Seconds

The second is the base unit of time in the International System of Units (SI). It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. This atomic definition, adopted in 1967, provides a precise, reproducible standard for measuring time across science, industry, and daily life.

Seconds can be subdivided into smaller units, such as milliseconds (one thousandth of a second), microseconds (one millionth), and nanoseconds (one billionth). Larger time units are built from seconds: 60 seconds make a minute, 60 minutes make an hour, and 24 hours make a day. In civil time, a day is nominally 86,400 seconds, though occasional leap seconds adjust UTC to align with Earth's rotation.

Timekeeping relies on clocks that accumulate seconds. Atomic clocks, using cesium, rubidium, or hydrogen-frequency standards, are the most accurate today and form the basis of international time scales such as International Atomic Time (TAI) and Coordinated Universal Time (UTC). Leap seconds are inserted irregularly to keep UTC within 0.9 seconds of UT1, a time scale based on Earth's rotation.

In addition to measuring durations, the word second appears in other contexts, such as seconds of arc, which describe angular distances, and the second hand on a clock. The concept underpins much of science, navigation, computing, and everyday scheduling.