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The Lunar Cycle

Students learn about the Moon's changing appearance and how orbital motion causes the Moon’s phases.

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Lesson Summary

Overview

The Moon, Earth's only natural satellite, is easily observed with the naked eye. Over the course of one month, you can observe its full range of appearances and its pattern of movement in the sky. The phases of the Moon (new, crescent, quarter, gibbous, and full) are a result not of a change in the actual shape of the Moon, but of the relative positions of the Sun, Moon, and Earth. That is, the apparent changes in the Moon's shape are simply changes in the portion of the lit side of the Moon that you can see from Earth.

However, misconceptions regarding the Moon's apparent change in shape still abound. Some believe that clouds or some other object cover up the Moon, blocking our view of part of it. This may happen, but it is not the mechanism behind the lunar cycle. Others believe that the shadow of Earth or some other object falls on the Moon's surface, causing the phases of the Moon. While the Moon does occasionally cross Earth's shadow, resulting in a lunar eclipse, shadows cast by Earth are not what cause lunar phases.

In this lesson, students learn about the Moon's changing appearance and its pattern of movement. Through class discussion, activities, and multimedia resources, students explore the phases of the Moon and are introduced to the concept of orbital motion.

Objectives

  • Identify the phases of the Moon
  • Understand that the Moon completes one revolution around Earth over the course of one month
  • Recognize that the Moon is visible sometimes at night, sometimes during the day, and sometimes not at all

Grade Level: 3-5

Suggested Time

Two to three class periods, separated by days and weeks

Multimedia Resources

Materials

  • Notebooks - one for each student
  • 1 lamp with exposed light bulb
  • 1 extension cord
  • Styrofoam balls -- one per pair of students

Before the Lesson

Arrange to begin the lesson when the Moon is in first quarter (it will be visible during the afternoon). Set up and try the model demonstration to make sure that the lighting in the classroom will be effective. The light source should be placed in a central location with room for students to move around it.

The Lesson

Part I: Observing the Moon

1. Ask students to share their ideas about the Moon. Encourage them to talk about what they have already observed and any conceptions that they may have. Ask:

  1. What does the Moon look like? Does it always look the same?
  2. When do you see it?
  3. In which part of the sky do you see it?
  4. What do you think the Moon is made of?
  5. Why do you think the Moon has phases?

2. Explain that the class is going to study the Moon based on their own observations. Ask:

  1. What does it mean to make observations?
  2. How can we make observations of the Moon?

3. Show the first minute of the Galileo: Discovering Jupiter's Moons, which discusses some of Galileo's observations of the Moon.

  1. What did Galileo observe about the Moon?
  2. When you look at the Moon without a telescope, what does its surface look like?

4. (Optional) Discuss what the Moon looks like up close. Remind students that humans have actually been on the Moon and have taken pictures and brought back samples of Moon rocks. Work though the Explore the Moon as a class.

5. As a class, look at the Phases of the Moon to see examples of how the Moon can appear from Earth. Discuss each screen as you go through the interactive.

6. Explain that the class will be making daily observations of the Moon and recording them in a Moon Journal. Their journal entries will include sketches of how the Moon looks each day (when visible), as well as the date, time, location of the Moon in the sky (direction and height above the horizon), weather conditions, and the name of the phase.

7. Distribute the notebooks for the Moon Journals. Have students make their first journal entry as a class. If the Moon is in first quarter, it should be visible during the afternoon. Go outside as a class and help students locate the Moon. Note: Remind students never to look directly at the Sun.

8. Assign students to continue to make daily observations of the Moon in their journals.

Part II: Investigating the Phases of the Moon

9. After a few days of observations, have students look at their journal entries. Ask:

  1. How has the appearance of the Moon changed?
  2. Does the Moon always appear to be in the same place? Does it always rise in the same direction? Does it move across the sky?
  3. How does the Moon's position change from day to day?
  4. Can you predict what the Moon will look like and where in the sky it will be at a given time tomorrow?

10. After a couple of weeks of observations (past the full phase), have students look at their journal entries again. Ask:

  1. What pattern do you notice in the Moon's changing appearance?
  2. Can you predict what the Moon will look like tomorrow? In a week? In a month?
  3. When do you think it will be in the new phase again?
  4. What do you notice about the curve of the visible part of the Moon?
  5. What do you think is causing the changes in the Moon's appearance?

11. Explain that the Moon does not give off any light of its own -- it only reflects sunlight. Because the Moon revolves around Earth, it looks different to us from day to day as it changes position in its orbit. For more information about how the Moon orbits Earth, show the Why Doesn't the Moon Fall Down?. Note: Even though Earth is sometimes positioned between the Sun and the Moon (such as at full moon), it is much smaller than the Sun, so it doesn't usually block the Sun's rays from reaching the Moon. However, occasionally, the three bodies line up just right so that Earth does block sunlight from reaching the Moon, causing a lunar eclipse.

12. Distribute the balls and have students work in pairs to model the Earth-Moon-Sun system. The light source, representing the Sun, should be placed in a central area with the students surrounding it. Each student represents Earth, and the ball (held at arm's length) represents the Moon. Half of the ball should be illuminated, and as the student rotates in place, the visible portion of the illuminated part will mimic the phases of the Moon. Have students explore this model for a few minutes. Note: To an observer on Earth, the curved side of the lit part of the Moon always faces the Sun (notice how it changes side before and after the new moon phase).

13. Review the phases of the Moon: new, crescent, quarter, gibbous, and full. Write the names of the phases on the board and also have students draw an illustration on the board of each phase.

14. Choose a phase and point to its illustration. Ask students to position their Moons so that they look like the picture. For example, if you ask for a full moon, the students need to manipulate their positions to simulate the full moon phase (i.e., the lamp is behind them and the fully lit side of the Moon is facing them). Have students explain the following:

  1. What portion of the Moon is illuminated?
  2. How much of the illuminated portion do you see?
  3. Does the Moon itself actually change shape?

15. Have students challenge their partners with the model -- one student acts as the model while the other asks for specific phases. Make sure each student has a chance to be both the model and the challenger. Then ask:

  1. What did this model demonstrate?
  2. What did you learn about how the Sun, Moon, and Earth move in relation to one another?
  3. How many times does the Moon travel around Earth to complete a full cycle of phases?

16. (Optional) Show The Origin of the Moon. This video segment from NOVA follows the Apollo 15 astronauts as they collect samples of ancient rock from the Moon's crust. The discovery of this ancient rock helps lead to a radical new theory about the Moon's origin. Note that this video segment is intended for middle school and high school students.

Check for Understanding

Have students discuss the following:

  1. What is the Moon?
  2. What does it mean to say that the Moon has phases? Can you name the phases?
  3. What do you think of the names for the phases? Do you have any better suggestions? If so, explain why you would change the names.
  4. How long does it take for the Moon to complete one lunar cycle?
  5. What would you tell someone who said that it was impossible to see the Moon during the day?
  6. Draw a diagram to show the relationship between the Sun, the Moon, and Earth.

The Digital Library for Earth System Education (www.dlese.org) offers access to additional resources on this topic.

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