In Lesson 5, 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 6 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.