in timekeeping, 60 seconds, now defined in terms of radiation emitted from atoms of the element cesium under specified conditions. The minute was formerly defined as the 60th part of an hour, or the 1,440th part (60 × 24 【hours】 = 1,440) of a mean solar day—i.e., of the average period of rotation of the Earth relative to the Sun. The minute of sidereal time (time measured by the stars rather than by the Sun) was a fraction of a second shorter than the mean solar minute. The minute of atomic time is very nearly equal to the mean solar minute in duration.