Measurement of Time :
Time plays an important role in our life. Any change, or movement which takes place, is understood in terms of a physical dimension called ‘time’.
There are many phenomenon by which we can understand time.
(i) The time between one sunrise and the next is called a solar day.
(ii) A month is measured from one new moon to the next.
(iii) The time taken by the earth to complete one revolution around the sun is called year.
The early scientist developed many devices to measures time intervals. All of them make use of some periodic motion.

Sundial :
People in older times measured intervals of time shorter than a day with a device called sundial The device worked on the principle that as the position of the sun in the sky changed, so did the position and length of the shadow cast by an object. The                 decrease and then increase in the length of the shadow during day time was used to measure time in sundials. The sundial has a triangular metallic plate called gnomon fixed vertically at the centre of a circular plate. The device is placed in the open in such a    way that gnomon points in the north south direction. The shadow of gnomon on the circular scale shows the time at that particular moment. Sundials indicate time quite accurately but the problem with them is that they cannot be used after sunset or on a            cloudy day they can also not be carried along to different places. One sundial can be seen at Jantar Mantar in New Delhi.

Candle Clocks and Sand Clocks :
Romans actually used sand clocks which worked on the principle that a definite amount of sand takes constant time to fall from the upper chamber to the lower chamber. That constant time was the unit of measurement of time commonly called an hour.         The sand clock is also called an hourglass. Once the upper chamber is emptied completely, the hourglass is turned upside down to record the time again.

Simple Pendulum :
Any event or motion which repeats itself after a constant time is called periodic motion.One of the most common periodic motion 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.
Figure (a) shows the pendulum at rest in its mean position. When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro, in figure (b). 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 O, moves from A to B and back to O. The pendulum also completes one oscillation when its bob moves from one extreme position A to the other extreme position B and comes back to A. The time taken by the pendulum to complete one oscillation is called its time period.

Units of Time :
The basic unit of time is second. Its symbol is s or sec. Other units are minutes (1 hour = 60 min),
(1 min = 60 sec), hour day (1 solar day = 24 hours), 1 year (365 days).
1 solar day = 24 × 60 × 60 = 86400 s
1 second = 1/86400th of a solar day

Note :
(i) The symbols of all units are written in singular.
(ii) Other smallest units of time 1 second = 106 microsecond; 1 sewcond = 109 nanosecond.