Early definitions of the second were based on the apparent motion of the sun around the earth. The solar day – the time for the Earth to spin once and face the Sun so the Sun is at the same latitude on the Earth's surface - was divided into 24 hours, each of which contained 60 minutes of 60 seconds each, so the second was 1⁄86 400 of the mean solar day.
Accurate astronomical observations in the nineteenth- and twentieth-century revealed that this average time is increasing, because the rate of rotation of the Eart is slowing due to internal tidal forces, and thus the sun/earth motion is no longer considered accurate enough to be a basis for defining the second. With the advance of technology and the invention of atomic clocksatomic clocks, it became feasible to define the second based on fundamental inherent properties of nature. Since 1967, the second has been defined to be
the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used, giving way to everyday units such as minute, hour, day, week etc.
The second is also the base unit of time in the SI system, which is designed so that physical equations are self consistent and need no conversion factors.