Glossary

Solar and Sidereal Month
 

In Lesson 1, you learned about the motion of the Moon. You might remember that it takes 27.3 days to complete one revolution around the Earth. But in Lesson 2 you learned that it takes around 29.5 days for the Moon to complete the phase cycle (15 days of waxing + 15 days of waning). If the Moon completes one revolution around the Earth in around 27 days why does its phase cycle take about 29 days?

Let us look at it more carefully. Suppose we begin observing the Moon on a Full Moon night. To keep track of its position, we identify a bright star nearby. Let us name that Star A. The next day, the Moon will rise a little later and is closer to another bright star, B. The third day, it rises even later and is closer to another bright star, C. In this manner, the Moon will pass through its orbit everyday, appearing to be closer to a different star. When it comes back to Star A again, it would have completed one revolution around the Earth, and we would see the Full Moon again. But we don’t! Instead we still see the waxing gibbous. Because the Earth has moved ahead in its orbit, the Moon needs another couple of days to catch up with the Earth. After a couple of days, the Moon is again exactly on the opposite side to the Sun, and we see the Full Moon again. At the time of the Full Moon, the Moon will be seen near Star C instead of Star A, as it was seen at the time of earlier Full Moon.

Let us do an activity to learn this.
 

 Role play :  Motion of the Moon with respect to Stars
Procedure:

1. We need a group of at least 6 students for this activity. As many as 30 students can be accommodated in this activity.

2. Three students will play the roles of the Sun, the Earth and the Moon. The rest will be stars. If there are 27 stars, they should all stand in a large circle around the Sun, Earth and the Moon. If there are only three stars, they should stand closeby on one side of the Sun-Earth-Moon as shown in Figure 2. All extra stars can stand around in the circle.

3. Take the Full Moon position such that Star A is seen close to the Moon in the background.

4. Now, the Moon should revolve around the Earth, and at the same time, the Earth should also start revolving very slowly around the Sun (Mind the direction in which the two move! They should be the same!). Make sure that the speeds of revolution of the Earth and the Moon are such that the Moon completes 12 revolutions around the Earth by the time the Earth completes one revolution around the Sun. (You can ignore the rotation of the Earth for this activity since it is not relevant here; also, the Earth will not be able to rotate so much!)

5. By the time the Moon comes back to where it started (near Star A), the Earth must have moved forward in its orbit. So the Moon will have to move a little ahead, close to Star C, so it is Full Moon again.

Draw a diagram explaining the difference between solar and sidereal month.

Use your notebook for drawing. Please write following on page where you are drawing- The Basic Astronomy Module: Unit 2: Lesson 4: Activity 2 & your Login ID

 See Figure 2 for reference.

Figure 2: The sidereal month is of 27.3 days and the synodic month is of 29.5 days.

The Moon completes one revolution with respect to the background stars in 27.3 days; so it is called a sidereal month (derived from Latin word ‘sidera’, which means star); the Moon  completes one phase cycle in 29.5 days, which is called a synodic month (derived from the Greek word sunodikos which means pertaining to the Sun and the Moon in this case). The old calendars use the synodic months, but at the same time, people also wanted to keep track of motion of the Moon with respect to the stars. So for each day a constellation (stars in a certain arrangement) near the Moon was identified. In Chaitri Panchanga (Hindu calendar which starts in the month of Chaitra) and in most of its variations, these specific constellations are called ‘Nakshatra’. Since the Moon takes a little more than 27 days to complete its revolution with respect to the stars, there are 27 Nakshtras.

The months are named after the Nakshatra in which the Moon is seen on the Full Moon night in that month. For example, the month in which the full Moon is seen near the Chitra nakshatra is called Chaitra. In the month of Vaishakha, the Moon is seen near the Vishakha Nakshatra. Table 1 gives the list of all Nakshatras. Can you identify the names of the months which are based on some of them?

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Identify the names of the months corresponding to some of the Nakshatras.

                                                                                                 Table 1: List of Nakshatras.

                                     
There are 27 Nakshatras. In one year the Moon completes a little more than 12 revolutions around the Earth; so there are 12 months. So the months are not named after all the Nakshatras. Only some Nakshatras, near which the Full Moon is seen get months named after them.
 

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