A source, natural or artificial, of events believed to occur at such uniform intervals of time that the frequency of the events can be used as a standard against which other frequencies or intervals of time can be compared. The Earth's rate of rotation with respect to the distant galaxies or fixed stars was at one time considered suitable as a frequency standard. The period of rotation was subdivided into smaller units by clocks. When the rate of rotation was found to be irregular, the rate of revolution of the Earth in 1900.0 was adopted. This standard was replaced in 1968 by the frequency of light emitted by the cesium atom in a specified transition between atomic energy levels. Frequency standards whose accuracy is highest are called primary standards; those of high precision but of lower accuracy are called secondary standards.