Time

Time has been understood differently by different cultures, by different philosophers and physicists, and in different contexts. This article will look at these differences in terms of three main approaches: the metaphysics of time, the perception of time, and the metric of time.

Metaphysics
Extensive reviews are available ; on the philosophical and metaphysical aspects of time.

Perception
A concise review on the perception of time is available. A book on time perception is also available.

Metric
Following the International System of Units (SI), the physical time is measured in seconds. A second is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at a temperature of 0 Kelvin" (, retrieved August 21st, 2007).

Many other measure units are in common use, which often do not obey the rules of the SI. Some common examples are:
 * 1) minute (min, 60 seconds)
 * 2) hour (hr, 60 minutes)
 * 3) day (d, 24 hours)
 * 4) week (wk, 7 days)
 * 5) month (mo, 28 to 31 days)
 * 6) year (yr, 12 months)

In geology, in order to describe even larger time intervals, other units are common as:
 * 1) kiloyear (ky, 1000 years)
 * 2) Megayear (My, 1 million years)

A further unit, Ma, is used to indicate elapsed time, thus, 1 Ma means one million years before present. It is common practice to set the present at year 1950.