The Sun-Earth-Moon System Earth s Moon Key Concepts How does the Moon move around Earth? Why does the Moon s appearance change? 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 3. The Moon was once a planet that orbited the Sun between Earth and Mars. 4. Earth s shadow causes the changing appearance of the Moon. Make an Outline As you read, make an outline to summarize the lesson. Use the main headings in the lesson as the main headings in the outline. Complete the outline with the information under each heading. Use the completed outline to review the lesson. 1. Name the hypothesis that best explains how the Moon formed. Seeing the Moon Why does the Moon shine? Why does its shape seem to change? The Moon does not give off light. Unlike the Sun, the Moon is a solid object. You see the Moon because it reflects light from the Sun. Data about the Moon s mass, size, and distance from Earth are shown in the table below. Mass 1.2% of Earth s mass Diameter 27% of Earth s diameter Moon Data Average distance from Earth Time for one rotation Time for one revolution 384,000 km 27.3 days 27.3 days The Moon s Formation The most widely accepted idea about how the Moon formed is the giant impact hypothesis. The giant impact hypothesis states that shortly after Earth formed, an object about the size of the planet Mars crashed into Earth. The impact caused a ring of vaporized rock to form around Earth. Eventually, the material in the ring cooled and clumped together and formed the Moon. 352 The Sun-Earth-Moon System Reading Essentials
The Moon s Surface Early in the Moon s history, various features formed on its surface. The main features on the Moon are craters, maria (MAR ee uh), and highlands. Craters The Moon s craters formed when objects from space, such as large rocks, crashed into the Moon. Light-colored streaks called rays run outward from some craters. Most of the Moon s craters formed about 3.5 billion years ago. This was long before dinosaurs lived on Earth. Earth also was hit by many large objects during this time. Wind, water, and plate tectonics erased these craters on Earth. The Moon has no atmosphere, no water, and no plate tectonics. Without these forces, Moon s craters, formed billions of years ago, have not changed much. Maria The large, dark, flat areas on the Moon are called maria. The maria (MAR ee uh) formed long after most impacts on the Moon s surface had ended. Maria formed when lava flowed through the Moon s crust and hardened. The hardened lava covered large portions of the Moon s surface, including many of its craters and other features. Highlands The light-colored areas on the Moon are the highlands. Highlands were not covered by the lava that formed the maria because they were too high for the lava to reach. Highlands are older than the maria and are covered with craters. The Moon s Motion As Earth revolves around the Sun, the Moon revolves around Earth. The pull of Earth s gravity causes the Moon to move in an orbit around Earth. Recall from Lesson 1 that if the pull of the Sun s gravity ended, Earth would fly in a straight line into space. The same would be true for the Moon if the pull of Earth s gravity ended. The Moon makes one revolution around Earth every 27.3 days. Like Earth, the Moon also rotates as it revolves. One complete rotation of the Moon also takes 27.3 days. Notice that this is the same amount of time it takes the Moon to make one complete revolution around Earth. The same side of the Moon always faces Earth because the Moon takes the same amount of time to orbit Earth and make one rotation. This side of the Moon that faces Earth is called the near side. The side of the Moon that cannot be seen from Earth is the far side of the Moon. 2. Explain How were maria produced? Key Concept Check 3. Restate What produces the Moon s revolution around Earth? 4. Compar e Earth s revolution and the Moon s revolution. Reading Essentials The Sun-Earth-Moon System 353
Key Concept Check 5. Identify What produces the phases of the Moon? Make a bound book to organize information about the lunar cycle. Each page should represent one week. Second Full First Week First Quarter 6. Name What are the waning phases of the Moon? Phases of the Moon The Sun is always shining on half the Moon, just as it is always shining on half of Earth. As the Moon moves around Earth, usually only one part of the Moon s near side is lit. The lit part of the Moon or a planet that can be seen from Earth is called a phase. The motion of the Moon around Earth causes the phase of the Moon to change. The phases follow a regular pattern that is called the lunar cycle. One lunar cycle takes 29.5 days, or slightly more than four weeks, to complete. Waxing Phases The waxing phases occur during the first half of the lunar cycle. During the waxing phases, more of the Moon s near side is lit each night. Week 1 First Quarter The lunar cycle begins. A sliver of light appears on the Moon s western edge. Each night, the lit part grows larger. By the end of the first week, the Moon reaches its first quarter phase. The entire western half of the Moon is now lit. Week 2 Full Moon During the second week in the lunar cycle, more and more of the near side of the Moon becomes lit. By the end of the second week, the Moon s near side is completely lit. It is at its full moon phase. Waning Phases After the Moon waxes, it is said to wane. During the waning phases, less of the Moon s near side is lit each night. As seen from Earth, the lit part is now on the Moon s eastern side. Week 3 Third Quarter During the third week in the lunar cycle, the lit part of the Moon becomes smaller. By the end of the third week, only the eastern half of the Moon is lit. This is the third quarter phase. Week 4 New Moon During the fourth week in the lunar cycle, less and less of the near side of the Moon is lit. When the Moon s near side is completely dark, it has reached the new moon phase. The entire lunar cycle is summarized in the figure on the next page. 354 The Sun-Earth-Moon System Reading Essentials
Full moon The Moon looks like this in the sky. Moon s orbit Third quarter You see the sunlit part. Full moon You don t see the shadowed part. First quarter Third quarter First quarter Earth The new moon phase is not visible from Earth. New moon Sunlight Visual Check 7. Circle the name of the moon phase that would be seen at the end of Week 2 of the lunar cycle. Reading Essentials The Sun-Earth-Moon System 355
Mini Glossary maria (MAR ee uh): the large, dark, flat areas on the Moon phase: the lit part of the Moon or a planet that can be seen from Earth waning phase: the part of the lunar cycle in which less of the Moon s near side is lit each night waxing phase: the part of the lunar cycle in which more of the Moon s near side is lit each night 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes the difference between waxing and waning. 2. Fill in the graphic organizers below to summarize the features of the Moon s surface and the phases of the Moon. Features of the Moon s Surface Moon s Phases Week 1 craters 3. How did making an outline help you understand what you read? 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? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. Week 2 full moon Week 3 Week 4 END OF LESSON 356 The Sun-Earth-Moon System Reading Essentials