Assertion : There are very small sporadic changes in the period of...

Normally when a body is moving in a straight line, whatever be the internal changes, the centre of mass continuous in its trajectory with the same velocity and a system that is rotating continues to rotate with the same angular velocity and changes occur in such a way that the angular momentum is conserved. Classical physics, taking the whole of the earth as a system does not allow sporadic changes in the speed of the earth. However, according to the information on the earth's rotation by U.S. Naval observatory sporadic changes of the speed of rotation of the earth have been observed and this is attributed to the influence of the moon on the tides. This makes it necessary to take the earth and the moon (at least) as one system. In the reason given, though normally when taking the earth as a single system, internal changes do not affect the motion, now that one has to take the earthmoon system, the reason that the earth's angular velocity can also change due to the events on earth alone cannot be ruled out. Then the total angular momentum of the earth-moon system will be conserved. The reason is also right, though according to the rules of the game for the $12^{\text{th }}$ class examinations, one can take only the earth as a whole as a system. In that case, both the assertion and reason are wrong.