The second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock. Early definitions of the second were based on the motion of the earth: 24 hours in a day meant that the second could be defined as of the average time required for the earth to complete one rotation about its axis. However, nineteenth- and twentieth-century astronomical observations revealed that this average tim… (More on Second)