SI definition of the second

The second is 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.

This definition refers to a caesium atom in its ground state at a temperature of 0 K.

The dimension of time is [T], its unit is the second and its symbol is s.

Definition of time

Funk & Wagnalls New Standard Dictionary of the English Language (Funk et al., 1946) defines time, inter alia, as:

A system of reckoning or measuring duration; as solar time; sidereal time; mean time.

Systems of time or time scales

There are two major systems of time: those based on the Earth's rotation and the orbital motions of the Earth, Moon and planets, known as dynamical time; and those based on atomic clocks and known as atomic time.

A time system or scale may be specified by two numbers, the origin from which the time intervals are to be measured and the number of predefined unit scale intervals measured since the time of origin (Leschiutta, 2001).

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.