The Oxford English Dictionary defines the word "realise" (also spelt "realize") as "to convert (something imagined, planned, etc.) into real existence or fact." The International vocabulary of metrology identifies three distinct ways in which this is done - the first being the realisation of a measurement unit from its definition, the second the reproduction of measurement standards and the third the process of actually adopting a particular artefact as a standard.
The realisation of time has gone through three phases. During both the first and second phases, man used Solar time—during the first phase, realisation of time was by observing the earth's rotation using such devices as the sundial or astrolabe. During the second phase actual timing devices such as hourglasses or clocks were used. If the user needed to know time-of-day rather than elapsed time, clocks were synchronized with astronomical time. The third phase made use of clocks that were sufficiently accurate that they could measure variations in the earth's rotation—such clocks taking over from the rotation of the earth as the prime measure of time.
Units of length, along with mass (or weight) and time, are one of the earliest quantities that was measured by man. Historically two distinct approaches were used - one was to use a naturally occurring phenomenon such as a particular seed or part of the human body, the other was to use a standard length that was held by a community leader.