Earth in Space. The Sun-Earth-Moon System

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1 in Space The --Moon System What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. Seasons are caused by the changing distance between and the. Key Concepts What causes seasons on? How does the Moon affect? How do solar and lunar eclipses differ? 2. The Moon has a dark side upon which the never shines. and the Universe Long ago, people studied the positions and motions of the, Moon, and other objects in the sky. They used patterns in the motions to predict future positions of sky objects. But they did not understand how the objects were related. They thought was the center of the universe. Today we know that is not the center of the universe. The Moon moves around, or orbits,. is one of eight planets that orbit the. The is one of billions of stars that make up the Milky Way galaxy. And the Milky Way is one of billions of galaxies in the universe. Objects orbit the because the has more than 99 percent of the solar system s mass. The has a huge gravitational pull. The is the biggest object in the solar system. As the figure shows, the s diameter is 100 times greater than s diameter and 10 times greater than Jupiter s. Size of the 1.4 million km Jupiter Building Vocabulary Work with another student to write a question about each vocabulary term in this lesson. Answer the questions and compare your answers. Reread the text to clarify the meaning of the terms. 1. Point Out What is the s diameter? Reading Essentials in Space 17

2 Make an envelope book and draw an image of the on the inside center. On the inside tabs, draw the position of for each season. Fall Winter Summer Spring 2. Define What is an astronomical unit? Motions of Have you ever flown in an airplane? Airplanes travel faster than 900 km/h. Yet as you sit in one, you hardly feel like you are moving. Living on is like traveling in an airplane. It seems as if is still and the and stars move around it. But is not still. moves in space. s Orbit As you read, is moving around the because of the s huge gravitational pull. Without the s pull, would move off into space in a straight line, as shown below. s orbit is nearly round, or elliptical. The orbit of an object around another object is revolution. It takes days one year to revolve around the once. As shown below, the distance between and the is not always the same. An astronomical unit (AU) is the average distance between and the. One AU is nearly 150 million km. Scientists often use AUs to measure distances to planets and other objects within the solar system. s Orbit s motion without the s gravitational pull In early July, is farther from the. 3. Discover When is closest to the? Gravitational pull of the 152 million km 147 million km s orbit In early January, is closest to the. s Rotation Imagine a rod pushed through the center of from the North Pole to the South Pole. The images of in the figure at the top of the next page show this. The rod represents s axis. spins, or rotates, on its axis like a top. Rotation is the spin of an object around its axis. Rotation is what causes day and night. The side of facing the is in daylight, and the side of facing away from the is in darkness. makes one full rotation every 24 h. 18 in Space Reading Essentials

3 s Rotation Axis Fall s orbit N S Winter Summer N N S S 4. Examine What is the degree of tilt of s rotation axis? s rotation axis is tilted N S Spring s Tilt and Seasons s changing distance from the does not cause the seasons. Look again at the figure on the previous page. is closest to the in January, when it is winter in the northern hemisphere. Seasons occur because s tilt does not change as orbits the, as shown in the figure above. If you drew a line perpendicular to s orbital path, the angle of tilt between s axis and that line would be As moves, this angle of tilt stays the same. The North Pole and the South Pole always point in the same directions. But the position of s tilt as it relates to the does change, as shown in the figure below. Spring and Fall An equinox (EE kwuh nahks) occurs when s rotation axis is tilted neither toward nor away from the. Equinox means equal night. Hours of daylight equal hours of darkness during an equinox. An equinox occurs two days of the year, one in March and one in September. These days are used to signify the beginning of spring or fall, as shown below. March equinox s axis s Tilt and the Seasons September June equinox solstice s axis At two points in s orbit the March and September equinoxes s axis does not point either toward or away from the. Light is distributed equally in the northern and southern hemispheres. s axis 5. Recognize The tilt of s axis. (Circle the correct answer.) a. does not change as moves around the b. has no effect on s seasons c. changes to create the seasons on 6. Identify How are the beginnings of spring and fall similar? December solstice s axis At two points in s orbit the June and December solstices s axis points the most toward or away from the. Light is not distributed equally in the northern and southern hemispheres. Reading Essentials in Space 19

4 Key Concept Check 7. Explain What causes seasons? 8. State What created the Moon s craters? 9. Explain Why does the same side of the Moon always face? Summer and Winter When s rotation axis is tilted directly toward or away from the a solstice (SAHL stuhs) occurs. Solstices happen in June and December. When the North Pole is toward the, the northern hemisphere has summer. The northern hemisphere receives more direct sunlight, and there are more hours of sunlight during the day. At the same time the northern hemisphere is having summer, the South Pole is tilted away from the, and it is winter in the southern hemisphere. The southern hemisphere receives less direct sunlight and there are fewer hours of sunlight. Six months later, the seasons are reversed. s Moon You can probably guess what force holds the Moon in orbit around. It s the same force that holds in orbit around the gravity! The Moon is about one-fourth the size of. The Moon is a dry, airless object made mostly of rock. Early in the Moon s history, many asteroids and comets crashed into it, leaving huge craters on its surface. The Moon also has mountains and smooth, dark lava plains from ancient volcanoes. Formation of the Moon Scientists propose that the Moon formed when a Marssized object collided with soon after formed. This collision threw debris into orbit around. Gravity pulled the debris together, which formed the Moon. Motions of the Moon Like, the Moon moves in different ways. It rotates on its axis, and it revolves around. It orbits once every 27.3 days. That is also how long it takes the Moon to rotate once. Because one revolution of the Moon around takes the same amount of time as one rotation of the Moon on its axis, the same side of the Moon always faces, as shown in the figure. The side of the Moon that does not face is called the far side. You cannot see it from. The Moon s Motion Moon s rotation North Pole 20 in Space Reading Essentials

5 Phases of the Moon Full moon Waning gibbous Waxing gibbous Moon s orbit Last quarter You don t see the shadowed part. You see the sunlit part. First quarter The new moon phase is not visible from. Waning crescent Waxing crescent light Phases of the Moon The Moon does not create its own light. The Moon is visible only because it reflects sunlight. As the Moon orbits, the half of the Moon facing the is in sunlight and the half facing away is in shadow, as shown in the figure above. But as the Moon orbits, the part of the Moon that can be seen from seems to change shape. These shapes are the phases of the Moon. The Moon completes a cycle of phases every 29.5 days. Refer to the figure above as you read about each of the Moon s phases. New Moon and Waxing Phases When the Moon is between and the, the sunlit half of the Moon faces away from. The half facing is dark because it is in shadow, as shown above. This phase is called a new moon. During the two weeks following a new moon, more of the Moon becomes visible. As the lit portion of the Moon becomes larger, the Moon is waxing. The waxing phases are waxing crescent, first quarter, and waxing gibbous. 10. Identify When does the Moon appear to get larger? When does it appear to get smaller? Reading Essentials in Space 21

6 11. State When does a full moon occur? Key Concept Check 12. Summarize How does the Moon cause tides on? 13. Point Out In the figure below, highlight the parts of the diagram that show how s oceans are affected by the gravitational pull of the Moon. Spring tides Full moon Full Moon and Waning Phases When is between the Moon and the, the entire sunlit half of the Moon faces. This phase is called a full moon. During the two weeks following a full moon, less of the sunlit side of the Moon is visible. As the lit portion of the Moon becomes smaller, the Moon is waning. The waning phases are waning gibbous, last quarter, and waning crescent. Tides Water levels of the ocean change. Tides are the periodical rise and fall of the oceans surfaces caused by the gravitational force between and the Moon and the. The Moon has about twice as much influence on tides as the because it is so much closer to. Effect of the Moon Locations on closest to and farthest from the Moon have the greatest tidal effect. As shown in the figure below, water on bulges slightly at these locations. These are the places where high tides occur. In contrast, locations on halfway between the two high-tide regions have low tides. As rotates, the locations of high and low tide change in predictable ways. Most coastlines have two high tides and two low tides each day. But water depth, coastline shape, and weather also affect tides. Effect of the When and the Moon are in line with the, the s gravitational pull adds to the Moon s gravitational pull. As a result, high tides are higher than usual. Tides at this time are called spring tides. Spring tides occur during full moon and new moon phases as shown in the figure below. New moon 22 in Space Reading Essentials

7 Neap tides First quarter moon Last quarter moon During the first and third quarter moons, the gravitational pull of the Moon is perpendicular to the gravitational pull of the. High tides are lower than usual. Tides at these times are called neap tides and are shown in the figure above. Eclipses Throughout human history, people have interpreted eclipses as signs of war or disaster. However, there is nothing mysterious about eclipses. They are natural events. An eclipse is the movement of one solar system object into the shadow of another object. You can view solar and lunar eclipses from. As shown in the figure below, the type of eclipse depends on the positions of the Moon,, and the. Solar Eclipses A solar eclipse can only occur during a new moon. During a solar eclipse, a small part of is in the Moon s shadow. The Moon appears to completely or partially cover the. Moon Solar eclipse During a total solar eclipse, only a small part of is covered by the Moon s shadow. 14. Distinguish What is the difference between spring tides and neap tides? 15. Define What is an eclipse? 16. Locate Where would you have to be on to see this total solar eclipse? Reading Essentials in Space 23

8 Lunar eclipse Moon During a total lunar eclipse, the Moon is completely covered by s shadow. Key Concept Check 17. Contrast How do solar and lunar eclipses differ? Lunar Eclipses A lunar eclipse can occur only during a full moon. During a lunar eclipse, s shadow completely or partially covers the Moon. The Moon is visible during a total lunar eclipse. Light changes direction as it passes through s atmosphere. The light that reaches the Moon appears red. 24 in Space Reading Essentials

9 Mini Glossary eclipse: the movement of one solar system object into the shadow of another object equinox (EE kwuh nahks): occurs when s rotational axis is tilted neither toward nor away from the revolution: the orbit of an object around another object rotation: the spin of an object around its axis solstice (SAHL stuhs): occurs when s rotation axis is tilted directly toward or away from the tides: the periodical rise and fall of the oceans surfaces caused by the gravitational force between and the Moon and the waning: Moon phase when the lit portion of the Moon becomes smaller waxing: Moon phase when the lit portion of the Moon becomes larger 1. Review the terms and their definitions in the Mini Glossary. Write a sentence in your own words using one of the Mini Glossary terms. 2. Identify each model as a solar or a lunar eclipse. Write the name of each solar system object on the blank lines. a. b. d. e. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? c. f. ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials in Space 25