EVERYDAY SCIENCE

Detailed explanation-1: -The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s-1.

Detailed explanation-2: -second, fundamental unit of time, now defined in terms of the radiation frequency at which atoms of the element cesium change from one state to another. The second was formerly defined as 1/86, 400 of the mean solar day-i.e., the average period of rotation of the Earth on its axis relative to the Sun.

Detailed explanation-3: -Also referred to as the metric system, the System of Units is commonly abbreviated as SI, which comes from the original French name, Système international d’unités.

Detailed explanation-4: -The amount of time it takes for the Earth to turn once about its axis, or for it to rotate once about the sun, is fairly stable, and for much of human history, it sufficed as a way of marking the passage of time.