The far side of the Moon is the side that never faces Earth, and it is not always dark. In fact, a little more than half of the Moon’s surface is lit all the time. From Earth, you can only see about half of the Moon’s surface and it is always the same.
So how does the Moon orbit around Earth so that only one side of the Moon faces Earth?
Only one side of the Moon’s surface can be viewed from Earth because the Moon makes one rotation on its axis as it revolves around Earth.
- axis An imaginary line, from top to bottom, through the center of a sphere.
- rotate To spin about an axis. This is much like the spinning of a toy top.
- revolve To move in a path about another body. Orbit is another term used for revolve. The Moon revolves (orbits) around Earth.
It is difficult for me to make a mental picture of these two Moon movements. So, I designed an investigation to determine how only one side of the Moon’s surface is visible from Earth.
Discover for Yourself
1. Draw a circle in the center of a piece of paper and label it EARTH. Lay this paper on the floor.
2. Mark an X in the center of a second sheet of paper. Secure this paper to a wall as shown.
3. Using diagram A as a reference, stand in position 1. In this position you are facing the X on the wall as well as the Earth.
5. Continue to move around the Earth, so that you are always facing the Earth. Notice that as you revolve (orbit) the Earth, the same side of your body faces the Earth at all times.
Why? To continue facing the Earth, you had to turn your body slightly as you revolved. During the revolution–movement about the Earth, your body made one complete rotation. This means that you turned around one time. The Moon makes one rotation each time it orbits (revolves) around Earth.
6. Using diagram B as a reference, stand in position 1. As before, you are facing the Earth as well as the X on the wall.
7. Walk from position 1 to position 2, but continue facing the X.
8. Continue to move around the Earth, so that you are always facing the X . Notice that as you revolve (orbit) the Earth, different sides of your body face the Earth.
Why? The movement in diagram B exposes every side of your body to viewers on Earth. If the Moon did not make one rotation during the time it orbits Earth, its surface facing Earth would continually change. We would not see “The Man in the Moon.” Actually, I think it looks more like “The Rabbit in the Moon.”
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