## Second |

second a pendulum-governed of a clock, ticking every secondescapement general information unit system si base unit unit of time symbol s the

**second**(symbol:**s**, abbreviation:**sec**) is the ofbase unit in thetime (si), commonly understood and historically defined as international system of units ^{1}⁄_{86400}of a – this factor derived from the division of the day first into 24day , then to 60hours and finally to 60 seconds each.minutes andanalog clocks often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. digital clocks and watches often have a two-digit seconds counter. the second is also part of several other units of measurement like meters per second forwatches , meters per second per second forvelocity , and cycles per second foracceleration .frequency although the historical definition of the unit was based on this division of the earth's rotation cycle, the formal definition in the international system of units (

) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆si *ν*_{cs}, the unperturbedground-state of thehyperfine transition frequency atom, to be 9192631770 when expressed in the unitcaesium-133 , which is equal to shz ^{−1}.^{[1]}^{[2]}because the earth's rotation varies and is also slowing ever so slightly, a is periodically added toleap second clock time ^{[nb 1]}to keep clocks in sync with earth's rotation.multiples of seconds are usually counted in hours and minutes. fractions of a second are usually counted in tenths or hundredths. in scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. an everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. camera

are often expressed in fractions of a second, such as shutter speeds ^{1}⁄_{30}second or ^{1}⁄_{1000}second. divisions of the day from a calendar based on astronomical observation have existed since the third millennium bc, though they were not seconds as we know them todaysexagesimal ^{[citation needed]}. small divisions of time could not be measured back then, so such divisions were mathematically derived. the first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. starting in the 1950s, became better timekeepers than earth's rotation, and they continue to set the standard today.atomic clocks - clocks and solar time
- events and units of time in seconds
- other units incorporating seconds
- timekeeping standards
- optical lattice clock
- history of definition
- si multiples
- see also
- notes
- references
- external links

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A pendulum-governed | |

General information | |

Unit of | |

Symbol | s |

The **second** (symbol: **s**, abbreviation: **sec**) is the ^{1}⁄_{86400} of a

Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (*ν*_{Cs}, the unperturbed ^{−1}.^{[1]}^{[2]}
Because the Earth's rotation varies and is also slowing ever so slightly, a ^{[nb 1]} to keep clocks in sync with Earth's rotation.

Multiples of seconds are usually counted in hours and minutes. Fractions of a second are usually counted in tenths or hundredths. In scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. An everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. Camera ^{1}⁄_{30} second or ^{1}⁄_{1000} second.

^{[citation needed]}. Small divisions of time could not be measured back then, so such divisions were mathematically derived. The first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s,