Modeling Moon Phases

Activity:
Students model moon phases using a flashlight and a racquet ball.

Science Standards:
Patterns of Change and Scientific Models (Conceptual)

Objectives:

  • The student will create a model of Moon phases.
  • The student will identify different phases of the Moon.
  • The student will describe the cause of Moon phases.
  • The student will describe the synchronous rotation of the Moon.
  • Materials:

  • flashlight
  • racquet ball (tennis balls have too much albedo)
  • copies of student answer sheets


  • Introduction:
    The cyclical period of the Moon is not only striking, but it quite is easy to observe. Throughout history, many cultures have used the lunar cycle as a method to measure increments of time; a lunar calendar. Nomadic peoples regulated their calendar based entirely on the 29 1/2 day synodic period of the Moon. Every time a slender crescent of a new moon appeared in the western evening sky, a new month of 29 - 30 days began at the evening hour. The lunar cycle was quite practical because of its short duration and its ease in use.
    The phases of the Moon are due to the changing relative positions of the Sun, the Moon, and the Earth. One half of the Moon is always lit by the Sun. As the Moon revolves around the Earth, we see first more and more of the lit side and then, less and less. The Moon must rotate on its axis with the same period that it revolves around the Earth because the same side always points toward the Earth.

    Directions:
    1. Students model the phases of the Moon.
    2. Students complete the student answer sheets provided.

    Modeling Moon Phases - Student Answer Sheets

    "The Moon has been created for the counting of days."
    From the Jewish Midrash

    Procedure: Answer each of the following questions in complete sentences after discussion within your laboratory group. You may compare your work with an adjacent laboratory group as necessary to arrive at a group consensus.

    At the front of the room are some rubber spheres (racquet balls) and a light source (a flashlight). In a group of at least three people, one individual should hold the flashlight (be the Sun), one individual should hold the rubber sphere (be the Moon), and the remaining member(s) of the group should stand together (on the Earth). The Sun should stand about 4 paces in front of the Earth and remain stationary.

    Procedure:

    1. The Moon should stand between the Sun and the Earth. Note how the side opposite the Earth is the side which is lit. This phase is called new moon .

    2. As a group, the Earth should make 4 complete rotations (yes that means turn around four times). During these rotations, the Moon should move 1/8 of a rotation around the Earth. Notice that the Earth must turn a little past its previous orientation in order to face the Moon. What phase of the Moon do you observe ?
     

    Which side of the Moon is lit (right or left) ?
     
     

    3. As a group, the Earth should make 3 more complete rotations (yes that means turn around three times). During these rotations, the Moon should move 1/4 of a rotation around the Earth. How many days into the lunar month does this occur ?
     

    Which side of the Moon is lit ?
     

    Why is it called first quarter ?
     
     

    4. As a group, the Earth should make 4 more complete rotations. During these rotations, the Moon should move 3/8 of a rotation around the Earth. Notice in each scenario, one half of the Moon is always lit by the Sun's light and one half is not. What phase of the Moon do you observe ?
     

    Which side of the Moon is lit ?
     

    If you were laying on the floor what phase of the Moon would you observe ?

    5. As a group, the Earth should make 3 more complete rotations. During this rotation, the Moon should move to where it is exactly opposite the Sun from the Earth. Notice that the Earth must turn a little past its previous orientation in order to face the Moon. What phase of the Moon do you observe ?
     

    Which side of the Moon is lit ?
     
     
     

    If the Moon is not being held over the top of the Earth, what occurs ?
     
     
     

    6. At this point, the folks holding the Moon and the Sun should switch with folks on the Earth so that everyone can get a chance to spin until they are slightly disoriented. As a group, the Earth should make 4 complete rotations (yes that means turn around four times). During these rotations, the Moon should move 5/8 of a rotation around the Earth. Notice that the Earth must turn a little past its previous orientation in order to face the Moon. What phase of the Moon do you observe ?
     
     

    7. As a group, the Earth should make 3 more complete rotations. During these rotations, the Moon should move 3/4 of a rotation around the Earth. Why is this phase called third quarter ?
     
     
     

    Which side of the Moon is bright?
     
     

    If you were visiting Australia, which side of the Moon would be right?
     
     
     

    If you were on the Moon at this point, how would the Earth appear ?
     

    8. The synodic lunar month (time between successive new moons) is actually 29 1/2 days compared to 27 days for the Moon to orbit the Earth once, why do you think that is? Compare your explanation with an another laboratory group who appear confused also.



    9. Listed below are some explanations people have for the cause of the changing shape of the Moon. As a group, discuss each of these explanations in turn. Briefly note the reasons for accepting or rejecting each explanation*.

    a. Clouds block the Moon.
     
     
     

    b. The Moon has moved in or out of the Earth's shadow.
     
     
     

    c. The moon is black and white and rotates on its axis.
     
     
     

    d. We're seeing more or less of the side of the
    Moon that is facing the Sun.
     
     
     

    10. The moon always shows the same face to the Earth. Appoint one member of your group to be the Moon (this person should be type cast as being slightly loony ). Instruct the Moon to continuously face the direction East and orbit the desk (representing the Earth). Would an observer on the desk (the Earth) always see the same side of the orbiting group member (the Moon)?
     
     

    Now instruct the loony person (maybe we should just refer to this person as being far-out) to again orbit the Earth (desk) but this time continuously face the Earth (desk). Would an observer on the desk (the Earth) always see the same side of the orbiting group member (the Moon)?
     
     
     

    In which situation is the Moon revolving? How many times does this individual rotate during a single orbit (revolution)?
     

    Is the Moon rotating on its axis as it revolves around the Earth? Explain.
     

    How fast is the Moon rotating on its axis (if it is indeed rotating)?
     
     

    Astronomers describe the Moon as being "tidally coupled " or "in synchronous rotation " to the Earth due to the gravitational attraction between them. This has not always been the situation. Over time the rotation of the Moon has slowed so that the same side always faces the Earth. This gravitational attraction between the two also has the effect of slowing the rotation of the Earth. Over time, the Earth and Moon will become totally coupled and the Moon will one day reside over only one portion of the Earth's surface (of course the Sun will disintegrate the Earth and Moon before this has time to actually occur).