In the previous section, we studied the motion of the sun with respect to the stars. There is one more prominent object in the sky. Yes, we are talking about the moon. Let us now look at the motion of the moon with respect to the stars.
We learnt that the sun completes one revolution along the ecliptic in 365 1/4 days. The moon also revolves around the earth. What is the time taken by the moon to complete one revolution? It is 27.3217 days or 27 days, 7 hours and 43 minutes.
Thus the time taken by the moon to complete one revolution around the earth along the ecliptic in the easterly direction is called the sidereal period of revolution of the moon. It is equal to 27.3217 days. You might wonder why this is also called as "sidereal period". It is called so because this is also related to the stars.
You know very well that a circle is divided into 360o. In one hour, the minute hand of the clock covers 360 o. In half an hour, it covers 180 o and so on. The moon completes one full rotation in 27.3217 days. That is, it covers 360o in 27.3217 days. Hence in one day it covers (360/27.3217)o or 13.17o along the ecliptic per day.
On a similar basis, the sun moves by 1o along the ecliptic per day. Thus, the motion of the moon is 13 times faster than that of the sun. We can notice the motion of the moon even in 2-3 hours. Let a moon be near a particular star at 9 p.m. After a few hours the moon would have moved eastward with respect to that star.