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Measurement Of Time: Time is a moment or duration in which things occur. The knowledge of time is essential for carrying on our daily life activities. In ancient times people used some natural events which repeated regularly after fixed time intervals to measure time. Like the sun rises everyday in the morning. The time between one sunrise and the next was called a day. Similarly, a month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun. The important time measuring devices used in ancient times were:

1. Sundial: The sundial measures time by the position of the shadow cast by the sun.
2. Sand clock: This is also called as sand glass or hour glass. The sand clock uses the flow of sand to measure time. The length of time taken by the sand to pass from one bulb of the sand clock to the other gives a constant time interval.
3. Water clock: This device uses the rate at which water drips from one vessel to another to measure time intervals. The time taken by the entire water to drip from upper vessel to lower vessel was used for measuring time intervals.

   One of the most well-known periodic motions is that of a simple pendulum.

   A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread. The metallic ball is called the bob of the pendulum is free to swing back and forth.

   Galileo discovered the important principle of pendulum. The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion. The pendulum is said to have completed one oscillation when its bob, starting from its mean position moves to extreme position and back to mean position. The pendulum also completes one oscillation when its bob moves from one extreme position to the other extreme position and comes back to first extreme position. The time taken by the pendulum to complete one oscillation is called its time period.