Time

Time The unit of time, the second (s), was historically defined in terms of the rota-

tion of Earth and was equal to of the mean solar day. However,

scientists have observed that the rate of rotation of Earth is gradually slowing down.

The second is now defined in terms of a characteristic frequency associated with the

cesium atom. All atoms, after absorbing energy, emit light with frequencies and

wavelengths characteristic of the particular element. There is a set of frequencies and

wavelengths for each element, with a particular frequency and wavelength associ-

ated with each energy transition within the atom. As far as we know, these frequen-

cies remain constant. The second is now defined so that the frequency of the light

from a certain transition in cesium is exactly 9192631770 cycles per second.