Universe Cycle - Earth (4)

  • Exploring the Earth/Moon system.
  • Differentiating rotation from revolution.
  • apogee
  • attraction
  • axis/axes
  • circular
  • elliptical
  • gravity
  • orbit
  • perigee
  • revolution
  • rotation

Students explore motions in the Earth/Moon system


The Moon and the Earth are held together by gravity.  The Earth is much more massive than the Moon causing  the Moon to orbit the Earth. The Moon revolves (orbits) eastward looking in the sky from Earth.  Each orbit takes 27.3 days. The Moon also rotates, or spins on an internal axis once every 27.3 days. The rotation and revolution take the same amount of time generally due to gravitational attraction of the Moon to the Earth. It makes one rotation per revolution. The Earth/Moon system also revolves around the Sun, taking 365.25 days (or a year) to complete one orbit.

The Moon’s orbit around the Earth is slightly elliptical or oval-shaped. At its closest point (perigee), the Moon is 363,000 kilometers from the Earth. At its maximum distance (apogee), the Moon is 405,000 kilometers away.

The elliptical orbit of the Moon may reflect its origin. Current evidence suggests that the Moon formed after the collision of the Earth with a protoplanet early in the Solar System’s history. The debris from this collision coalesced to form the Moon. Computer models suggest that the early orbit of the Moon may have been highly elliptical, and became rounder with time.  Due to the rotation, both the Earth and Moon are slightly wider at the equator than between the poles. They are not perfect spheres, which makes their orbits a little erratic. The Moon’s internal structure is slightly uneven, which would also contribute to an elliptical orbit. Finally,

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. Note: there is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term "dark side" in the past may have referred to the far side as "dark" in the sense of "unknown" (i.e. "darkest Africa") but even that meaning is no longer valid today!

  1. This lab comprises 4 stations, each of which has questions for the students to answer. Each station helps to understand a different concept in the relationship of the Earth/Moon system. Set up the stations as described in the Lab below. You may want to make multiple sets for each stations, to allow the students to work more quickly.
  2. Explain the rotations and revolutions of the Earth/Moon system to the students. The Moon revolves around the Earth due to the gravitational attraction between the two.   
  3. Have the students work in groups, and answer the questions on the worksheet.
    STATION A. stretchy substance with a handle
    1. An elliptical orbit can be created at a high speed.
    2. A circular orbit forms at lower speeds.
    3. The Moon’s orbit is elliptical, but not eccentric (more elongated). The Moon orbits the Earth because the latter is much more massive. The reason for the shape of the orbit is unclear, but relates to how the Earth/Moon system has evolved through time.
    4. Gravity holds the Earth and Moon together.

    STATION B. plain styrofoam ball, styrofoam ball with pin

    1. The students should realize that a spherical object will roll in a straight line, but once a sphere becomes oblate (its center of mass is offset from its center), its travel path will change. This is like the Earth/Moon relationship; both bodies are not perfectly spherical, so their rotations and revolutions are slightly distorted or they wobble.

    STATION C. 2 styrofoam balls with handles

    The students should visualize that the Moon revolves around the Earth, and both the Earth and the Moon rotate.

    1. The small styrofoam ball is the Moon.
    2. The Earth is the large ball.
    3. Students may have difficulty showing revolution, until they try putting one ball upside down.

    STATION D. Solar System (Illuminated Orbiter)

    Students should be able to see that this planetarium does not show distances correctly. However, it does show the correct Moon/Earth relationships. They may also note that only Venus, Earth, and the Moon are included; the other planets are missing. They may also make silly comments, such as the Sun is not a light bulb!

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