A second is officially 9,192,631,770 ticks of a caesium atom
For most of history a second was a slice of Earth's spin, one 86,400th of a day. Since 1967 it has been defined by caesium-133 atoms, whose ground-state transition oscillates exactly 9,192,631,770 times each second. Earth, slowing slightly and irregularly, now needs occasional leap seconds to keep pace with the clocks.
The traditional second comes from carving the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds. The names trace this: minute comes from Latin pars minuta prima, the first small part of the hour, and second from pars minuta secunda, the second small part produced by dividing again by sixty. The Earth-based definition survives in the astronomical timescale called UT1.
Base-sixty divisions of the day go back to the third millennium BC, but nobody could measure such tiny intervals then, so they were worked out mathematically. Seconds could not be counted accurately until pendulum clocks arrived in the seventeenth century. From the 1950s atomic clocks outperformed the planet's rotation, and by 1967 it was practical to anchor the unit to a fundamental property of nature.
Before good mechanical clocks, sundials were the only reliable timekeepers, and they track apparent time, the Sun's actual position. Because Earth's axis is tilted relative to its orbit, apparent time drifts against the uniform mean time a clock keeps, with the gap accumulating to as much as 15 minutes over parts of the year even though consecutive days barely differ. Astronomers had noticed this in antiquity, but until the mid-seventeenth century sundial time was the accepted standard.
Today, tiny fractions take SI prefixes: milliseconds, microseconds, nanoseconds and smaller, with prefixes spanning 10 to the minus 30 up to 10 to the 30 seconds. A one-gigahertz processor completes a cycle each nanosecond, and camera shutters open for a thirtieth or a thousandth of a second. Kiloseconds are possible but hardly anyone uses them; longer spans are written as hours, minutes and seconds separated by colons, a notation that can be ambiguous, since the same style shows hours and minutes.
Source: Second
