Modeling the Moon’s Rotation

Trying to visualize two of the Moon’s movement, rotation and revolution, can be difficult.

The diagram shows four locations of the Moon in its eastward revolution (orbiting) about the Earth.  The red line on the small circles represents one position on the Moon. 

Question: What parts of the Moon would an observer from Earth see if the Moon did not revolve on its axis?

Answer:

In the diagram, the  small circles connected by the black line represent the movement of the Moon around the Earth if the Moon did not rotate on its axis.  

In position 1, the red line points toward the East. If the Moon did not rotate on its axis, when it moved in its orbit around the Earth, the red line would always point toward the East.

Discover for Yourself

You could model this using anything to represent the Earth. Place your Earth model where you have enough room to walk around it. Letting each wall in the room represent a compass direction, stand so that you are facing the Earth Model as well as facing the east wall.

Now start your rotation around the Earth, but continue to face the east wall. Make note of the part of your body that faces the Earth in each of the four positions.

Position one–you face the Earth
Position two- your left side faces the Earth
Position three–your back side faces the Earth
Position four- your right side faces the Earth

Question: What parts of the Moon would an observer from Earth see if the Moon revolves once on its axis as it orbits the Earth? 

Answer:In the diagram, the circles closest to the Earth represent  the movement of the Moon around the Earth if the Moon Rotates ONCE during the orbit. Notice that the part of the Moon’s surface pointed to by the red line always faces the Earth.

Discover  for Yourself

Repeat the previous activity, but this time you must continue facing the Earth Model.  For each of the positions, which compass direction are you facing?

Using the information from this investigation, explain why an observer on Earth only sees one side of the Moon.