Time – Time is a measure that is used to determine the period between two events. Time is considered one of the fundamental properties that can be used to define other quantities. For example velocity, which is defined as the rate of change of length with time and acceleration that is defined as the rate of change of velocity with time. The unit of time was originally taken from the solar day (the interval that elapses between two consecutive passages of the sun across the meridian (i.e. the time between true noon one day and the next) is called a solar day. Unfortunately the length of the solar day is not constant but depends on the time of the year. The units of time have also been determined from the period of the swing of a pendulum and even the heart beat. Today however, the unit of time is obtained from the caesium clock in which the time interval is based on radioactivity). The caesium clock is so accurate that it will neither gain nor loose one second in 80 million years.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links