Glossary

As you know, the Moon revolves around the Earth. It rotates around its own axis while revolving around the Earth. The direction of its rotation as well as its revolution is the same as the direction of the rotation of the Earth (anticlockwise if seen from above the North Pole). You can find this using the Right Hand Thumb Rule. Interestingly, the Moon takes the same time to complete one rotation around itself as it takes to revolve around the Earth. Let us see how this motion looks.

 Role Play :  Motion of the Moon Around the Earth
Procedure:

1. Stand in pairs, at least a couple of feet apart.

2. In each pair, the student on the left will be the Earth, and the student on the right will be the Moon. The Moon and the Earth should face each other.

3. Rotation Only: First, the Moon should make one rotation slowly. Can Nitu (the person on the nose) see all sides of the Moon (front and back)?

4. Revolution Only: Now the Moon should make one revolution around the Earth without rotation. The Moon has to keep its face in the same direction. Can the Earth see all sides of the Moon (front and back)?

5. Rotation + Revolution: The Moon should complete one rotation and one revolution at the same time. Break this motion into 4 quarters. In each quarter the Moon should rotate exactly by 90^°. Can the Earth see all sides of the Moon (front and back)?

6. Perform a few more revolutions. Notice that only the face (one side) of the Moon is visible from the Earth.

  Can you draw a diagram to explain why we see only one face of the Moon?

Use your notebook for drawing. Please write following on page where you are drawing- The Basic Astronomy Module: Unit 2: Lesson 1: Activity 1 & your Login ID

Suppose we put a flag at the centre of the part of the Moon visible to us. By the time the Moon completes quarter of its rotation, it will rotate by 90^° so the flag will remain at the centre of the visible part (see Figure 3). The Moon continues its motion in this fashion, and the flag will always remain pointed towards the Earth.
 

Figure 3: Motion of the Moon

The Moon takes 27.3 days to complete one rotation (and revolution). That means the Earth has to rotate 27 times while the Moon completes one revolution.

The Moon shown in Figure 1 is the side of the Moon we always see from the Earth. Figure 4 is a photograph of the other side of the Moon (the side which we never see from the Earth).
 

Figure 4: The other side of the Moon
(Image Credit: By NASA/GSFC/Arizona State University - http://wms.lroc.asu.edu/lroc_browse/view/WAC_GL180 (see also http://photojournal.jpl.nasa.gov/catalog/PIA14021), Public Domain, https://commons.wikimedia.org/w/index.php?curid=14842928)