Second

The second (symbol: s) is the base unit of time in the International System of Units (SI). It is the most common measure for short time intervals and serves as the foundation for all other time units: 60 seconds make a minute, 60 minutes make an hour, and 24 hours make a day.

The second is not merely a measure of time; it also appears in the definition of many other units, such as the meter (defined as the distance light travels in a fraction of a second) and units of energy and frequency. Consequently, the precision of the second directly affects the precision of scientific measurements in general.

Origin of the name and division of time

The division of an hour into 60 minutes and a minute into 60 parts derives from the sexagesimal (base-60) numbering system used in ancient Mesopotamia. The European term 'second' comes from the Latin phrase meaning 'the second division'—referring to the second subdivision when an hour was divided. In Somali, the word 'ilbiriqsi' originally refers to a blink of the eye, an image representing an extremely short duration.

Earlier astronomical definition

For a long time, the second was defined by Earth's rotation: it was one of 86,400 parts of an ordinary solar day. When it became clear that Earth's rotation is not uniform—it gradually slows and undergoes small variations—this definition proved inadequate for precise measurement needs. For a period, a definition based on Earth's orbit around the Sun (the year) was used.

Atomic definition

Since the late 1950s, the second has been defined by the motion of electrons within an atom. The current definition is based on the number of cycles of radiation emitted during a particular transition in the caesium-133 atom. When a specified number of cycles is counted, one second is obtained. Atomic clocks operating on this principle are now the most accurate, and they serve as the basis for Coordinated Universal Time (UTC).

Because Earth's rotation does not synchronize precisely with atomic clocks, a 'leap second' is occasionally added to UTC to keep the difference between atomic time and astronomical time within acceptable bounds. This practice has provoked debate among those who manage computer and telecommunications systems, and proposals exist to discontinue the use of leap seconds in the future.