average distance of are shaped as they are. 149,600,000 km. Its orbit, like those of all the planets, moons, asteroids, and many comets, N is shaped like an ellipse. An ellipse is an elongated, closed Van Allen belts curve with two foci. The Sun is not located at the center of the ellipse, but at one of its two foci.