In classical mechanics, time is isolated from space, motion and mass. It considered an absolute, unchanging quantity that flows uniformly from past to future. It is measured in seconds, which is the base unit of time in the International System of Units.
How to define and measure time?
Time is a concept that has been essential to human understanding since the beginning of civilization. It is a measure of the duration of events and is commonly defined by the natural phenomena that happens over and over again in a regular fashion.
Day and night were probably the first natural phenomenon to which humans were exposed in a cyclical manner. Thus day and night were first used to define time. The time interval between the first noon and the second noon was taken for granted as a day. Then humans wanted to know the exact moment of the day, so they invented various timekeeping tools, or clocks.
One of the earliest ways in which humans measured time was by observing the motion of the sun across the sky. The Egyptians and Babylonians used sundials to measure the time of day, while the Greeks used water clocks. Later, pendulum clocks and mechanical watches were invented. A pendulum clock uses the periodic oscillation of a pendulum to record time, while a mechanical watch uses the periodic vibration of a spring to record time. These devices are much more accurate than the ancient sundials and water clocks and are sufficiently precise for daily life.
However, these methods were not highly accurate, and it wasn't until the development of electronic oscillators and atomic clocks in the 20th century that timekeeping became much more precise.
Electronic oscillators work by producing a regular, repeating signal that can be used as a reference for measuring time. Atomic clocks, on the other hand, use the natural vibration of atoms to keep time. A second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. These incredibly accurate atomic clocks are insensitive to surrounding environment and are now used as the standard for defining the time.