Time is measured using **SI Unit** **Second**, which is defined as duration of **9192631770 periods** of the radiation corresponding to transition between two hyperfine levels of ground state of the **caesium 133 atom**. Here caesium atom is in its ground state at temperature **0 kelvin**.

Dimension of time is **[T]**

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## Different Time Scales

There are two major systems for measuring time one is **Dynamical time** which is based on orbital motions of Earth, Moon and planets; other one is **Atomic time** which is based upon atomic clocks.

### Dynamical Time

**Dynamical time** may be thought of as the independent variable in the equations that describe the motions of the bodies in the Solar System. It was developed from the natural system of timekeeping based on the apparent motion of the Sun across the celestial sphere.

### Atomic Time

By the middle of the twentieth century, the relative accuracy of the unit **Dynamical time** interval was, at best, around **1 part in 10 ^{8}**, which was proving inadequate for particular applications in astronomy, spectroscopy and telecommunications.

This accuracy limit was imposed by the irregularities in the period of the Earth’s rotation. The continued developments of atomic clocks in the 1950s led to an improvement in the accuracy of time measurement to 1 part in 10^{9}, which in turn led to the discovery of the variability in the Earth’s rotation period. Currently, the most accurate atomic clock is a type known as a **caesium fountain atomic clock**, with a measurement uncertainty of around 5 parts in 10^{16}.

## Time Systems Currently in use in the World

Time System | Definition |
---|---|

Atomic Time | It is defined in terms of duration of a specified number of cycles of radiation corresponding to transition of two hyperfine levels of the ground state of at absolute zero.^{133}Cs |

Internation Atomic Time(TAI) | It is calculated from statistical analysis of individual frequency standards and time scales based on atomic clocks situated throughout the world. |

Universal Time(UA) | Universal Time is based on Mean Solar Day and UA is used for all civil timekeeping. UT0 is rotational time for a particular location on the Earth, uncorrected for shifts in longitude due to polar motion. UT1 is the rotational time corrected for such shifts, though it is still non-unifom |

Coordinated Universal Time(UTC) | UTC is the time scale distributed by means of radio signals, satellites, radio and TV broadcasts. UTC differs from Internation Atomic Time(TAI) by an integral number of seconds and is kept to within 0.9 seconds of UT1 by means of the irregular introduction of integer leap seconds. |

Terrestrial Dynamic Time(TDT) | TDT is idealized time based on the rotation of the geoid. |

Barycentric Dynamical Time(TDB) | TDB is relativistically transformed time for referring equations of motion to the barycentre of the Solar System. |

Geocentric Coordinate Time(TCG) | Geocentric Coordinate Time is a coordinate time related to the centre of mass of the Earth as its spatial origin. |

## Relationships Between Different Time Systems

**Terrestrial Dynamic Time(TDT)**is ahead of**Internation Atomic Time(TAI)**by**32.184 seconds**. So mathematically,**TDT = TAI + 32.184 seconds****Geocentric Coordinate Time(TCG)**is related to**Terrestrial Dynamic Time(TDT)**by following formula.

Here in the formula**JD**is Julain Day Number, which is equal to number of days that have elapsed since Greenwich Mean Noon on 1 January 4713 BCE.