The second is the base unit of time in the International System of Units and is also a unit of time in other systems of measurement. Between 1000 and 1960 the second was defined as 1/86,400 of a mean solar day. Between 1960 and 1967, it was defined in terms of the period of the Earth's orbit around the Sun in 1900, but it is now defined more precisely in atomic terms. Seconds may be measured using mechanical, electric or atomic clocks. Astronomical observations of the 19th and 20th centuries revealed that the mean solar day is slowly but measurably lengthening and the length of a tropical year is not entirely predictable either; thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be: SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond, the microsecond, and the nanosecond. Though SI prefixes may also be used to form multiples of the second such as kilosecond, such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day. Wikipedia [ - ]

The second is the base unit of time in the International System of Units and is also a unit of... [ + ]