Glossary

 

Activity 1a: Phases of the Moon (Model)

Note: This experiment needs to be performed in a dark room.

Material: Small spherical object (a ball or a lemon), stiff stick or a cycle spoke, light torch/bulb

Procedure:

1. Attach the ball firmly to the stick. This ball will act as a Moon.

2. Now stretch your hand, bring the hand up to the shoulder level and move the ball around yourself. (Remember to rotate according to the Right Hand Thumb Rule!) Only one side of the Moon will be visible to you. This is how the Moon looks from the Earth. So your head will be the Earth.

3. Now ask your friend to stand about 2 metres away from you and flash the torch in your direction. These are sunrays.

4. Again stretch your hand such that the Moon is between the Earth and the Sun. Do you see any of the lit part of the Moon¹? No! This is the position of the new Moon.

5. From here, start rotating anticlockwise to complete one full rotation and observe what you see.

   • After a slight rotation a small lit part of the Moon will be visible. Notice its shape. Is it same as a crescent? The crescent will get bigger while you continue your rotation till 90^° from the original position.

   • At 90^° you will be able to see half of the lit part. This is the position for the half Moon.

   • Continue your rotation and you will start seeing more than half of the lit part of the Moon. This shape will be the same as the gibbous Moon, and it will get bigger while you complete your rotation by another 90^°.

   • When the Moon is on exactly the opposite side to the Sun (torch), you will be able to see the entire half sphere which is lit. (If your head is blocking the torch light, move the Moon little above your head). This is the position the full Moon. This completes the waxing of the Moon.

   • Continue your rotation, and you will notice the gibbous Moon again, and it will get smaller while you rotate for another 90^°.

   • At 90^° you will again see the half Moon. But this will be the waning half Moon. Notice that, this time, a different half is visible from the last time (left half of the Moon was lit at the time of waxing, and this time, the right half is lit).

   • Continue the rotation and you should be able to see the crescent getting smaller and smaller.

   • When you complete the rotation, none of the lit part will be visible again. You are back to the new Moon position!

¹As we already discussed in Lesson 1, on the Earth there is scattered light from atmosphere, so even in a dark room you will be able to see the part of the Moon which is not directly lit. But in space, there is no atmosphere to scatter the light so the dark part of the Moon is invisible.

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Activity 1b: Phases of the Moon (Role play)

Note: This role play works best if it is done outdoors when the the Sun is a little lower in the sky, that is, early morning (around 9 am) or late afternoon (4 pm).

If you are doing it indoors, make sure that the light is coming from one side; either by opening the windows only on one of the walls or keeping a light source (e.g. a projector) on one side.

Procedure:

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

2. In each pair, the students on the left will be the Earth and the students on the right will be the Moon.

3. The Moon should stand between the Earth and the light source (the Sun). Is there any direct light on the Moon’s face? No! The back of her head should be lit, but it is not visible from the Earth.

4. The Moon should slowly rotate and revolve around the Earth, just as you did in the last class.

5. The Earth should carefully observe how much of the Moon’s face is lit. As the Moon starts rotating, the light will start falling on her left cheek, resembling the shape of a crescent and gradually getting bigger.

6. When the Moon completes quarter of its revolution (stop and let the Earth watch), exactly half of the Moon’s face should be lit. (Note that half of the Moon is lit, out of which half is visible to the earth the remaining half (the lit part of the head) is not visible from the Earth.)

7. As the Moon continues its revolution, more than half of the face will be lit, resembling the gibbous Moon. It will get bigger and bigger till the Moon revolves by another 90^°.

8. When the Moon is exactly on the opposite side of the Earth, the entire face is lit (the Earth should bend down a little if she is blocking the light). This is the position of the Full Moon. It completes the waxing of the Moon.

9. As the Moon continues its revolution, the lit part of the face will start getting smaller, resembling the Gibbous.

10. When the Moon completes its revolution by another 90^° (three fourths of its complete revolution), you will see that half of the face is lit. Notice that this time, a different half is visible from the last time (left cheek of the Moon was lit at the time of waxing half, and right cheek this time).

11. Now as the Moon continues the last quarter of its revolution, you will see less than half of the face is lit, resembling a crescent, and the crescent will get thinner and thinner.

12. Finally, when the Moon completes its one revolution, we are back with the new Moon position when there is no light on the face. This completes waning of the Moon.

Can you draw what you have learnt? See Figure 2 for your reference. It shows the Earth-Moon system as seen from above the North Pole. The Moon is shown at eight positions in its orbit. Notice that half of the Moon is lit at any given point. The image next to the Moon, outside the orbit shows how the Moon would look from from the Earth when the Moon is at that position.
 

Figure 2: Explanation of Phases of the Moon
 

New moon to New Moon completes one phase cycle. (You can count from Full moon to Full moon or from any other phase to the same phase). It takes around 29½ days to complete the phase cycle, which is why, in many ancient calendars, there are of 30 days in a month. In the Indian calendars which define a month based on the phase cycle, each day (tithi) is referred to by the corresponding phase of the Moon. For example, today (when I am writing this lesson) it is Shukla Chaturdashi (14th day of waxing) of the month of Shravan and tomorrow, on the full Moon of Shravana, I will celebrate Raksha Bandhan (also known as Rakhi Pornima, Jhulan Purnima, Narali Pornima, Salono, Janai Purnima or Rishitarpani in different parts of India). Many of the festivals celebrated in India are based on the different phases of the Moon. Can you find such festivals and note them in your notebook?
 
Find out what is today’s tithi for you.

Activity 1c: 

1. Can all people on Earth see the same phase of the Moon on any given day?
2. Do the people in the southern hemisphere see the same phase of the Moon as those in the northern hemisphere?
3. Imagine that you are on the Moon. How would the Earth appear from the Moon? As we have already discussed in Lesson 2, the apparent size of the Sun and the Moon is almost the same from the Earth. Would the size of the Sun and the Earth appear the same from the Moon? If not, would the Earth appear smaller than the Sun or bigger than the Sun? Would you be able to see the phases of the Earth?
   1. How would the Earth appear from the Moon when we see the full Moon from the Earth?
   2. How would the Earth appear from the Moon when we see the new Moon from the Earth?
   3. How would the Earth appear from the Moon when we see the half Moon from the Earth?
   4. When we see lunar eclipse from the Earth, what would you see from the Moon?