# time

Time is an observed phenomenon, by means of which human beings sense and record changes in the environment and in the universe. A literal definition is elusive. Time has been called an illusion, a dimension, a smooth-flowing continuum, and an expression of separation among events that occur in the same physical location.

Time is a practical convenience in modern life. Numerous standards have been set up, allowing people to coordinate events and, in general, keep their lives running smoothly. The earth has been divided into so-called time zones that reflect the fact that high noon occurs at different times at different places on the planet. All of these time zones are referenced to the time at the longitude of Greenwich, England. A universal standard, coinciding almost exactly with the time at Greenwich, is known as Coordinated Universal Time (UTC). There are various other time standards.

The fundamental unit of time in the International System of Units (SI) is the second (symbolized s or sec). One second elapses during the occurrence of exactly 9,192,631,770 (9.192631770 x 10^{9}) cycles of the radiation produced by the transition between two levels of the cesium 133 atom. Other common units of time include the hour, the mean solar day, and the synodic year (sun-based earth year). The table below shows the relationship among the second, the hour, the mean solar day, and the synodic year.

Unit(and symbol) |
To convert time in given unitto time in seconds, multiply by: |
To convert time in secondsto time in given unit, multiply by: |

hour (hr) | 3600 | 2.7778 x 10^{-4} |

mean solar day (dy) | 8.6400 x 10^{4} |
1.1574 x 10^{-5} |

synodic year (yr) | 3.1558 x 10^{7} |
3.1688 x 10^{-8} |

Isaac Newton believed that time is continuous, and that it flows at an unchanging rate everywhere in the universe. This was accepted by most scientists until the Michelson-Morley experiment around the end of the 19th century, from which it was discovered that the speed of light is the same regardless of the direction of propagation, and regardless of the motion of the source. Albert Einstein considered this result an axiom, from which he derived the special and general theories of relativity. According to relativistic physics, the rate at which time passes depends on the relative motion between observers, and also on the strength of a gravitational or acceleration field.

Time is one of three primary phenomena in the meter/kilogram/second (mks) and centimeter/gram/second (cgs) systems of measurement. It is one of seven fundamental quantities or phenomena in the International System of Units.

Also see mks system of units, cgs system of units or small-unit metric system, the International System of Units (SI), and our Table of Physical Units.

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