What is Time?
Time is a fundamental property of the universe, and can be considered a fourth dimension along with the three spacial dimensions, whilst also being inherently different. It is used as a measurement of the sequence of events, or to compare the intervals between those events and to calculate the motions of objects. In science, philosophy and religion, time lies at the heart of all understanding. Time is essential to relate physical properties and their rates of change. The sense of the efluxion of time is the very essence of human experience and consciousness. And yet in all these fields of study, time has no clear or unambiguous definition.
A purely practical definition of time can be obtained by observing a particular number of repetitions of one cyclical event such as the passage of a free-swinging pendulum, or the rotation of the earth about its axis. The SI unit of time is the second. Larger units such as the minute, hour and day are all defined in terms of the second. The larger units are "non-SI" units because they do not use the decimal system. However, they are formally accepted within the International System of units. The relation between seconds and months or years is not straightforward as these are astronomically based and have variations in length. This can be due to perturbations in orbit or because, for example, the time taken for the earth to orbit around the sun is not an exact multiple of the time the earth takes to rotate about its axis.
Several types of devices have been invented to measure time. Horology is the study of these devices. In ancient days, people used incense sticks, candles, sun dial, water clock, and hour glasses or astronomical observation for calculating the time. In the 11th century Chinese engineers invented the first mechanical clocks to be driven by an escapement mechanism. In the 18th Century the demand for accurate timekeeping was driven by the need for accurate naval navigation, and very accurate mechanical clockwork chronometers were produced, most notably by John Harrison.
Clockwork devices continued to be the main technology for timekeeping until electronic watches and clocks became widely available from the 1970’s, based on oscillating quartz crystals. Before this only reference clocks used these devices. Nowadays most time-keeping is based on these devices, except for reference clocks which have moved on to even more accurate technologies such as atomic clocks, which are accurate to within fractions of a second per millennium. These are now mostly based on the periodic properties of the caesium atom. Since 1967 the properties of the caesium atom have been the actual definition of the SI unit of time, the second.
As well as measuring time second by second, longer periods are measured in calendar time which is aligned with the motion of the earth. To keep these longer time periods synchronised with the seasons of the year (and to a lesser extent, day and night), periodic corrections are made to the calendar such as leap years and leap seconds.