Glossary

In the last lesson, we learned that the Moon rotates and revolves around the Earth at the same time. Its rotation period and orbital period (duration to complete one revolution) are the same, which is why we see only one face of the Moon.

You must have noticed that the shape of the Moon is not the same every day. How does it change?
 

Draw the different shapes of the Moon. Try to draw them in the same order in which the Moon changes its shape every day.

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

You might have noticed that, on a certain day, the Moon is not visible at all. This is called the new Moon. A crescent Moon appears on the next day. It gets bigger and bigger every day and on the eighth day appears as a half Moon. It continues to get bigger, and after another 7 days you see the full Moon. The change in the Moon’s shape from new Moon to full Moon is called the waxing of the Moon.

Then, in the reverse order, the Moon starts getting smaller and smaller. We see a gibbous (less than full but more than half) Moon for seven days, then we see a half Moon on the eighth day. Then we see the crescent getting smaller and smaller for another 7 days, and finally, there will be no Moon in the sky for a day. The change in the Moon’s shape from Full Moon to New Moon is called the waning of the Moon.

The different shapes of the Moon are known as phases of the Moon. Certainly, the Moon does not actually change its shape! Why then does the Moon’s shape appear to change?

As you know, the Moon does not have light of its own. It shines because it reflects the sunlight. At any given time how much of the Moon is lit? Just as the Earth, half of the Moon is always lit by sunlight (Figure 1). The phase of the Moon depends upon how much of this lit part is visible from the Earth. Let us try to see it with an activity.

Figure 1: Half of the Moon is lit at any given point