second

a unit of measurement of plane angles. It is equal to 1/3600 of a degree or 1/60 of a minute and is denoted by the symbol”.

a unit of time. It is one of the seven base units of the International System of Units. The following definition was adopted by the 13th General Conference on Weights and Measures, held in 1967: the second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. The second defined in this manner is known as the atomic second. It can be reproduced by means of cesium frequency and time standards, which permit the determination of the frequency of the radiation of cesium-133 atoms when transitions occur between two fixed energy levels (seeQUANTUM FREQUENCY STANDARDS).

Besides the atomic second, such sciences as astronomy and geodesy make use of the second whose definition is based on the period of revolution of the earth about the sun. This unit is called the ephemeris second, and it is determined from astronomical observations. The ephemeris second is taken as 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time—that is, the year beginning at noon on Dec. 31, 1899. The precise date is indicated in the definition of the ephemeris second because the tropical year is not a constant.

Before the introduction of the ephemeris second in 1956, the time standard was the second defined as 1/86,400 of the mean solar day. This unit, however, was insufficiently stable because of variations in the speed of rotation of the earth. The introduction of the ephemeris second and then of the atomic second permitted the precision of the time standard to be improved by several orders of magnitude. When the atomic second is reproduced by cesium standards, the error at the present time is approximately ± 1 × 10^-12.