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.

 Question:  Why does the Sun and the Moon follow us every were we go?

 Answer: Sky watchers from the beginning of time have observed the presence of the Sun and Moon no matter where they travel.

Early astronomers’ explanation of this phenomenon was that Earth is flat and has a dome over it. The dome acted as a background for all celestial bodies ( the natural things seen in the sky including the Moon, the Sun, and the stars). The celestial bodies moved from one side of the dome to another, much like actors entering from one side of the stage, crossing the stage, and then exiting on the opposite side.  You can see actors on a stage from any seat in the theater, so the Sun and Moon could be seen from any location on Earth.

 This  explanation sounds rather dumb to us now, but this is because we know so much more about our universe. Never the less, I do find it fun to dream up imaginary explanation for the things we see. For example, imagine two very long invisible strings, one attached to the Moon and the other attached to the Sun. As you move the two celestial bodies must follow, much like helium balloons attached to strings. While this is a fun idea, it obviously is not true. Actually, the idea that the Sun or Moon is following you isn’t true, but since you see it and it seems to be true it is called an illusion.

 The illusion that the Sun or Moon is following you is due to their great distances from Earth. The Sun is about 93 million miles from Earth and the Moon is 238,000 miles away. No matter where you move on Earth, there is so little change in your distance from the Sun or Moon that you don’t notice it. Because of this, you never move past or away from these celestial bodies. 

Discover for Yourself
Distant objects seem to follow you, while objects near-by move out of view. Outdoors, pick out a distant tree.

1. Walk toward the tree and notice that things you pass, drop out of view but the tree remains directly in front of you.
2. Turn right and walk a short distance. Again, notice that some things drop out of view. The tree is still in view, but it is on your left side instead of being straight in front of you. Has the tree moved? No, you turned so that your left side faces it.
3. Turn right again and walk a short distance. As before things fall out of view and while the Tree is not visible you can still see it if you look behind you. Has the tree moved? No, you turned away from it.
4. Turn right again and walk a short distance. Again, some things fall out of view. The tree is now in view on your right side. Did the tree move? No, you again changed your direction so your right side faces the tree.